


 Magnetic harddirection ordering in anisotropic Kondo systems,
M. P. Kwasigroch, Huanzhi Hu, Frank Krüger, and A. G. Green,
Phys. Rev. B 105, 224418 (2022).
We present a generic mechanism that explains why many Kondo materials show magnetic ordering along directions that are not
favoured by the crystalfield anisotropy. Using a renormalizationgroup (RG) analysis of single impurity Kondo models with
singleion anisotropy, we demonstrate that strong fluctuations above the Kondo temperature drive a moment reorientation over
a wide range of parameters, e.g. for different spin values S and number of Kondo channels N. In tetragonal systems this can
happen for both easyplane or easy axis anisotropy. The characteristic crossing of magnetic susceptibilities is not an artefact
of the weakcoupling RG treatment but can be reproduced in bruteforce perturbation theory. Employing numerical renormalization
group (NRG), we show that for an underscreened moment (S=1, N=1) with easyplane anisotropy, a crossing of magnetic
susceptibilities can also occur in the strongcoupling regime (below the Kondo temperature). This suggests that collective
magnetic ordering of such underscreened moments would develop along the magnetic hard axis.
Our work won the first prize in the poster competition at the International Conference on Strongly Correlated Electron Systems
(SCES2022) in Amsterdam (2529 July 2022).
 Interplay of interactions and disorder at the chargedensity wave transition of twodimensional Dirac semimetals,
Mikolaj D. Uryszek and Frank Krüger,
Phys. Rev. B 105, 075143 (2022).
We consider the effects of weak quenched fermionic disorder on the quantumphase transition between the Dirac semimetal and
charge density wave (CDW) insulator in two spatial dimensions. The symmetry breaking transition is described by the
GrossNeveuYukawa (GNY) theory of Dirac fermions coupled to an Ising order parameter field. Treating the disorder using the
replica method, we consider chemical potential, vector potential (gauge), and random mass disorders, which all arise from
nonmagnetic charge impurities. We selfconsistently account for the Landau damping of longwavelength orderparameter
fluctuations by using the nonperturbative RPA resummation of fermion loops, and compute the renormalizationgroup (RG)
flow to leading order in the disorder strength and 1/N (N the number of Dirac fermion flavors). We find two fixed points,
the clean GNY critical point which is stable against weak disorder, and a dirty GNY multicritical point, at which the chemical
potential disorder is finite and the other forms of disorder are irrelevant. We investigate the scaling of physical observables at
this finitedisorder multicritical point which breaks Lorentz invariance and gives rise to distinct nonFermi liquid behavior.
 NonFermi liquid behavior below the Néel temperature in the frustrated heavy fermion magnet UAu_{2},
Christopher D. O'Neill, Julian L. Schmehr, Harry D. J. Keen, Luke Pritchard Cairns, Dmitry A. Sokolov,
Andreas Hermann, Didier Wermeille, Pascal Manuel, Frank Krüger, and Andrew D. Huxley
Proceedings of the National Academy of Sciences, Dec 2021, 118 (49) e2102687118.
The term Fermi liquid is almost synonymous with the metallic state. The association is known to break down at quantum critical
points (QCPs), but these require precise values of tuning parameters, such as pressure and applied magnetic field, to exactly
suppress a continuous phase transition temperature to the absolute zero. Threedimensional nonFermi liquid states, apart from
superconductivity, that are unshackled from a QCP are much rarer and are not currently well understood. Here, we report that the
triangular lattice system uranium diauride (UAu_{2}) forms such a state with a nonFermi liquid lowtemperature heat capacity
C/T~ log(1/T) and electrical resistivity ρ(T)  ρ(0) ~ T^{1.35} far below its Néel temperature. The magnetic order itself has a novel
structure and is accompanied by weak charge modulation that is not simply due to magnetostriction. The charge modulation
continues to grow in amplitude with decreasing temperature, suggesting that charge degrees of freedom play an important role
in the nonFermi liquid behavior. In contrast with QCPs, the heat capacity and resistivity we find are unusually resilient in
magnetic field. Our results suggest that a combination of magnetic frustration and Kondo physics may result in the emergence
of this novel state.
 FieldInduced Modulated State in the Ferromagnet PrPtAl,
Christopher D. O'Neill, Gino AbdulJabbar, Didier Wermeille, Philippe Bourges,
Frank Krüger, and Andrew D. Huxley
Phys. Rev. Lett. 126, 197203 (2021).
The theory of quantum orderbydisorder (QOBD) explains the formation of modulated magnetic states at the boundary
between ferromagnetism and paramagnetism in zero field. PrPtAl has been argued to provide an archetype for this.
Here, we report the phase diagram in magnetic field, applied along both the easy aaxis and hard baxis. For field aligned
to the baxis, we find that the magnetic transition temperatures are suppressed and at low temperature there is a single
modulated fan state, separating an easy aaxis ferromagnetic state from a field polarized state. This fan state is well
explained with the QOBD theory in the presence of anisotropy and field. Experimental evidence supporting the QOBD
explanation is provided by the large increase in the T^{2} coefficient of the resistivity and direct detection of enhanced
magnetic fluctuations with inelastic neutron scattering, across the field range spanned by the fan state. This shows that
the QOBD mechanism can explain field induced modulated states that persist to very low temperature.
 Fermionic criticality of anisotropic nodal point semimetals away from the upper critical dimension:
exact 1/N_{f} exponents,
Mikolaj D. Uryszek, Frank Krüger and Elliot Christou
Phys. Rev. Research 2, 043265 (2020).
We consider the fermionic quantum criticality of anisotropic nodal point semimetals in d = d_{L} + d_{Q} spatial dimensions that
disperse linearly in d_{L} dimensions, and quadratically in the remaining d_{Q} dimensions. When subject to strong interactions,
these systems are susceptible to semimetalinsulator transitions concurrent with spontaneous symmetry breaking. Such quantum
critical points are described by effective field theories of anisotropic nodal fermions coupled to dynamical order parameter fields.
We analyze the universal scaling in the physically relevant spatial dimensions, generalizing to a large number N_{f} of fermion
flavors for analytic control. Landau damping by gapless fermionic excitations gives rise to nonanalytic selfenergy corrections
to the bosonic orderparameter propagator that dominate the longwavelength behavior. We show that perturbative momentum
shell RG leads to nonuniversal, cutoff dependent results, as it does not correctly account for this nonanalytic structure. In turn,
using a completely general soft cutoff formulation, we demonstrate that the correct IR scaling of the dressed bosonic propagator
can be deduced by enforcing that results are independent of the cutoff scheme. Using this soft cutoff approach, we compute the
exact critical exponents for anisotropic semiDirac fermions (d_{L}=1, d_{Q}=1) to leading order in 1/N_{f}, and to all loop orders.
Applying the same method to relativistic Dirac fermions, we reproduce the critical exponents obtained by other methods,
such as conformal bootstrap. Unlike in the relativistic case, where the UVIR connection is reestablished at the upper critical
dimension, nonanalytic IR contributions persist near the upper critical line 2d_{L} + d_{Q}=4 of anisotropic nodal fermions.
We present εexpansions in both the number of linear and quadratic dimensions. The corrections to critical exponents are
nonanalytic in ε, with a functional form that depends on the starting point on the upper critical line.
 Nonlinear soliton confinement in weakly coupled antiferromagnetic spin chains,
H. Lane, C. Stock, S.W. Cheong, F. Demmel, R. A. Ewings, and F. Krüger,
Phys. Rev. B 102, 024437 (2020).
We analyze the lowenergy dynamics of quasi one dimensional, largeS quantum antiferromagnets with easyaxis anisotropy,
using a semiclassical nonlinear sigma model. The saddle point approximation leads to a sineGordon equation which supports
soliton solutions. These correspond to the movement of spatially extended domain walls. Longrange magnetic order is a
consequence of a weak interchain coupling. Below the ordering temperature, the coupling to nearby chains leads to an energy
cost associated with the separation of two domain walls. From the kinkantikink twosoliton solution, we compute the effective
confinement potential. At distances large compared to the size of the solitons the potential is linear, as expected for pointlike
domain walls. At small distances the gradual annihilation of the solitons weakens the effective attraction and renders the potential
quadratic. From numerically solving the effective one dimensional Schrödinger equation with this nonlinear confinement potential
we compute the soliton bound state spectrum. We apply the theory to CaFe_{2}O_{4}, an anisotropic S=5/2 magnet based upon
antiferromagnetic zigzag chains. Using inelastic neutron scattering, we are able to resolve seven discrete energy levels for spectra
recorded slightly below the Néel temperature T_{N}≈ 200K. These modes are well described by our nonlinear confinement model
in the regime of large spatially extended solitons.
 Criticality of Dirac fermions in the presence of emergent gauge fields,
Elliot Christou, Fernando de Juan, and Frank Krüger,
Phys. Rev. B 101, 155121 (2020).
We consider spontaneous symmetrybreaking transitions of strongly interacting twodimensional Dirac fermions minimally coupled
to mass and emergent gauge field order parameters. Using a renormalization group analysis, we show that the presence of gauge
fields leads to fermion induced quantum (multi)criticality where the putative emergent Lorentz invariance is violated. We
illustrate this with the example of translational symmetry breaking due to chargedensity wave order on the honeycomb lattice.
Finally, we identify that topological phase transitions are well described by this effective field theory.
 Quantum Criticality of SemiDirac Fermions in 2+1 Dimensions,
Mikolaj D. Uryszek, Elliot Christou, Akbar Jaefari, Frank Krüger, and Bruno Uchoa
Phys. Rev. B 100, 155101 (2019).
Twodimensional semiDirac fermions are quasiparticles that disperse linearly in one direction and quadratically in the other.
We investigate instabilities of semiDirac fermions towards charge and spindensity wave and superconducting order, driven by
shortrange interactions. We analyse the critical behaviour of the Yukawa theories for the different order parameters using Wilson
momentum shell RG. We generalise to a large number N_{f} of fermion flavours to achieve analytic control in 2+1 dimensions and
calculate critical exponents at oneloop order, systematically including 1/N_{f} corrections . The latter depend on the specific form
of the bosonic IR propagator in 2+1 dimensions, which needs to be included to regularise divergencies. The 1/N_{f} corrections
are surprisingly small, suggesting that the expansion is well controlled in the physical dimension. The order parameter correlations
inherit the electronic anisotropy of the semiDirac fermions, leading to correlation lengths that diverge along the spatial directions
with distinct exponents, even at the meanfield level. We conjecture that the proximity to the critical point may stabilize novel
modulated order phases.
 Hidden Charge Order of Interacting Dirac fermions on the Honeycomb Lattices,
Elliot Christou, Bruno Uchoa, and Frank Krüger,
Phys. Rev. B 98, 161120(R) (2018).
We consider the extended halffilled Hubbard model on the honeycomb lattice for second nearest neighbors interactions.
Using a functional integral approach, we find that collective fluctuations suppress topological states and instead favor charge ordering,
in agreement with previous numerical studies. However, we show that the critical point is not of the putative semimetalMott insulator
variety. Due to the frustrated nature of the interactions, the ground state is described by a
hidden metallic charge order with semiDirac
excitations. We conjecture that this transition is not in the GrossNeveu universality class.
 Quantum OrderbyDisorder in Strongly Correlated Metals,
A. G. Green, G. Conduit, and F. Krüger,
Annual Reviews of Condensed Matter Physics 9, pp. 5977 (2018).
Entropic forces in classical manybody systems, e.g. colloidal suspensions, can lead to the formation of new phases. Quantum
fluctuations can have similar effects: spin fluctuations drive the superfluidity of Helium3 and a similar mechanism operating in metals
can give rise to superconductivity. It is conventional to discuss the latter in terms of the forces induced by the quantum fluctuations.
However, focusing directly upon the free energy provides a useful alternative perspective in the classical case and can also be applied
to study quantum fluctuations. Villain first developed this approach for insulating magnets and coined the term orderbydisorder to
describe the observed effect. We discuss the application of this idea to metallic systems, recent progress made in doing so, and the
broader prospects for the future.
 Topological Triplon Modes and Bound States in a ShastrySutherland Magnet,
P. A. McClarty, F. Krüger, T. Guidi, S. F. Parker, K. Refson, A. W. Parker, D. Prabakharan, and R. Coldea
Nature Physics 13, 736 (2017).
The twin discoveries of the quantum Hall effect, in the 1980's, and of topological band insulators, in the 2000's, were landmarks in
physics that enriched our view of the electronic properties of solids. In a nutshell, these discoveries have taught us that quantum
mechanical wavefunctions in crystalline solids may carry nontrivial topological invariants which have ramifications for the observable
physics. One of the side effects of the recent topological insulator revolution has been that such physics is much more widespread
than was appreciated ten years ago. For example, while topological insulators were originally studied in the context of electron
wavefunctions, recent work has led to proposals of topological insulators in bosonic systems: in photonic crystals, in the vibrational
modes of crystals, and in the excitations of ordered magnets. Here we confirm the recent proposal that, in a weak magnetic field, the
dimerized quantum magnet SrCu_{2}(BO_{3})_{2} is a bosonic topological insulator with nonzero Chern number in the triplon bands and
topologically protected chiral edge excitations.
 Entanglement entropies and fermion signs of critical metals,
N. Kaplis, F. Krüger, and J. Zaanen
Phys. Rev. B 95, 155102 (2017).
The fermion sign problem is often viewed as a sheer inconvenience that plagues numerical studies of strongly interacting
electron
systems. Only recently, it has been suggested that fermion signs are fundamental for the universal behavior of critical metallic
systems and crucially enhance their degree of quantum entanglement.In this work we explore potential connections between
emergent scale invariance of fermion sign structures and scaling properties of bipartite entanglement entropies. Our analysis
is based on a wavefunction ansatz that incorporates collective, longrange backflow correlations into fermionic Slater determinants.
Such wavefunctions mimic the collapse of a Fermi liquid at a quantum critical point. Their nodal surfaces  a representation of the
fermion sign structure in manyparticle configurations space  show fractal behavior up to a length scale ξ that diverges at a critical
backflow strength. We show that the Hausdorff dimension of the fractal nodal surface depends on ξ, the number of fermions,
and the exponent of the backflow. For the same wavefunctions we numerically calculate the second Rényi entanglement entropy S_{2}.
Our results show a crossover from volume scaling, S_{2} ~ℓ^θ (θ=2 in d=2 dimensions), to the characteristic Fermiliquid
behavior
S_{2} ~ ℓ ln ℓ on scales larger than ξ. We find that volume scaling of the entanglement entropy is a robust feature of critical
backflow
fermions, independent of the backflow exponent and hence the fractal dimension of the scale invariant sign structure.
 Stability of a spintriplet nematic state near to a quantum critical point,
G. Hannappel, C. J. Pedder, F. Krüger, and A. G. Green
Phys. Rev. B 93, 235105 (2016).
We analyze a model of itinerant electrons interacting through a quadrupole densitydensity repulsion in three
dimensions. At the
mean field level, the interaction drives a continuous Pomeranchuk instability towards dwave, spintriplet nematic order, which
simultaneously breaks the SU(2) spinrotation and spatial rotational symmetries. This order results in spin antisymmetric, elliptical
deformations of the Fermi surfaces of up and down spins. We show that the effects of quantum fluctuations are similar to those in
metallic ferromagnets, rendering the nematic transition firstorder at low temperatures. Using the fermionic quantum orderbydisorder
approach to selfconsistently calculate fluctuations around possible modulated states, we show that the firstorder transition is
preempted by the formation of a nematic state that is intertwined with a helical modulation in spin space. Such a state is closely
related to dwave bond density wave order in squarelattice systems. Moreover, we show that it may coexist with a modulated,
pwave superconducting state.
 Incommensurate spindensitywave antiferromagnetism in CeRu_{2}Al_{2}B,
A. Bhattacharyya, D. D. Khalyavin, F. Krüger, D. T. Adroja, A. M. Strydom, W. A. Kockelmann, and A. D. Hillier,
Phys. Rev. B 93, 060410(R) (2016).
The newly discovered Isingtype ferromagnet CeRu_{2}Al_{2}B exhibits an additional phase transition at T_{N}= 14.2 K before going to the
ferromagnetic ground state at T_{C} = 12.8 K. We clarify the nature of this transition through high resolution neutron diffraction
measurements. The data reveals the presence of a longitudinal incommensurate spindensity wave (SDW) in the temperature range
between T_{C} and T_{N}. The propagation vector q~(0,0,0.148) is nearly temperature independent in this region and discontinuously
locks into q=0 at T_{C}. Meanfield calculations of an effective Ising model indicate that the modulated SDW phase is stabilized by
a strong competition between ferromagnetic and antiferromagnetic exchange interactions. This makes CeRu_{2}Al_{2}B a particularly
attractive model system to study the global phase diagram of ferromagnetic heavyfermion metals under influence of magnetic
frustration.
 Bose and Mott Glass Phases in Dimerized Quantum Antiferromagnets,
S. J. Thomson and F. Krüger,
Phys. Rev. B 92, 180201(R) (2015).
We examine the effects of disorder on dimerized quantum antiferromagnets in a magnetic field, using the mapping to a lattice
gas
of hardcore bosons with finiterange interactions. Combining a strongcoupling expansion, the replica method, and a oneloop
renormalization group analysis, we investigate the nature of the glass phases formed. We find that away from the tips of the Mott lobes,
the transition is from a Mott insulator to a compressible Bose glass, however the compressibility at the tips is strongly suppressed. We
identify this finding with the presence of a rare Mott glass phase not previously described by any analytic theory for this model and
demonstrate that the inclusion of replica symmetry breaking is vital to correctly describe the glassy phases. This result suggests that
the formation of Bose and Mott glass phases is not simply a weak localization phenomenon but is indicative of much richer physics.
We discuss our results in the context of both ultracold atomic gases and spindimer materials.
 Modulated magnetism in PrPtAl,
G. AbdulJabbar, D. A. Sokolov, C. O'Neill, C. Stock, D. Wermeille, F. Demmel, F. Krüger, A. G. Green,
F. LevyBertrand, B. Grenier, and A. Huxley,
Nature Physics 11, 321 (2015).
The transition between paramagnetism and ferromagnetism is the paradigm for a continuous phase transition at
finite temperature.
When such a transition is tuned to zero temperature in clean materials, the growth of low energy zeropoint fluctuations potentially
drives an array of phenomena, including the formation of novel states such as nonconventional superconductivity. Experimentally,
the growth of the fluctuations is however curtailed and the transition becomes discontinuous as its temperature is reduced. This
is understood to arise from nonanalytic corrections to the free energy that always occur. In a recent theory changes of the excitation
spectrum are selfconsistently considered alongside the ground state. This analysis reveals that a transition to a new state may be
an alternative outcome. Since the excitation spectrum (the "disorder") is pivotal to promoting the new "order" this mechanism is
referred to as "order by disorder". Here, we report the discovery of modulated order in PrPtAl, consistent with complex spirals, at
the boundary between paramagnetism and ferromagnetism that is the first clear experimental realisation of such a state.
 Symmetry of reentrant tetragonal phase in Ba(1x)Na(x)Fe(2)As(2): magnetic versus orbital ordering mechanism,
D.D. Khalyavin, S.W. Lovesey, P. Manuel, F. Krüger, S. Rosenkranz, J.M. Allred, O. Chmaissem, and R. Osborn,
Phys. Rev. B 90, 174511 (2014).
Magnetostructural phase transitions in Ba(1x)A(x)xFe(2)As(2) (A = K, Na) materials are discussed for both magnetically and orbitally
driven mechanisms, using a symmetry analysis formulated within the Landau theory of phase transitions. Both mechanisms predict
identical orthorhombic spacegroup symmetries for the nematic and magnetic phases observed over much of the phase diagram, but
they predict different tetragonal spacegroup symmetries for the newly discovered reentrant tetragonal phase in Ba(1x)Na(x)xFe(2)As(2)
(x ~ 0.240.28). In a magnetic scenario, magnetic order with moments along the caxis, as found experimentally, does not allow any type
of orbital order, but in an orbital scenario, we have determined two possible orbital patterns, specified by P4/mnc1' and I4221' space
groups, which do not require atomic displacements relative to the parent I4/mmm1' symmetry and, in consequence, are indistinguishable
in conventional diffraction experiments. We demonstrate that the three possible space groups are however, distinct in resonant Xray
Bragg diffraction patterns created by TempletonTempleton scattering. This provides an experimental method of distinguishing between
magnetic and orbital models.
 Fluctuation driven magnetic hardaxis ordering in metallic ferromagnets,
F. Krüger, C. J. Pedder, and A. G. Green,
Phys. Rev. Lett. 113, 147001 (2014).
We demonstrate that the interplay between soft electronic particlehole fluctuations and magnetic anisotropies
can drive ferromagnetic
moments to point along a magnetic hard axis. As a proof of concept, we show this behavior explicitly for a generic twoband model with
local Coulomb and Hund's interactions, and a spinorbitinduced easy plane anisotropy. The phase diagram is calculated within the
fermionic quantum orderbydisorder approach, which is based on a selfconsistent freeenergy expansion around a magnetically ordered
state with unspecified orientation. Quantum fluctuations render the transition of the easyplane ferromagnet firstorder below a tricritical
point. At even lower temperatures, directionally dependent transverse fluctuations dominate the magnetic anisotropy and the moments
flip to lie along the magnetic hard axis. We discuss our findings in the context of recent experiments that show this unusual ordering
along the magnetic hard direction.
 Replica symmetry breaking in the Bose glass,
S. J. Thomson and F. Krüger,
EPL (Europhysics Letters) 108, 30002 (2014).
We investigate the nature of the Bose glass phase of the disordered BoseHubbard model in d>2 and demonstrate the existence
of a
glasslike replica symmetry breaking (RSB) order parameter in terms of particle number fluctuations. Starting from a strongcoupling
expansion around the atomic limit, we study the instability of the Mott insulator towards the formation of a Bose glass. We add some
infinitesimal RSB, following the Parisi hierarchical approach in the most general form, and observe its flow under the momentumshell
renormalization group scheme. We find a new fixed point with onestep RSB, corresponding to the transition between the Mott insulator
and a Bose glass phase with hitherto unseen RSB. The susceptibility associated to infinitesimal RSB perturbation in the Mott insulator
is found to diverge at the transition with an exponent of gamma=1/d. Our findings are consistent with the expectation of glassy behavior
and the established breakdown of selfaveraging. We discuss the possibility of measuring the glasslike order parameter in optical
lattice experiments as well as in certain spin systems that are in the same universality class as the BoseHubbard model.
 Resummation of fluctuations near ferromagnetic quantum critical points,
C. J. Pedder, F. Krüger, and A. G. Green,
Phys. Rev. B 88, 165109 (2013).
We present a detailed analysis of the nonanalytic structure of the free energy for the itinerant ferromagnet near the quantum
critical point in two and three dimensions. We analyze a model of electrons with an isotropic dispersion interacting through a contact
repulsion. A fermionic version of the quantum orderbydisorder mechanism allows us to calculate the free energy as a functional of
the dispersion in the presence of homogeneous and spiralling magnetic order. We resum the leading divergent contributions, to derive
an algebraic expression for the nonanalytic contribution to free energy from quantum fluctuations. Using a recursion which relates
subleading divergences to the leading term, we calculate the full T=0 contribution in d=3. We propose an interpolating functional form,
which allows us to track phase transition lines at temperatures far below the tricritical point and down to T=0. In d=2, quantum
fluctuations are stronger and nonanalyticities more severe. Using a similar resummation approach, we find that despite the different
nonanalytic structures, the phase diagrams in two and three dimensions are remarkably similar, exhibiting an incommensurate spiral
phase near to the avoided quantum critical point.
 Breakdown of selfaveraging in the Bose glass,
A. Hegg, F. Krüger, and P. Phillips,
Phys. Rev. B 88, 134206 (2013).
We study the squarelattice BoseHubbard model with bounded random onsite energies at zero temperature. Starting from a dual representation obtained from a strongcoupling expansion around the atomic limit, we employ a realspace block decimation scheme.
This approach is nonperturbative in the disorder and enables us to study the renormalizationgroup flow of the induced randommass
distribution. In both insulating phases, the Mott insulator and the Bose glass, the average mass diverges signaling short range
superfluid correlations. The relative variance of the mass distribution distinguishes the two phases, renormalizing to zero in the Mott
insulator and diverging in the Bose glass. Negative mass values in the tail of the distribution signal the presence of rare superfluid
regions in the Bose glass. The breakdown of selfaveraging is evidenced by the divergent relative variance and increasingly
nonGaussian distributions. We determine an explicit phase boundary between the Mott insulator and Bose glass.
 Helical glasses near ferromagnetic quantum criticality,
S. J. Thomson, F. Krüger, and A. G. Green,
Phys. Rev. B 87, 224203 (2013).
We study the effects of quenched charge disorder on the phase reconstruction near itinerant ferromagnetic quantum critical points in three spatial dimensions. Combining a replica disorder average with a fermionic version of the quantumorderby disorder mechanism,
we show that weak disorder destabilizes the ferromagnetic state and enhances the susceptibility towards incommensurate, spiral
magnetic ordering. The Goldstone modes of the spiral phase are governed by a 3dXY model. The induced disorder in the pitch of the
spiral generates a random anisotropy for the Goldstone modes, inducing vortex lines in the phase of the helical order and rendering
the magnetic correlations short ranged with a strongly anisotropic correlation length.
 Disordered driven coupled cavity arrays: Nonequilibrium stochastic meanfield theory,
G. Kulaitis, F. Krüger, F. Nissen, and J. Keeling,
Phys. Rev. A 87, 013840 (2013).
We study the interplay of disorder with pumping and decay in coupled qubitcavity arrays, the JaynesCummingsHubbard model. We find that relatively weak disorder can wash out the bistability present in the clean pumped system, and that moreover, the combination
of disorder in onsite energies and decay can lead to effective phase disorder. To explore these questions, we present a nonequilibrium
generalisation of StochasticMeanField theory, providing a simple tool to address such questions. This technique is developed for
rather general forms of lightmatter coupling, driving, dissipation, and onsite disorder, making it applicable to a wide range of systems.
 Quantum orderbydisorder driven phase reconstruction in the vicinity of ferromagnetic quantum critical points,
U. Karahasanovic, F. Krüger, and A. G. Green,
Phys. Rev. B 85, 165111 (2012).
The formation of new phases close to itinerant electron quantum critical points has been observed experimentally in many
compounds. We present a unified analytical model that explains the emergence of new types of order around itinerant ferromagnetic
quantum critical points. The central idea of our analysis is that certain Fermisurface deformations associated with the onset
of the competing order enhance the phasespace available for lowenergy quantum fluctuations and so selfconsistently lower
the free energy. We demonstrate that this quantum orderbydisorder mechanism leads to instabilities towards the formation
of spiral and dwave spin nematic phases close to itinerant ferromagnetic quantum critical points in three spatial dimensions.
 Quantum orderbydisorder near criticality and the secret of partial order in MnSi,
F. Krüger, U. Karahasanovic, and A. G. Green,
Phys. Rev. Lett. 108, 067003 (2012).
The vicinity of quantum phase transitions has proven fertile ground in the search for new quantum phases. We propose a physically
motivated and unifying description of phase reconstruction near metallic quantumcritical points using the idea of quantum
orderbydisorder. Certain deformations of the Fermi surface associated with the onset of competing order enhance the phase
space available for lowenergy, particlehole fluctuations and selfconsistently lower the free energy. Applying the notion of quantum
orderbydisorder to the itinerant helimagnet MnSi, we show that near to the quantum critical point, fluctuations lead to an increase
of the spiral ordering wave vector and a reorientation away from the lattice favored directions. The magnetic ordering pattern in this
fluctuationdriven phase is found to be in excellent agreement with the neutron scattering data in the partially ordered phase of MnSi.
 Spinwave excitations in the ferromagneticmetallic and in the charge, orbital and spin ordered states
in
Nd(1x)Sr(x)MnO(3)$ with x~0.5,
H. Ulbrich, F. Krüger, A. A. Nugroho, D. Lamago, Y. Sidis, M. Braden,
Phys. Rev. B 84, 094453 (2011).
Inelastic neutron scattering experiments have been performed on single crystals of Nd(1x)Sr(x)MnO(3) with x~0.5. Colossal
magnetoresistance (CMR) in the manganites arises from the interplay between a ferromagnetic metallic and antiferromagnetic charge
and orbital ordered insulating state. Therefore, it appears important to compare these phases concerning their underlying magnetic
interaction parameters. Our investigations of the spinwave disperion in the AFM ordered state of Nd(0.5)Sr(0.5)MnO(3) exhibits a
strongly anisotropic stiffness. The sign of the anisotropy is characteristic for the sitecentered model for charge and orbital ordering in
halfdoped manganites. Within this model, linear spinwave theory yields a perfect description of the experimental dispersion.
Furthermore, magnetic excitations in the ferromagnetic metallic state of Nd(1x)Sr(x)MnO(3) with x=0.49 and x=0.50 exhibit
nearly the same magnon dispersion which can be described with a Heisenberg model including nearestneighbor interactions.
 Two distinct MottInsulator to Boseglass transitions and breakdown of self averaging in the disordered
BoseHubbard model,
F. Krüger, Seungmin Hong, and P. Phillips,
Phys. Rev. B 84, 115118 (2011).
We investigate the instabilities of the Mottinsulating phase of the weakly disordered BoseHubbard model within a renormalization group
analysis of the replica field theory obtained by a strongcoupling expansion around the atomic limit. We identify a new order parameter
and associated correlation length scale that is capable of capturing the transition from a state with zero compressibility, the Mott insulator,
to one in which the compressibility is finite, the Bose glass. The order parameter is the relative variance of the disorderinduced mass
distribution. In the Mott insulator, the relative variance renormalizes to zero, whereas it diverges in the Bose glass. The divergence of
the relative variance signals the breakdown of selfaveraging. The length scale governing the breakdown of selfaveraging is the distance
between rare regions. This length scale is finite in the Bose glass but diverges at the transition to the Mott insulator with an exponent of
$\nu=1/D$ for incommensurate fillings. Likewise, the compressibility vanishes with an exponent of $\gamma=4/D1$ at the transition. At
commensurate fillings, the transition is controlled by a different fixed point
at which both the disorder and interaction vertices are relevant.
 Orbital Ordering and Unfrustrated (pi,0) Magnetism from Degenerate Double Exchange in the Pnictides,
Weicheng Lv, F. Krüger, and P. Phillips,
Phys. Rev. B 82, 045125 (2010).
The magnetic excitations of the iron pnictides are explained within a degenerate doubleexchange model. The localmoment spins are
coupled by superexchanges J1 and J2 between nearest and nextnearest neighbors, respectively, and interact with the itinerant electrons
of the degenerate d(xz) and d(yz) orbitals via a ferromagnetic Hund exchange. The latter stabilizes (pi,0) stripe antiferromagnetism due to
emergent ferroorbital order and the resulting kinetic energy gain by hopping preferably along the ferromagnetic spin direction. Taking the
quantum nature of the spins into account, we calculate the magnetic excitation spectra in the presence of both, super and double
exchange. A dramatic increase of the spinwave energies at the competing Néel ordering wave vector is found, in agreement with recent
neutron scattering data. The spectra are fitted to a spinonly model with a strong spatial anisotropy and additional longer ranged couplings
along the ferromagnetic chains. Over a realistic parameter range, the effective couplings along the chains are negative corresponding to
unfrustrated stripe antiferromagnetism.
 Anomalous suppression of the Bose glass at commensurate fillings in the
disordered BoseHubbard model,
F. Krüger, Jiansheng Wu, and P. Phillips,
Phys. Rev. B 80, 094526 (2009),
Virtual Journal of Atomic Quantum Fluids 1, (4) (2009).
We study the weakly disordered BoseHubbard model on a cubic lattice through a oneloop renormalization group analysis of the
corresponding effective field theory which is explicitly derived by combining a strongcoupling expansion with a replica average over
the disorder. The method is applied not only to generic uncorrelated onsite disorder but also to simultaneous hopping disorder
correlated with the differences of adjacent disorder potentials. Such correlations are inherent in finegrained optical speckle potentials
used as a source of disorder in optical lattice experiments. As a result of strong coupling, the strength of the replica mixing disorder
vertex, responsible for the emergence of a Bose glass, crucially depends on the chemical potential and the Hubbard repulsion and
vanishes to leading order in the disorder at commensurate boson fillings. As a consequence, at such fillings a direct transition
between the Mottinsulator and the superfluid in the presence of disorder cannot be excluded on the basis of a oneloop
calculation. At incommensurate fillings, at a certain length scale, the Mott insulator will eventually become unstable towards
the formation of a Bose glass. Phase diagrams as a function of the microscopic parameters are presented and the finitesize
crossover between the Mottinsulating state and the Bose glass is analyzed.
 Phase diagram of the frustrated, spatially anisotropic S=1
antiferromagnet on a square lattice,
H. C. Jiang, F. Krüger, J. E. Moore, D. N. Sheng, J. Zaanen, and Z. Y. Weng,
Phys. Rev. B 79, 174409 (2009).
We study the S=1 square lattice Heisenberg antiferromagnet with spatially anisotropic nearest neighbor couplings J1x, J1y
frustrated by a nextnearest neighbor coupling J2 numerically using the densitymatrix renormalization group (DMRG) method and
analytically employing the SchwingerBoson meanfield theory (SBMFT). Up to relatively strong values of the anisotropy, within both
methods we find quantum fluctuations to stabilize the Neel ordered state above the classically stable region. Whereas SBMFT
suggests a fluctuationinduced first order transition between the Néel state and a stripe antiferromagnet for 1/3<J1x/J1y<1
and an intermediate paramagnetic region opening only for very strong anisotropy, the DMRG results clearly demonstrate that the two
magnetically ordered phases are separated by a quantum disordered region for all values of the anisotropy with the remarkable implication
that the quantum paramagnetic phase of the spatially isotropic J1J2 model is continuously connected to the limit of decoupled
Haldane spin chains. Our findings indicate that for S=1 quantum fluctuations in strongly frustrated antiferromagnets are crucial and
not correctly treated on the semiclassical level.
 Spinorbital frustrations and anomalous metallic state in ironpnictide superconductors,
F. Krüger, S. Kumar, J. Zaanen, and J. van den Brink,
Phys. Rev. B 79, 054504 (2009),
Virtual Journal of Applications of Superconductivity 16, (4) (2009).
We develop an understanding of the anomalous metal state of the parent compounds of recently discovered iron based superconductors
starting from a strong coupling viewpoint, including orbital degrees of freedom. On the basis of an intermediatespin (S=1) state for the
Fe(2+) ions, we derive a KugelKhomskii spinorbital Hamiltonian for the active t(2g) orbitals. It turns out to be a highly complex
model with frustrated spin and orbital interactions. We compute its classical phase diagrams and provide an understanding for the stability
of the various phases by investigating its spinonly and orbitalonly limits. The experimentally observed spinstripe state is found to be
stable over a wide regime of physical parameters and can be accompanied by three different types of orbital orders. Of these the orbital
ferro and orbitalstripe orders are particularly interesting since they break the inplane lattice symmetry a robust feature of the undoped
compounds. We compute the magnetic excitation spectra for the effective spin Hamiltonian, observing a strong reduction of the ordered
moment, and point out that the proposed orbital ordering pattern can be measured in resonant Xray diffraction.
 Fermionic quantum criticality and the fractal nodal surface,
F. Krüger and J. Zaanen,
Phys. Rev. B 78, 035104 (2008).
The complete lack of theoretical understanding of the quantum critical states found in the heavy fermion metals and the normal states
of the highTc superconductors is routed in deep fundamental problem of condensed matter physics: the infamous minus signs
associated with FermiDirac statistics render the path integral nonprobabilistic and do not allow to establish a connection with critical
phenomena in classical systems. Using Ceperley's constrained pathintegral formalism we demonstrate that the workings of scale
invariance and FermiDirac statistics can be reconciled. The latter is selfconsistently translated into a geometrical constraint structure.
We prove that this "nodal hypersurface" encodes the scales of the Fermi liquid and turns fractal when the system becomes quantum
critical. To illustrate this we calculate nodal surfaces and electron momentum distributions of Feynman backflow wave functions and indeed
find that with increasing backflow strength the quasiparticle mass gradually increases, to diverge when the nodal structure becomes fractal.
Such a collapse of a Fermi liquid at a critical point has been observed in the heavyfermion intermetallics in a spectacular fashion.
Triggered by our paper, the visualization of nodal surfaces of backflow
wavefunctions has been implemented as a
MATHEMATICA demonstration project.
Synopsis: "Fractals and quantum criticality" by Sarma Kancharla
 Pacifying the Fermiliquid: battling the devious fermion signs,
J. Zaanen, F. Krüger, J.H. She, D. Sadri, S. I. Mukhin,
Iranian Journal of Physics Research, Vol. 8, No. 2, 39 (2008).
The fermion sign problem is studied in the path integral formalism. The standard picture of Fermi liquids is first critically analyzed, pointing
out some of its rather peculiar properties. The insightful work of Ceperley in constructing fermionic path integrals in terms of constrained
worldlines is then reviewed. In this representation, the minus signs associated with FermiDirac statistics are self consistently translated
into a geometrical constraint structure (the nodal hypersurface) acting on an effective bosonic dynamics. As an illustrative example we use
this formalism to study 1+1dimensional systems, where statistics are irrelevant, and hence the sign problem can be circumvented. In this
lowdimensional example, the structure of the nodal constraints leads to a lucid picture of the entropic interaction essential to one
dimensional physics. Working with the path integral in momentum space, we then show that the Fermi gas can be understood by analogy
to a Mott insulator in a harmonic trap. Going back to real space, we discuss the topological properties of the nodal cells, and suggest
a new holographic conjecture relating Fermi liquids in higher dimensions to softcore bosons in one dimension. We also discuss some
possible connections between mixed Bose/Fermi systems and supersymmetry.
 Magnetic fluctuations in ntype highTc superconductors reveal breakdown of fermiology:
Experiments and Fermiliquid/
RPA calculations,
F. Krüger, S. D. Wilson, L. Shan, S. Li, Y. Huang, H. H. Wen, S. C. Zhang, Pengcheng Dai, J. Zaanen,
Phys. Rev. B 76, 094506 (2007).
By combining experimental measurements of the quasiparticle and dynamical magnetic properties of optimally electrondoped
Pr(0.88)LaCe(0.12)CuO4 with theoretical calculations we demonstrate that the conventional fermiology approach cannot possibly account
for the magnetic fluctuations in these materials. In particular, we perform tunneling experiments on the very same sample for which a
dynamical magnetic resonance has been reported recently and use photoemission data by others on a similar sample to characterize the
fermionic quasiparticle excitations in great detail. We subsequently use this information to calculate the magnetic response within the
conventional fermiology framework as applied in a large body of work for the holedoped superconductors to find a profound disagreement
between the theoretical expectations and the measurements: this approach predicts a steplike feature rather than a sharp resonance
peak, it underestimates the intensity of the resonance by an order of magnitude, it suggests an unreasonable temperature dependence
of the resonance, and most severely, it predicts that most of the spectral weight resides in incommensurate wings which are a key
feature of the holedoped cuprates but have never been observed in the electrondoped counterparts. Our findings strongly suggest that
the magnetic fluctuations reflect the quantummechanical competition between antiferromagnetic and superconducting orders.
 Is deconfined quantum criticality in frustrated antiferromagnets ruled out by generic fluctuation induced
firstorder behavior?
F. Krüger,
J. Supercond. Nov. Magn. 20, 575 (2007).
Proceeding for International Conference Stripes2006, Rome, Italy.
 Frustrated Heisenberg antiferromagnets:
fluctuation induced first order vs deconfined quantum criticality,
F. Krüger and S. Scheidl,
Europhys. Lett. 74, 896 (2006).
Recently it was argued that quantum phase transitions can be radically different from classical phase transitions with as a highlight the
'deconfined critical points' exhibiting fractionalization of quantum numbers due to Berry phase effects. Such transitions are supposed to
occur in frustrated ('J1J2') quantum magnets. We have developed a novel renormalization approach for such systems which is
fully respecting the underlying lattice structure. According to our findings, another profound phenomenon is around the corner:
a fluctuation induced (orderoutofdisorder) first order transition. This has to occur for large spin and we conjecture that it is responsible
for the weakly first order behavior recently observed in numerical simulations for frustrated S=1/2 systems.
 Spinwave dispersion in orbitally ordered La$_{1/2}$Sr$_{3/2}$MnO$_4$,
D. Senff, F. Krüger, S. Scheidl, M. Benomar, Y. Sidis, S. Demmel, and M. Braden,
Phys. Rev. Lett. 96, 257201 (2006).
The magnon dispersion in the charge, orbital and spin ordered phase in La(1/2)Sr(3/2)MnO4 has been studied by means of
inelastic neutron scattering. We find an excellent agreement with a magnetic interaction model basing on the CEtype superstructure.
The magnetic excitations are dominated by ferromagnetic exchange parameters revealing a nearlyone dimensional character at high
energies. The nearest neighbor ferromagnetic interaction in La(1/2)Sr(3/2)MnO4 is significantly larger than the one in the
metallic ferromagnetically ordered manganites. The large ferromagneticinteraction in the charge/orbital ordered phase appears to be
essential for the capability of manganites to switch between metallic and insulating phases.
 The spin excitation spectrum in striped bilayer compounds,
F. Krüger and S. Scheidl,
Phys. Rev. B 70, 064421 (2004),
Virtual Journal of Applications of Superconductivity 7, (5) (2004).
The spin dynamics of bilayer cuprate compounds are studied in a basic model. The magnetic spectral properties are calculated in linear
spinwave theory for several stripe configurations which differ by the relative location of the stripes in the layers. We focus on the bilayer
splitting of the magnon bands near the incommensurate low energy peaks as well as near the pi resonance, distinguishing between the
odd and even channel. We find that a xshaped dispersion near the pi resonance is generic for stripes. By comparison of our results to
neutron scattering data for YBa(2)Cu(3)O(6+x) we conclude that the stripe model is consistent with characteristic features of
bilayer highTc compounds.
 Spin dynamics of stripes,
F. Krüger and S. Scheidl,
Phys. Rev. B. 67, 134512 (2003).
The spin dynamics of stripes in hightemperature superconductors and related compounds is studied in the framework of a spinwave
theory for a simple spinonly model. The magnon dispersion relation and the magnetic structure factor are calculated for diagonal and
vertical stripes. Acoustical as well as optical bands are included in the analysis. The incommensurability and the pi resonance
appear as complementary features of the band structure at different energy scales. The dependence of spinwave velocities and
resonance frequencies on the stripe spacing and coupling is calculated. At low doping, the resonance frequency is found to scale
roughly inversely proportional to the stripe spacing. The favorable comparison of the results with experimental data suggests
that the spinonly model provides a suitable and simple basis for calculating and understanding the spin dynamics of stripes.
 Spin and charge ordering transitions in stripes,
F. Krüger and S. Scheidl,
J. Phys. IV France 12, Pr9259 (2002).
Proceeding for International Workshop on Electronic Crystal (ECRYS), St. Flour, France.
 Nonuniversal ordering of spin and charge in stripe phases,
F. Krüger and S. Scheidl,
Phys. Rev. Lett. 89, 095701 (2002).
We study the interplay of topological excitations in stripe phases: charge dislocations, charge loops, and spin vortices. In two dimensions
these defects interact logarithmically on large distances. Using a renormalizationgroup analysis in the Coulomb gas representation of
these defects, we calculate the phase diagram and the critical properties of the transitions. Depending on the interaction parameters,
spin and charge order can disappear at a single transition or in a sequence of two transitions (spincharge separation). These transitions
are nonuniversal with continuously varying critical exponents. We also determine the nature of the points where three phases coexist.
 Bonddisordered spin systems:
Theory and application to doped high$T_\textrm{c}$ compounds,
F. Krüger and S. Scheidl,
Phys. Rev. B 65, 224502 (2002).
We examine the stability of magnetic order in a classical Heisenberg model with quenched random exchange couplings. This system
represents the spin degrees of freedom in highTc compounds with immobile dopants. Starting from a replica representation
of the nonlinear sigma model, we perform a renormalizationgroup analysis. The importance of cumulants of the disorder distribution
to arbitrarily high orders necessitates a functional renormalization scheme. From the renormalization flow equations we determine the
magnetic correlation length numerically as a function of the impurity concentration and of temperature. From our analysis follows that
twodimensional layers can be magnetically ordered for arbitrarily strong but sufficiently diluted defects. We further consider the
dimensional crossover in a stack of weakly coupled layers. The resulting phase diagram is compared with experimental data for
La(2x)Sr(x)CuO(4).



