Nonlinear transport phenomena offer an exciting probe into the band geometry and symmetry properties of a system. While most studies on nonlinear transport have looked at second-order nonreciprocal responses in noncentrosymmetric systems, the reciprocal third-order effects that are dominant in centrosymmetric systems remain largely uncharted. Here, we uncover two significant contributions to the third-order charge conductivity: one affecting longitudinal resistance and another impacting the Hall effect. We demonstrate that these relatively unexplored contributions arise in time-reversal symmetry-broken systems from the band geometric quantities such as the Berry curvature and the symplectic connection. We prescribe a detailed symmetry dictionary to facilitate the discovery of these fundamental transport coefficients. Additionally, we unify our quantum kinetic results with the semiclassical wave-packet formalism to unveil all contributions to the third-order charge transport. We illustrate our results in antiferromagnetic monolayer ${\mathrm{SrMnBi}}_2$. Our comprehensive study significantly advances the fundamental understanding of reciprocal nonlinear responses.

We propose an approach to study the transition between different topological states, based on the assessment of the vibrational resonances in infrared spectra. We consider the Haldane and Kane-Mele models, finding that Born effective charges are nearly quantized, with a discontinuous jump concomitant with the topological phase transition. In particular, Born effective charges display a finite value in the trivial phase and a null one in the nontrivial one. This is rooted in the connection between Born effective charges and electronic Berry curvature at the band edges. Finally, at the topological phase transition of the Haldane model, we also observe a nearly quantized jump of the chiral splitting of the zone-center phonon frequencies, induced by time-reversal symmetry breaking.

Neutron-scattering measurements were employed to investigate the magnetic ordering in ${\mathrm{Tb}}_{1-x}{\mathrm{Gd}}_x{\mathrm{Mn}}_2{\mathrm{O}}_5$ ($x=0.485$) enriched with the ${}^{160}\mathrm{Gd}$ isotope, a member of multiferroic $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ ($R=\mathrm{rare}\text{−}\mathrm{earth}$ element) compounds. This material, at low temperatures, exhibits commensurate magnetic (CM) ordering, which drives electric polarization. Notably, electric polarization diminishes as the temperature decreases further, while the CM ordering persists. Magnetic structure analyses revealed that a cycloidal magnetic structure along the $c$ axis transforms into a sinusoidal-like structure, resulting in the nonferroelectric phase, whereas a collinear magnetic structure in the $ab$ plane exhibits no significant change. The findings suggest that the cycloidal magnetic structure plays a key role in inducing electric polarization in the CM ordering of the present material.

The highly tunable nature of synthetic quantum materials—both in the solid-state and cold atom contexts—invites examining which microscopic ingredients aid in the realization of correlated phases of matter such as superconductors. Recent experimental advances in moiré materials suggest that unifying the features of the Fermi-Hubbard model and quantum Hall systems creates a fertile ground for the emergence of such phases. Here, we introduce the “double Hofstadter” model, a minimal 2D lattice model that incorporates exactly these features: time-reversal symmetry, band topology, and strong repulsive interactions. By using infinite cylinder density matrix renormalization group methods (cylinder iDMRG), we investigate the ground state phase diagram of this model. We find that it hosts an interaction-induced quantum spin Hall insulator and demonstrate that weakly hole-doping this state gives rise to a superconductor at a finite circumference, with indications that this behavior persists on larger cylinders. At the aforementioned circumference, the superconducting phase is surprisingly robust to perturbations including additional repulsive interactions in the pairing channel. By developing a technique to probe the superconducting gap function in iDMRG, we phenomenologically characterize the superconductor. Namely, we demonstrate that it is formed from the weak pairing of holes atop the quantum spin Hall insulator. Furthermore, we determine the pairing symmetry of the superconductor, finding it to be $p$-wave—reminiscent of the unconventional superconductivity reported in experiments on twisted bilayer graphene (TBG). Motivated by this, we elucidate structural similarities and differences between our model and those of TBG in its chiral limit. Finally, to provide a more direct experimental realization, we detail an implementation of our Hamiltonian in a system of cold fermionic alkaline-earth atoms in an optical lattice.

Quantum state transfer (QST) describes the coherent passage of quantum information from one node in a network to another. Experiments on QST span a diverse set of platforms and currently report transport across up to tens of nodes in times of several hundred nanoseconds with fidelities that can approach 90% or more. Theoretical studies examine both the lossless time evolution associated with a given (Hermitian) lattice Hamiltonian and methods based on the master equation that allows for losses. In this paper, we describe Monte Carlo techniques which enable the discovery of a Hamiltonian that gives high-fidelity QST. We benchmark our approach in geometries appropriate to coupled optical cavity-emitter arrays and discuss connections to condensed matter Hamiltonians of localized orbitals coupled to conduction bands. The resulting Jaynes-Cummings-Hubbard and periodic Anderson models can, in principle, be engineered in appropriate hardware to give efficient QST.

Competing interactions in low-dimensional materials can produce nearly degenerate electronic and structural phases. We investigate structural phase transitions in layered ${\mathrm{IrTe}}_2$ for which a number of potential transition mechanisms have been postulated. The spatial coexistence of multiple phases on the micron scale has prevented a detailed analysis of the electronic structure. By exploiting micro-angle-resolved photoemission spectroscopy obtained with synchrotron radiation we extract the electronic structure of the multiple structural phases in ${\mathrm{IrTe}}_2$ in order to address the mechanism underlying the phase transitions. We find direct evidence of lowered energy states that appear in the low-temperature phases, states previously predicted by ab initio calculations and extended here. Our results validate a proposed scenario of bonding and antibonding states as the driver of the phase transitions.

We propose a minimal “three-patch model” for the anomalous Hall crystal (AHC), a topological electronic state that spontaneously breaks both time-reversal symmetry and continuous translation symmetry. The proposal for this state is inspired by the recently observed integer and fractional quantum Hall states in rhombohedral multilayer graphene at zero magnetic field. There, interaction effects appear to amplify the effects of a weak moiré potential, leading to the formation of stable, isolated Chern bands. It has been further shown that Chern bands are stabilized in mean-field calculations even without a moiré potential, enabling a realization of the AHC state. Our model is built on the dissection of the Brillouin zone into patches centered around high-symmetry points. Within this model, the wave functions at high-symmetry points fully determine the topology and energetics of the state. We extract two quantum geometrical phases of the noninteracting wave functions that control the stability of the topologically nontrivial AHC state. The model predicts that the AHC state wins over the topological trivial Wigner crystal in a wide range of parameters, and agrees very well with the results of full self-consistent Hartree-Fock calculations of the rhombohedral multilayer graphene Hamiltonian.

The inherent complexity of system-bath interactions often requires making critical approximations, which we here show to have a radical influence on the renormalization group flow and the resulting phase diagram. Specifically, for the Caldeira-Leggett model Schmid and Bulgadaev (SB) predicted a phase transition, whose experimental verification in resistive superconducting circuits is currently hotly debated. For normal metal and Josephson junction array resistors, we show that the mapping to Caldeira-Leggett is only exact when applying approximations which decompactify the superconducting phase. We show that there exist treatments that retain phase compactness, which immediately lead to a phase diagram depending on four instead of two parameters. While we still find an SB-like transition in the transmon regime, the critical parameter is controlled exclusively by the capacitive coupling. In contrast, the Cooper pair box maps to the anisotropic Kondo model, where a pseudoferromagnetic phase is not allowed for regular electrostatic interactions.

Mott insulators with spin-orbit entangled $j=1/2$ moments host intriguing magnetic properties. The $j=1/2$ wave function requires cubic symmetry, while a noncubic crystal field mixes $j=1/2$ and 3/2 character. Spectroscopic studies of $5d^5$ iridates typically claim noncubic symmetry, e.g., based on a splitting of the excited $j=3/2$ quartet. A sizable splitting is particularly puzzling in antifluorite-type ${\mathrm{K}}_2{\mathrm{IrCl}}_6$, a frustrated fcc quantum magnet with global cubic symmetry. It raises the fundamental question about the stability of $j=1/2$ moments against magnetoelastic coupling. Combining resonant inelastic x-ray scattering with optical spectroscopy, we demonstrate that the multi-peak line shape in ${\mathrm{K}}_2{\mathrm{IrCl}}_6$ reflects a vibronic character of the $j=3/2$ states rather than a noncubic crystal field. The quasimolecular crystal structure with well separated ${\mathrm{IrCl}}_6$ octahedra explains the existence of well-defined sidebands that are usually smeared out in solids. Our results highlight the spin orbital lattice entangled character of cubic ${\mathrm{K}}_2{\mathrm{IrCl}}_6$ with ideal $j=1/2$ moments.

The exchange interactions in insulators depend on the orbital state of magnetic ions, obeying certain phenomenological principles, known as Goodenough-Kanamori-Anderson rules. Particularly, the ferro order of alike orbitals tends to stabilize antiferromagnetic interactions, while the antiferro order of unlike orbitals favors ferromagnetic interactions. The Kugel-Khomskii theory provides a universal view on such coupling between spin and orbital degrees of freedom, based on the superexchange processes: namely, for a given magnetic order, the occupied orbitals tend to arrange in a way to further minimize the exchange energy. Then, if two magnetic sites are connected by the spatial inversion, the antiferro orbital order should lead to the ferromagnetic coupling and break the inversion symmetry. This constitutes the basic idea of our work, which provides a pathway for designing ferromagnetic ferroelectrics: the rare but fundamentally and practically important multiferroic materials. After illustrating the basic idea on toy-model examples, we propose that such behavior can be indeed realized in the van der Waals ferromagnet ${\mathrm{VI}}_3$, employing for this analysis the realistic model derived from first-principles calculations for magnetic $3d$ bands. We argue that the intra-atomic interactions responsible for Hund's second rule, acting against the crystal field, tend to restore the orbital degeneracy of the ionic $d^2$ state in ${\mathrm{VI}}_3$ and, thus, provide a necessary flexibility for activating the Kugel-Khomskii mechanism of the orbital ordering. In the honeycomb lattice, this orbital ordering breaks the inversion symmetry, stabilizing the ferromagnetic-ferroelectric ground state. The symmetry breaking leads to the canting of magnetization, which can be further controlled by the magnetic field, producing a huge change of electric polarization.

It is known that the surface of a three-dimensional topological insulator (3D TI) supports a chiral Majorana edge mode at the interface between a superconductor and a magnetic insulator. The complexity of the materials combination is such that this state has not yet been observed. Here we show that a helical Majorana edge mode appears even in the absence of the magnetic insulator, if the Fermi level of the massless surface electrons is at the Dirac point. Specular Andreev reflection of Dirac fermions is at the origin of the effect. The simplified geometry may favor experimental observation of the helical Majorana mode, although it lacks the topological protection of its chiral counterpart.

We have generated elliptically polarized femtosecond pulses from a seeded free-electron laser (FEL) in the spectral range 700–800 eV and conducted demonstrative experiments based on magnetic circular dichroism, at the Fe $L_3$ edge. This was achieved by making use of the radiation produced at the third harmonic of the fundamental FEL wavelength λ. A planar radiator emits linearly polarized harmonics along the undulator axis, while a helical undulator produces off-axis harmonic radiation with circular polarization. However, off-axis circularly polarized light is characterized by a rather low flux and, as a result, it is unsuitable for most experiments that rely on circular dichroism. Here, we demonstrate that on-axis radiation with a substantial degree of circular polarization at λ/3 can be generated using a variable-polarization (APPLE-type) radiator, finely tuned in a hybrid-polarization mode. The experiments, which demonstrate the potential of the adopted configuration for magnetic dichroism studies, were carried out at the FERMI FEL facility in Trieste (Italy). These results pave the way to experiments at the $2p\text{−}3d$ resonances of the transition metals most relevant for magnetic studies (Fe, Co, Ni) and make it possible to envisage investigations, e.g., spectroscopic studies with different core-hole lifetimes, covering both the $M$ (50–70 eV) and $L$ (600–900 eV) edges, at the same seeded FEL source.

It was recently argued that Bernal stacked bilayer graphene (BLG) exposed to a two-dimensional superlattice (SL) potential exhibits a variety of intriguing behaviors [Ghorashi et al., Phys. Rev. Lett. 130, 196201 (2023)]. Chief among them is the appearance of flat Chern bands that are favorable to the appearance of fractional Chern insulator states. Here, we explore the application of spatially periodic out-of-plane orbital magnetic fields to the model of Ghorashi et al. to find additional means of inducing flat Chern bands. We focus on fields that vary on length scales much larger than the atomic spacing in BLG, generating what we refer to as magnetic SLs. The magnetic SLs we investigate either introduce no net magnetic flux to the SL unit cell or a single quantum of flux. We find that magnetic SLs acting on their own can induce topological flat bands, but richer behavior, such as the appearance of flat and generic bands with high Chern numbers, can be observed when the magnetic SLs act in conjunction with commensurate electric SLs. Finally, we propose a method of generating unit-flux-quantum magnetic SLs along with concomitant electric SLs. The magnetic SL is generated by periodic arrays of flux vortices originating from type II superconductors, while the electric SL arises due to a magnetic SL-induced charge density on the surface of a magnetoelectric material. Tuning the vortex lattice and the magnetoelectric coupling permits control of both SLs, and we study their effects on the band structure of BLG.

The topotactic reduction of $(\text{La},\text{Sr}){\text{NiO}}_3$ to superconducting $(\text{La},\text{Sr}){\text{NiO}}_2$ using ${\mathrm{CaH}}_2$ as a detergent has revealed new routes to synthesize nickelates. Here we look into the possibility of a similar reduction of ${(\text{La},\text{Sr})}_2{\mathrm{NiO}}_4$ to ${(\text{La},\text{Sr})}_2{\mathrm{NiO}}_3$. We perform density functional theory plus dynamical mean-field theory calculations. Our results show that chemical reduction (via, e.g., ${\mathrm{CaH}}_2$) is expected to remove in-plane oxygens instead of the apical ones, leaving vertical planar ${\mathrm{NiO}}_4$ squares that form one-dimensional NiO chains. ${\left({\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\right)}_2{\mathrm{NiO}}_3$ with a $3d^9$ electronic configuration is an antiferromagnetic, single-band Mott insulator. Hole doping suppresses this antiferromagnetic order and turns the system metallic. The crystal and electronic structure of ${(\text{La},\text{Sr})}_2{\mathrm{NiO}}_3$ bears quite some similarity with the cuprate superconductor ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$.

We develop a theory of probing phonon modes of van der Waals materials using the quantum twisting microscope. While elastic tunneling dominates the tunneling current at small twist angles, the momentum mismatch between the $K$ points of tip and sample at large twist angles can only be bridged by inelastic scattering. This allows for probing phonon dispersions along certain lines in reciprocal space by measuring the tunneling current as a function of twist angle and bias voltage. We illustrate this modality of the quantum twisting microscope by developing a systematic theory for graphene-graphene junctions. We show that beyond phonon dispersions, the tunneling current also encodes the strength of electron-phonon couplings. Extracting the coupling strengths for individual phonon modes requires careful consideration of various inelastic tunneling processes. These processes are associated with the intralayer and interlayer electron-phonon couplings and appear at different orders in a perturbative calculation of the tunneling current. We find that the dominant process depends on the particular phonon mode under consideration. Our results inform the quest to understand the origin of superconductivity in twisted bilayer graphene and provide a case study for quantum-twisting-microscope investigations of collective modes.

In transition metal dichalcogenide semiconductor monolayers the spin dynamics of electrons is controlled by the original spin-valley locking effect resulting from the interplay between spin-orbit interaction and inversion asymmetry. As a consequence, for electrons occupying bottom conduction bands, a carrier spin flip occurs only if there is a simultaneous change of valley. However, very little is known about the intravalley spin relaxation processes, including the possible role of chiral phonons. In this work we perform stationary and time-resolved photoluminescence measurements on high-quality ${\mathrm{WSe}}_2$ monolayers. Our experiments highlight an efficient relaxation from bright to dark excitons due to a fast intravalley electron transfer from the top to the bottom conduction band with opposite spins. A combination of experiments and theoretical analysis allows us to infer a spin relaxation time of about ${\tau }_s\sim 10\mathrm{ps}$, driven by the interplay between $\mathrm{\Gamma }$-valley chiral phonons and spin-orbit mixing.

Motivated by recent experiments reporting unconventional collective modes in the charge density wave (CDW) state of rare earth tritellurides $R{\mathrm{Te}}_3$, we derive a CDW Ginzburg-Landau theory that allows for nontrivial orbital order from a multiorbital microscopic model on the square net. Our analysis reveals unconventional CDWs where order parameters with distinct orbital character coexist due to an approximate symmetry of the low-energy model, which becomes exact in the limit of nearest-neighbor-only hopping and decoupled $p_x,p_y$ orbitals. Because of this coexistence, the resulting CDW pattern displays an orbital texture that generally breaks additional symmetries of the lattice in addition to those explicitly broken by the CDW wave vector. We find two competing phases that differ in whether they break or preserve inversion and vertical mirror symmetries. We explain the mechanisms that favor each outcome and discuss experimental probes that can distinguish the different phases.

The origin of the “hidden” quadrupolar and unconventional magnetic low-temperature orders observed in the spin-orbit double perovskite ${\mathrm{Ba}}_2{\mathrm{MgReO}}_6$ defies explanation through standard experimental and theoretical techniques. Here we address this problem by deriving and solving an ab initio low-temperature effective Hamiltonian including intersite electronic exchange and vibronic (electron-lattice) couplings between $J_{\mathrm{eff}}=3/2$ Jahn-Teller-active rhenium states. Our findings disclose the nature of these elusive states, attributing it to intertwined exchange and electron-lattice couplings, thus diverging from the conventional dichotomy of purely electronic or lattice driving mechanisms. Our results indicate the resilience of the quadrupolar hidden order under pressure, yet its rapid suppression under uniaxial strain suggests that external or lattice-induced distortions play a pivotal role in determining the relative stability of competing phases in ${\mathrm{Ba}}_2{\mathrm{MgReO}}_6$ and similar $d^1$ double perovskites.

Field-induced superconductivity has long presented a counterintuitive phenomenon and a pivotal challenge in condensed matter physics. In this paper, we introduce a mechanism for achieving field-induced superconductivity wherein the sublattice degree of freedom and the Coulomb interaction are tightly entwined. Our multipole-resolved analysis elucidates that lifting the fluctuation degeneracy results in an unconventional Cooper pairing channel, thereby realizing field-induced superconductivity. This research substantively augments the exploration of the latent potential of strongly correlated electron systems with sublattice degrees of freedom.

Semiconducting transition metal dichalcogenides of group VI are well known for their prominent excitonic effects and the transition from an indirect to a direct band gap when reduced to monolayers. While considerable efforts have elucidated the Rydberg series of excitons in monolayers, understanding their properties in multilayers remains incomplete. In these structures, despite an indirect band gap, momentum-direct excitons largely shape the optical response. In this work, we combine magnetoreflectance experiments with theoretical modeling based on the k$·$p approach to investigate the origin of excitonic resonances in ${\mathrm{WSe}}_2$ bi, tri, and quadlayers. For all investigated thicknesses, we observe a series of excitonic resonances in the reflectance spectra, initiated by a ground state with an amplitude comparable to the ground state of the $1s$ exciton in the monolayer. Higher-energy states exhibit a decrease in intensity with increasing energy, as expected for the excited states of the Rydberg series, although a significant increase in the diamagnetic shift is missing in tri and quadlayers. By comparing the experimental observations with theoretical predictions, we discover that the excitonic resonances observed in trilayers originate from two Rydberg series, while quadlayers exhibit four such series, and bilayers host a single Rydberg series similar to that found in monolayers.