We report a study of isoelectronic chemical substitution in the recently discovered quantum critical ferromagnet ${\mathrm{CeRh}}_6{\mathrm{Ge}}_4$. Upon silicon doping, the ferromagnetic ordering temperature of ${\mathrm{CeRh}}_6{\left({\mathrm{Ge}}_{1-x}{\mathrm{Si}}_x\right)}_4$ is continuously suppressed, and no transition is observed beyond $x_c\approx 0.125$. Non-Fermi-liquid behavior with $C/T\propto \ln (T^{*}/T$) is observed close to $x_c$, indicating the existence of strong quantum fluctuations, while the $T$-linear behavior observed upon pressurizing the parent compound is absent in the resistivity, which is likely a consequence of the disorder induced by silicon doping. Our findings show the effects of disorder on the unusual ferromagnetic quantum criticality in ${\mathrm{CeRh}}_6{\mathrm{Ge}}_4$, and provide further evidence for understanding the origin of this behavior.

Single-crystal elastic and inelastic neutron-scattering experiments are performed on the frustrated ferro-antiferromagnet $\mathrm{SrZnVO}({\mathrm{PO}}_4{)}_2$ in high magnetic fields. The fully polarized state, the presaturation phase and the columnar-antiferromagnetic phase just below the presaturation phase were investigated. The observed renormalization of spin-wave bandwidths, re-distribution of intensities between different branches and nonlinearities in the magnetization curve are all indicative of strong deviations from classical spin-wave theory. The previously observed presaturation transition is attributed to a staggered pattern of Dzyaloshinskii-Moriya interactions.

The relationship among unconventional superconductivity, antiferromagnetism, and nematic order in iron-based superconductors (FeSCs) is still highly debated. In many FeSCs superconductivity is in proximity of a nematically and magnetically ordered state. LiFeAs is an exceptional stoichiometric FeSC becoming superconducting below 18 K without undergoing a structural or magnetic transition. However, some recent experimental studies suggested the existence of finite nematic fluctuations and even a nematic superconducting state. In this paper, we employ elastoresistance as a measure of nematic fluctuations in pristine LiFeAs and compare the findings with the elastoresistance of LiFeAs at low Co and V doping levels as well with that of magnetically and nematically ordering NaFeAs. We find LiFeAs and cobalt-doped LiFeAs far away from a nematic instability.

The structure of the potentially scintillating high-pressure phase of $\beta \text{−}{\mathrm{MgMoO}}_4$ ($\gamma \text{−}{\mathrm{MgMoO}}_4$) has been solved by means of high-pressure single-crystal x-ray diffraction. The phase transition occurs above 1.5 GPa and involves an increase of the Mo coordination from fourfold to sixfold accommodated by a rotation of the polyhedra and a concommitant bond stretching resulting in an enlargement of the $c$ axis. A previous high-pressure Raman study had proposed such changes with a symmetry change to space group $P2/c$. Here it has been found that the phase transition is isosymmetrical ($C2/m⟶C2/m$). The bulk moduli and the compressibilities of the crystal axes of both the low- and the high-pressure phase, have been obtained from equation of state fits to the pressure evolution of the unit-cell parameters which were obtained from powder x-ray diffraction up to 12 GPa. The compaction of the crystal structure at the phase transition involves a doubling of the bulk modulus $B_0$ changing from 60.3(1) to 123.7(8) GPa and a change of the most compressible crystal axis from the (0, $b$, 0) direction in $\beta \text{−}{\mathrm{MgMoO}}_4$ to the ($0.9a$, 0, $0.5a$) direction in $\gamma \text{−}{\mathrm{MgMoO}}_4$. The lattice dynamical calculations performed here on $\gamma \text{−}{\mathrm{MgMoO}}_4$ served to explain the Raman spectra observed for the high-pressure phase of $\beta \text{−}{\mathrm{MgMoO}}_4$ in a previous work demonstrating that the use of internal modes arguments in which the ${\mathrm{MoO}}_n$ polyhedra are considered as separate vibrational units fails at least in this molybdate. The electronic structure of $\gamma \text{−}{\mathrm{MgMoO}}_4$ was also calculated and compared with the electronic structures of $\beta \text{−}{\mathrm{MgMoO}}_4$ and ${\mathrm{MgWO}}_4$ shedding some light on why ${\mathrm{MgWO}}_4$ is a much better scintillator than any of the phases of ${\mathrm{MgMoO}}_4$. These calculations yielded for $\gamma \text{−}{\mathrm{MgMoO}}_4$ a $Y_2\mathrm{\Gamma }⟶\mathrm{\Gamma }$ indirect band gap of 3.01 eV in contrast to the direct bandgaps of $\beta \text{−}{\mathrm{MgMoO}}_4$ (3.58 eV at $\mathrm{\Gamma }$) and ${\mathrm{MgWO}}_4$ (3.32 eV at $Z$).

Triangular lattice quasi-two-dimensional Mott insulators based on the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecule and its analogies present a possibility to produce exotic phases by coupling charge and spin degrees of freedom. In this work we discuss magnetic properties of one such material, $\kappa$-(BEDT-${\mathrm{TTF})}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Cl}$, which is found at the border of the phase transition between a Mott insulator into a charge ordered state. Our magnetic susceptibility and cantilever magnetization measurements demonstrate how the charge degree of freedom defines magnetic properties for few different charge phases observed in this material as a function of temperature. Between $T_{\text{CO}}=30\text{K}$ and $T_{\text{S}}=24\text{K}$ we observe charge and spin separation due to one-dimensional charge stripes formed in this material below $T_{\text{CO}}=30\text{K}$. Below $T_{\text{S}}=24\text{K}$ charge and spin degrees of freedom demonstrate coupling. Spin-singlet correlations develop below 24 K, however, the melting of charge order below 15 K prevents the spin-singlet-state formation, leaving the system in the inhomogeneous state with charge ordered spin-singlet domains and charge and spin fluctuating ones.

We use the dynamical structure factors of the quantum Hall states at $\nu =1/3$ and $1/2$ in the lowest Landau level to study their excitation spectrum. Using the density matrix renormalization group in combination with the time-dependent variational principle on an infinite cylinder geometry, we extract the low-energy properties. At $\nu =1/3$, a sharp magnetoroton mode and the two-roton continuum are present and the finite-size effects can be understood using the fractional charge of the quasiparticle. At $\nu =1/2$, we find low-energy modes with linear dispersion and the static structure factor $\overline{s}\left(q\right)\sim {\left(q\ell \right)}^3$ in the limit $q\ell \to 0$. The properties of these modes agree quantitatively with the predictions of the composite-fermion theory placed on the infinite cylinder.

We present a particle-number-conserving theory for many-body effects in mesoscopic superconducting islands connected to normal electrodes, which explicitly includes quantum fluctuations of Cooper pairs in the condensate. Beyond previous BCS mean-field descriptions, our theory can precisely treat the pairing and Coulomb interactions over a broad range of parameters by using the numerical renormalization group method. On increasing the ratio of pairing interactions to Coulomb interactions, the low-energy physics of the system evolves from the spin Kondo regime to the mixed-valence regime and eventually reaches an anisotropic charge Kondo phase, while a crossover from $1e$- to $2e$-periodic Coulomb blockade of transport is revealed at high temperatures. For weak pairing, the superconducting condensate is frozen in the local spin-flip processes but fluctuates in the virtual excitations, yielding an enhanced spin Kondo temperature. For strong pairing, massive fluctuations of Cooper pairs are crucial for establishing charge Kondo correlations whose Kondo temperature rapidly decreases with the pairing interaction. Surprisingly, a charge-exchange-induced local field may occur even at the charge degenerate point, thereby destroying the charge Kondo effect. These are demonstrated in the spectral and transport properties of the island.

We derive a criterion under which splitting of all eigenstates of an open $\mathrm{XYZ}$ Hamiltonian with boundary fields into two invariant subspaces, spanned by chiral shock states, occurs. The splitting is governed by an integer number, which has the geometrical meaning of the maximal number of kinks in the basis states. We describe the generic structure of the respective Bethe vectors. We obtain explicit expressions for Bethe vectors, in the absence of Bethe roots, and those generated by one Bethe root, and we investigate the single-particle subspace. We also describe in detail an elliptic analog of the spin-helix state, appearing in both the periodic and the open $\mathrm{XYZ}$ model, and we derive the eigenstate condition. The elliptic analog of the spin-helix state is characterized by a quasiperiodic modulation of the magnetization profile, governed by Jacobi elliptic functions.

Topologically ordered phases of matter display a number of unique characteristics, including ground states that can be interpreted as patterns of closed strings in the case of general ${\mathbb{Z}}_2$ string liquids. In this paper, we consider the problem of detecting and distinguishing closed strings in Ising spin configurations sampled from the classical ${\mathbb{Z}}_2$ gauge theory. We address this using the framework of persistent homology, which computes the size and frequency of general loop structures in spin configurations via the formation of geometric complexes. Implemented numerically on finite-size lattices, we show that the first Betti number of the Vietoris-Rips complexes achieves a high density at low temperatures in the ${\mathbb{Z}}_2$ gauge theory. In addition, it displays a clear signal at the finite-temperature deconfinement transition of the three-dimensional theory. We argue that persistent homology should be capable of interpreting prominent loop structures that occur in a variety of systems, making it a useful tool in theoretical and experimental searches for topological order.

We undertake a detailed high-resolution diffraction study of a plain band insulator, ${\mathrm{La}}_2{\mathrm{MgO}}_4$, which may be viewed as a structural surrogate system of the undoped end-member of the high-$T_c$ superconductor family ${\mathrm{La}}_{2-x-y}{A}_x^{2+}{R}_y^{3+}{\mathrm{CuO}}_4$ ($A=\mathrm{Ba},\mathrm{Sr}$; $R=\text{rare}\text{earth}$). We find that ${\mathrm{La}}_2{\mathrm{MgO}}_4$ exhibits the infamous low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase transition that has been linked to the suppression of superconductivity in a variety of underdoped cuprates, including the well-known ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ ($x=0.125$). Furthermore, we find that the LTO-to-LTT phase transition in ${\mathrm{La}}_2{\mathrm{MgO}}_4$ occurs for an octahedral tilt angle in the $4^{\circ }$–$5^{\circ }$ range, similar to that which has previously been identified as a critical tipping point for superconductivity in these systems. We show that this phase transition, occurring in a system lacking spin correlations and competing electronic states such as charge density waves and superconductivity, can be understood by simply navigating the density functional theory ground-state energy landscape as a function of the order parameter amplitude. This result calls for a careful reinvestigation of the origins of the phase transitions in high-$T_c$ superconductors based on the hole-doped, $n=1$ Ruddlesden-Popper lanthanum cuprates.

We study 't Hooft anomalies and the related anomaly inflow for subsystem global symmetries. These symmetries and anomalies arise in a number of exotic systems, including models with fracton order such as the $X$-cube model. As is the case for ordinary global symmetries, anomalies for subsystem symmetries can be canceled by anomaly inflow from a bulk theory in one higher dimension; the corresponding bulk is therefore a nontrivial subsystem symmetry protected topological (SSPT) phase. We demonstrate these phenomena in several examples with continuous and discrete subsystem global symmetries, as well as time-reversal symmetry. For each example we describe the boundary anomaly, and present classical continuum actions for the corresponding bulk SSPT phases, which describe the response of background gauge fields associated with the subsystem symmetries. Interestingly, we show that the anomaly does not uniquely specify the bulk SSPT phase. In general, the latter may also depend on how the symmetry and the associated foliation structure on the boundary are extended into the bulk.

It is shown that the ideal boundary between a perfectly conducting electrode and electron liquid state acts as a contact whose conductance per unit area is higher than the fundamental Sharvin conductance by a numerical coefficient $2\alpha$, where $\alpha$ is slightly smaller than unity and depends on the dimensionality of the system. If the boundary has a finite curvature, an additional correction to the boundary conductance appears, which is parametrically small as a product of the curvature by the electron-electron mean free path length. The relation of the normal current density to the voltage between the electrode and electron liquid represents itself a hydrodynamic boundary condition for current-penetrable boundary. Calculations of the conductance and potential distribution in microstructures by means of numerical solution of the Boltzmann equation show that the concept of boundary conductance works very good when the hydrodynamic transport regime is reached. The superballistic transport, when the device conductance is higher than the Sharvin conductance, can be realized in Corbino disk devices not only in the hydrodynamic regime, although requires that the electron-electron scattering rate must be higher than the momentum-relaxing scattering rate. The theoretical results for Corbino disks are consistent with recent experimental findings.

A particle with finite initial velocity in a disordered potential comes back and on average stops at the original location. This phenomenon, dubbed the “quantum boomerang effect” (QBE), has been recently observed in an experiment simulating the quantum kicked-rotor model [Sajjad et al., Phys. Rev. X 12, 011035 (2022)]. We provide analytical arguments that support the presence of the QBE in a wide class of disordered systems. Sufficient conditions to observe the real-space QBE are (a) Anderson localization, (b) the reality of the spectrum for the case of non-Hermitian systems, (c) the ensemble of disorder realizations $\left\{H\right\}$ being invariant under the application of $\mathcal{R}\mathcal{T}$, and (d) the initial state being an eigenvector of $\mathcal{R}\mathcal{T}$, where $\mathcal{R}$ is a reflection $x\to -x$ and $\mathcal{T}$ is the time-reversal operator. The QBE can be observed in momentum space in systems with dynamical localization if conditions (c) and (d) are satisfied with respect to the operator $\mathcal{T}$ instead of $\mathcal{RT}$. These conditions allow the observation of the QBE in time-reversal-symmetry-broken models, contrary to what was expected from previous analyses of the effect, and in a large class of non-Hermitian models. We provide examples of the QBE in lattice models with magnetic flux breaking time-reversal symmetry and in a model with an electric field. Whereas the QBE straightforwardly applies to noninteracting many-body systems, we argue that a real-space (momentum-space) QBE is absent in weakly interacting bosonic systems due to the breaking of reflection–time-reversal (time-reversal) symmetry.

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state can emerge in superconductors for which the orbital critical field exceeds the Pauli limit. Here, we present angular-resolved specific-heat data of the quasi-two-dimensional organic superconductor $\kappa$-(${\mathrm{ET})}_2\mathrm{Cu}{\left(\mathrm{NCS}\right)}_2$, with a focus on high fields in the regime of the FFLO transition. For an increasing out-of-plane tilt of the applied magnetic field, which leads to an increase of orbital contributions, we found that the nature of the superconducting transition changes from second to first order and that a further transition appears within the high-field superconducting phase. However, the superconducting state above the Pauli limit is stable for field tilt of several degrees. Since any finite perpendicular component of the magnetic field necessarily leads to quantization of the orbital motion, the resulting vortex lattice states compete with the modulated order parameter of the FFLO state leading to complex high-field superconducting phases. By solving the linearized self-consistency equation within weak-coupling BCS theory, we show that our results are clear experimental evidence of an orbital-induced transformation of the FFLO order parameter into Abrikosov-like states of higher Landau levels.

The Hund's coupling rule is of fundamental importance in atomic physics. It also plays a key role in the determination of physical properties in strongly correlated electron systems where the Fermi energy lies in multiple bands. We report a thorough investigation of a rarely studied $4d$ perovskite oxide ${\mathrm{LaRuO}}_3$ by a suite of measurements including transport, magnetization, specific heat, thermoelectric power, thermal conductivity, and neutron and x-ray diffraction. Our results are consistent with theoretical predictions, which makes ${\mathrm{LaRuO}}_3$ an exemplary Hund metal.

We identify the bosonic excitations in ferroelectrics that carry electric dipoles from the phenomenological Landau-Ginzburg-Devonshire theory. The “ferron” quasiparticles emerge from the concerted action of anharmonicity and broken inversion symmetry. In contrast to magnons, the transverse excitations of the magnetic order, the ferrons in displacive ferroelectrics are longitudinal with respect to the ferroelectric order. Based on the ferron spectrum, we predict temperature-dependent pyroelectric and electrocaloric properties, electric-field-tunable heat and polarization transport, and ferron-photon hybridization.

Understanding non-Fermi liquids in dimensions higher than one has been a subject of great interest. Such phases may serve as parent states for other unconventional phases of quantum matter, in a similar manner that conventional broken symmetry states can be understood as instabilities of the Fermi liquid. In this work, we investigate the emergence of an exotic non-Fermi liquid in two dimensions, where the fermions with quadratic band-touching dispersion interact with Bose metal. The bosonic excitations in Bose metal possess an extended nodal-line spectrum in momentum space, which arises due to the subsystem symmetry or the restricted motion of bosons. Using renormalization group analysis and direct computations, we show that the extended infrared singularity of Bose metal leads to a line of interacting fixed points of an exotic non-Fermi liquids, where the anomalous dimension of the fermions varies continuously, akin to the Luttinger liquid in one dimension. Further, the generalization of the model with multiple low-energy excitations is used to explore other unusual features of the resulting ground state.

The Hubbard model on the kagome lattice is presently often considered as a minimal model to describe the rich low-temperature behavior of $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ compounds (with $A=\mathrm{K},\mathrm{Rb},\mathrm{Cs}$), including charge density waves (CDWs), superconductivity, and possibly broken time-reversal symmetry. Here, we investigate, via variational Jastrow-Slater wave functions, the properties of its ground state when both on-site $U$ and nearest-neighbor $V$ Coulomb repulsions are considered at the van Hove filling. Our calculations reveal the presence of different interaction-driven CDWs and, contrary to previous renormalization-group studies, the absence of ferromagnetism and charge- or spin-bond order. No signatures of chiral phases are detected. Remarkably, the CDWs triggered by the nearest-neighbor repulsion possess charge disproportionations that are not compatible with the ones observed in $A{\mathrm{V}}_3{\mathrm{Sb}}_5$. As an alternative mechanism to stabilize charge-bond order, we consider the electron-phonon interaction, modeled by coupling the hopping amplitudes to quantum phonons, as in the Su-Schrieffer-Heeger model. Our results show the instability towards a trihexagonal distortion with $2\times 2$ periodicity, in closer agreement with experimental findings.

Edge states play a major role in the electron dynamics of topological insulators as they are the only conducting part in such materials. In this work we consider the Haldane model for a two-dimensional topological insulator, subjected to an intense laser field. We compare the numerically simulated high-harmonic generation (HHG) in the bulk of the Haldane model to HHG in corresponding finite flakes with edge states present, and explain the differences. In particular, peaks for energies below the bulk band gap appear in the spectra for the finite flakes. The positions of these peaks show a strong dependence on the size of the flakes, which can be explained using the dispersion relation for the edge states.