We report a detailed investigation into the response of single ${\mathrm{BaTiO}}_3$ (BTO) nanocrystals under applied electric fields (E-field) using Bragg coherent diffraction imaging. Our study reveals pronounced domain wall migration and expansion of a sample measure in situ under applied electric field. The changes are most prominent at the surface of the nanocrystal, where the lack of external strain allows greater domain wall mobility. The observed domain shifts are correlated to the strength and orientation of the applied E-field, following a side-by-side domain model from Suzana et al. [Adv. Funct. Mater. 33, 2208012 (2023)]. Notably, we identified a critical electric field strength of 3 MV/m, which leads to irreversible structural changes, suggesting plastic deformation. The findings highlight how surface effects and intrinsic defects contribute to the enhanced dielectric properties of BTO at the nanoscale, in contrast to bulk materials, where strain limits domain mobility. These findings deepen our understanding of nanoscale dielectric behavior and inform the design of advanced nanoelectronic devices.

Recent experiments in ABC trilayer graphene detected superconductivity on the border of a phase transition to a symmetry-broken phase. In this work, we use unrestricted Hartree-Fock to study the nature of this phase. We find a close competition between two incommensurate intervalley coherent (IVC) phases: an IVC crystal where the ordering occurs at multiple wave vectors, and an IVC spiral with a single ordering wave vector. Focusing on one of the regimes where superconductivity is observed experimentally, we find a continuous (or very weakly first order) transition between a half-metallic phase to an IVC crystal, followed by a first-order transition into an IVC spiral. Using time-dependent Hartree-Fock, we study the collective mode spectrum in the half-metalic phase. We find a soft intervalley mode that can mediate superconductivity in a narrow sliver of density near the continuous transition, with a $T_c$ that can reach a few hundreds of mK and a sign-changing $s$-wave order parameter. The spin stiffness in the half-metal phase is found to be surprisingly low, of the order of a few degrees Kelvin.

The measurement of the electronic structure of metal alloys is hampered by the inapplicability of conventional quantum oscillation measurements owing to electron scattering by thermal or alloy disorder. Recent advancements in the study of the electronic contribution to the entropy suggest a path to ground transport properties in alloy equilibrium thermodynamics. Fe-Ni represents an interesting test case because it exhibits an intricate electronic structure, order-disorder transformations, and a large solid-solution region where equilibrium data can be obtained. Using cluster modeling, the electronic contribution to the entropy can be inferred from high-temperature thermodynamic data. Electronic transport property measurements can be used to independently evaluate the electronic contribution to the entropy. Reconciling these two approaches at high temperature supports this method to study electronic structure for metal alloys.

We develop a linear response theory for materials collectively coupled to a cavity that is valid in all regimes of light-matter coupling, including symmetry-broken phases. We present and compare two different approaches. First, we use a coherent path-integral formulation for the partition function to obtain thermal Green's functions. This approach relies on a saddle-point expansion for the action that can be truncated in the thermodynamic limit. Second, we formulate the equations of motion for the retarded Green's functions and solve them. We use a mean-field decoupling of high-order Green's functions to obtain a closed, solvable system of equations. Both approaches yield identical results in the calculation of response functions for the cavity and material. These are obtained in terms of the bare cavity and material responses. In combination, the two techniques clarify the validity of a mean-field decoupling in correlated light-matter systems and provide complementary means to compute finite-size corrections to the thermodynamic limit. The theory is formulated for a general model that encompasses most of the systems typically considered in the field of cavity QED materials. Finally, we provide a detailed application of the theory to the quantum Hall effect and to a collection of spin models.

Topologically protected spin textures, such as skyrmions and antiskyrmions, have garnered considerable attention in the past decade. Although theoretical models have proposed various mechanisms for stabilizing antiskyrmions, their experimental realization has predominantly been limited to systems exhibiting anisotropic Dzyaloshinskii-Moriya interactions and specific symmetry classes, such as $D_{2d}$ or $S_4$. Alternatively, the stabilization of antiskyrmions can be achieved through the manipulation of the interplay between long-range dipolar interactions and exchange interactions. In this paper, we report the observation of dipolar-stabilized skyrmions and antiskyrmions in the van der Waals magnet ${\mathrm{Fe}}_3{\mathrm{GaTe}}_2$ (FGT) across a broad temperature range (100–370 K) without the need of an external magnetic field. By manipulating thermal agitation to modulate the strength of perpendicular magnetic anisotropy, we were able to precisely control the formation of skyrmions and antiskyrmions, as observed using Lorentz transmission electron microscopy. We further demonstrate the presence of multitopological spin textures in FGT and their reversible transformations, which were controlled through a history-dependent magnetic field sweep process. These phenomena were characterized using magnetic force microscopy. In this paper, we provide experimental evidence for the existence of topological quasiparticles in two-dimensional systems.

Unconventional superconductivity in heavy-fermion systems appears often near magnetic quantum critical points (QCPs). This seems to be the case also for ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ ($T_{\text{c}} \approx 0.31$ K). ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ shows two superconducting (SC) phases, SC1 and SC2, for a magnetic field along the $c$ axis of the tetragonal unit cell, but only the SC1 phase is observed for a field along the basal plane. Furthermore, another ordered state (phase I) is observed below $T_0\approx 0.48\mathrm{K}$ whose nature is still unclear: Thermodynamic and magnetic measurements pointed to a nonmagnetic multipolar state, but recent $\mu \mathrm{SR}$ and nuclear quadrupole resonance/nuclear magnetic resonance (NMR) experiments have clearly detected antiferromagnetic (AFM) order below $T_{\text{0}}$. Also, quasi-two-dimensional AFM fluctuations were observed in NMR and neutron-scattering experiments above $T_{\text{0}}$. The proximity of a QCP is indicated by non-Fermi-liquid (NFL) behavior observed above the ordered states in both specific heat $C\left(T\right)/T\propto T^{-0.6}$ and resistivity $\rho \left(T\right)\propto \sqrt{T}$. These $T$ dependencies are not compatible with any generic AFM QCP. Because of the strong magnetic-field anisotropy of both the SC phase and phase I, it is possible to study a field-induced SC QCP as well as a phase-I QCP by varying the angle $\alpha$ between the field and the $c$ axis. Thus, by examining the behavior of the electronic specific-heat coefficient $C\left(T\right)/T$ across these QCPs, we can determine which phase is associated with the NFL behavior. Here, we present low-temperature specific-heat measurements taken in a magnetic field as high as 21 T applied at several angles $\alpha$. We observe that the NFL behavior very weakly depends on the field and the angle $\alpha$, a result that is at odds with observations in standard magnetic QCPs. This suggests a nonmagnetic origin of the quantum critical fluctuations.

Slow magnetization relaxation processes are an important time-dependent property of many magnetic materials. We show that magnetotransport measurements based on a well-established current-reversal method can be utilized to implement a simple and robust screening scheme for such relaxation processes. We demonstrate our approach considering the anomalous Hall effect in a $\mathrm{Pt}/\mathrm{Co}/{\mathrm{AlO}}_x$ trilayer model system, and then explore relaxation in $\tau \text{−}\mathrm{MnAl}$ films. Compared to magnetotransport experiments based on ac lock-in techniques, we find that the dc current-reversal method is particularly sensitive to relaxation processes which happen on timescales on the order of few to many seconds.

We suggest and develop an approach for describing the topological properties of a periodic system purely from the transfer matrix associated with a unit cell. Our approach uses the Iwasawa decomposition to parametrize the transfer matrix uniquely in terms of three real numbers. This allows us to obtain simple conditions for the existence of topologically protected edge states and to provide a visual illustration of all possible solutions. In order to demonstrate our method in action, we apply it to study some generalizations of the Su-Schrieffer-Heeger (SSH) model, such as the tetramer SSH4 model and a dimerized one-dimensional photonic crystal. Finally, we also obtained a simple pictorial proof of the Zak phase bulk-edge correspondence for any one-dimensional system using this approach.

Magnetic topological materials $\mathit{Ln}\mathrm{SbTe}$ ($\mathit{Ln}$ = lanthanide) have attracted intensive attention because of the presence of interplay between magnetism, topological, and electron correlations depending on the choices of magnetic $\mathit{Ln}$ elements. Varying Sb and Te composition is an efficient approach to control structural, magnetic, and electronic properties. Here, we report on the composition-dependent properties in ${\mathrm{PrSb}}_x{\mathrm{Te}}_{2-x}$. We identified the tetragonal-to-orthorhombic structure transitions in this material system and very large negative magnetoresistance in the $x=0.3$ composition, which might be ascribed to the coupling between magnetism and transport. Such unusual magnetotransport enables ${\mathrm{PrSb}}_x{\mathrm{Te}}_{2-x}$ topological materials as a promising platform for device applications.

We study numerically the Hessian of low-lying minima of vector spin glass models defined on random regular graphs. We consider the two-component (XY) and three-component (Heisenberg) spin glasses at zero temperature, subjected to the action of a randomly oriented external field. Varying the intensity of the external field, these models undergo a zero-temperature phase transition from a paramagnet at high field to a spin glass at low field. We study how the spectral properties of the Hessian depend on the magnetic field. In particular, we study the shape of the spectrum at low frequency and the localization properties of low-energy eigenvectors across the transition. We find that in both phases the edge of the spectral density behaves as ${\lambda }^{3/2}$: such a behavior rules out the presence of a diverging spin glass susceptibility ${\chi }_{\mathrm{SG}}=\langle 1/{\lambda }^2\rangle$. As to low-energy eigenvectors, we find that the softest eigenmodes are always localized in both phases of the two models. However, by studying in detail the geometry of low-energy eigenmodes across different energy scales close to the lower edge of the spectrum, we find a different behavior for the two models at the transition: in the XY case, low-energy modes are typically localized; at variance, in the Heisenberg case low-energy eigenmodes present a multimodal structure (sort of “delocalization”). These geometrically nontrivial excitations, which we call concentrated and delocalized low-energy modes, coexist with trivially localized excitations: we interpret their existence as a sign of critical behavior related to the onset of the spin glass phase.

The Wannier-Stark ladder (WSL) is a basic concept supporting periodic oscillation, widely used in many areas of physics. In this paper, we investigate the formations of WSL in generalized systems, including strongly correlated and non-Hermitian systems. We present a theorem on the existence of WSL for a set of general systems that are translationally symmetric before the addition of a linear potential. For a non-Hermitian system, the WSL becomes complex but maintains a real energy-level spacing. We illustrate the theorem using one-dimensional extended Bose-Hubbard models with both real and imaginary hopping strengths. It is shown that the Bloch-Zener oscillations of correlated bosons are particularly remarkable under resonant conditions. Numerical simulations for cases with boson numbers $N=2,3$, and 4 are presented. Analytical and numerical results for the time evolution of the $N$-boson-occupied initial state indicate that all evolved states exhibit quasiperiodic oscillations but with different profiles, depending on the Hermiticity and interaction strength.

For nearly 90 years, precession and relaxation processes have been thought to dominate the magnetization dynamics. Only recently has it been considered that, on short time scales, an inertia-driven magnetization dynamics should become relevant, leading to additional nutation of the magnetization vector. Here, we trigger magnetic nutation via a sudden excitation of a thin ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (Permalloy) film with an ultrashort optical pulse, that leads to an abrupt tilting of the effective field acting on the magnetic moments, separating the dynamics of the magnetization from that of its angular momentum. We investigate the resulting magnetization dynamics in the inertial regime experimentally by the time-resolved magneto-optical Kerr effect. We find a characteristic oscillation in the Kerr signal in the range ∼0.1 THz superimposed on the precessional oscillations with GHz frequencies. By comparison with atomistic spin dynamics simulations, we demonstrate that this observation cannot be explained by the well-known Landau-Lifshitz-Gilbert equation of motion but can be attributed to inertial contributions leading to nutation of the magnetization vector around its angular momentum. Hence, an optical and nonresonant excitation of inertial magnetization dynamics can trigger and control different magnetic processes, ranging from demagnetization via nutation to precession in a single device. These findings will have profound implications for the understanding of ultrafast spin dynamics and magnetization switching.

This study investigates a charge-driven feedback loop on single quantum dots (QDs) embedded in micropillar cavities under electrical readout. The coupled quantum–dot–microcavity system demonstrates a significant reduction in hysteresis under temperature sweep when the QD is in resonance with the cavity mode. To describe the experimental results, we develop a feedback model for the photocurrent response which incorporates a quadratic Stark shift to accurately fit the observed asymmetries and hysteresis in the spectra. Supported by this model, we attribute the observed reduction in hysteresis to an interplay between cavity-enhanced effective excitation power and the competition between radiative and nonradiative recombination by cavity quantum electrodynamics effects in the Purcell regime. This work provides important insights that can guide future optimization of QD-based devices for applications in quantum technologies.

We study how stronger noise can enhance the entanglement in inhomogeneously monitored quantum systems. We consider a free fermions model composed of two coupled chains—a system chain and an ancilla chain, each subject to its own different noise—and explore the dynamics of entanglement within the system chain under different noise intensities. Our results demonstrate that, contrary to the detrimental effects typically associated with noise, certain regimes of noise on the ancilla can significantly enhance entanglement within the system. Numerical simulations demonstrate the robustness of such entanglement enhancement across various system sizes and noise parameters. This enhancement is found to be highly dependent on the hopping strength in the ancilla, suggesting that the interplay between unitary dynamics and noise can be tuned to optimize the entanglement of a quantum system.

Utilizing the high-pressure low-temperature Raman and optical transmission/absorption spectroscopies, we have mapped out the phase diagram of ${\mathrm{H}}_2$-HD-${\mathrm{D}}_2$ alloy with the initial H:D concentration 50:50 up to 350 GPa between 10 and 300 K. We followed the phase lines between all known solid phases [I, II, III, IV, and ${\mathrm{IV}}^{\prime }$ ($\mathrm{IV}+\mathrm{V}$)] of pure ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ constraining their locations in the wide P-T space. We trace the phase boundary separating phases III and $\mathrm{IV}({\mathrm{IV}}^{\prime }$)/V demonstrating that this phase line has a steep negative slope ($-0.95$ K/GPa), which becomes shallower at above 250 GPa ($-0.55$ K/GPa). The phase line follows the trend observed in the pure species. Additionally, we find that the band gap of the mixture between 300 and 360 GPa is close to those of the pure species suggesting that alloying one isotope by another would not significantly shift the metallization pressure. These observations imply that hydrogen deuteride (HD) has very similar properties to those exhibited by the pure species.

Chiral one-dimensional transport can be realized in thin films of a surface-insulating ferromagnetic topological insulator called quantum anomalous Hall insulator (QAHI). When superconducting (SC) pairing correlations are induced in the surface of such a material by putting an $s$-wave superconductor on the top, the resulting topological superconductivity gives rise to chiral Majorana edge modes. A quantized two-terminal conductance of $\frac{1}{2}(e^2/h)$ was proposed as a smoking-gun evidence for the topological SC phase associated with a single chiral Majorana edge mode. There have been experiments to address this proposal, but the conclusion remains unclear. Here we formulate the edge transport in a multiterminal superconductor–QAHI heterostructure using the Landauer-Büttiker formalism. Compared to the original proposal for the $\frac{1}{2}(e^2/h)$ quantization based on a simple two-terminal model, our formalism allows for deeper understanding of the origin of the quantization. The analysis of our experiments on multiterminal devices unambiguously shows that the half-integer conductance quantization arises from the equilibration of the potentials of the incoming edge states at the SC electrode, and hence it is not of Majorana origin.

Recent seminal experiments have utilized time-resolved x-ray absorption spectroscopy (XAS) to investigate the ultrafast photoinduced renormalization of the electron interaction (“Hubbard $U$”) in Mott and charge transfer insulators. In this paper, we theoretically analyze the change of interactions due to dynamical screening as it is encoded in the XAS signal, using the nonequilibrium $GW+\mathrm{EDMFT}$ formalism. Our study shows that XAS is well-suited for measuring this change, but two aspects should be kept in mind if the screening processes are not substantially faster than the valence electron dynamics: (i) Screening in a photoexcited system can affect both the position and the lineshape of the absorption lines. (ii) In general, the effect cannot be captured by the modification of a single interaction parameter. Specifically, an estimate for $\mathrm{\Delta }U$ extracted from the shift of the XAS lines does not necessarily describe the related shift of the upper Hubbard band. We clarify these aspects using a minimal cluster model and the three-band Emery model for a charge transfer insulator.

We report on studies of the decoherence processes of a ballistic quantum interferometer subjected to a radio-frequency (RF) electromagnetic (EM) field and a surface acoustic wave (SAW). The device consists of an Aharonov-Bohm (AB) ring sandwiched between two interdigital transducers (IDTs), where the AB ring serves as a phase sensor and the IDTs function as a controlled source of environmental noise. By employing the IDTs with an RF-EM field to launch a SAW train through the ring, we extract the decoherence rate ${\mathrm{\Gamma }}_{\varphi }$ from the AB oscillation amplitude and investigate the dephasing process through the RF power $P_{\text{rf}}$ dependence of ${\mathrm{\Gamma }}_{\varphi }$, the electronic temperature $T_s$, and acoustoelectric current $I_{\text{ae}}$ spectrum. Our data reveal that the bandwidth-narrowing feature caused by Bragg reflections of the SAW is consistently found in the spectra of ${\mathrm{\Gamma }}_0,T_s$, and $I_{\text{ae}}$, suggesting that the piezoelectric field is the dominant EM field contributing to decoherence. At the resonance frequency, the decoherence rate follows $P_{\text{rf}}^{1/5}$ dependence due to thermal fluctuation of the alternating electric current by the EM field. At the off-resonance condition, we identify that the asymmetric ${\mathrm{\Gamma }}_0$ spectrum is induced by the crosstalk of SAW interference under the influence of a weak $P_{\text{rf}}$. Furthermore, we find the optimal conditions for operating the SAW to minimize dephasing without thermal heating. The underlying mechanism responsible for the decoherence is attributed to the enhancement of charge-charge interactions in the presence of the EM field and the SAW. These findings are crucial for the development of quantum electronic devices leveraging SAW technology.

Measurements of the Fermi surface are a fundamental technique for determining the electrical and magnetic properties of solids. In two-dimensional (2D) systems, the area and diameter of the Fermi surface are typically measured using Shubnikov–de Haas oscillations and commensurability oscillations respectively. However, these techniques are unable to detect changes in the parity of the Fermi surface [i.e., when $E(+k)\ne E(-k)]$. Here, we show that transverse magnetic focusing can be used to detect such changes, because focusing only measures a well defined section of the Fermi surface and does not average over $+k$ and $-k$. Furthermore, our results show that focusing is an order of magnitude more sensitive to changes in the Fermi surface than other 2D techniques. While we investigate a specific Fermi surface shift in this work, focusing could be used to investigate similar Fermi surface changes in other 2D systems.

We develop a microscopic theory for excitons and cavity exciton polaritons in transition metal dichalcogenide (TMD) monolayers under a perpendicular static magnetic field. We obtain numerically exact solutions for the ground and excited states, accounting for the interplay between arbitrarily large magnetic fields and light-matter coupling strengths. This includes the very strong coupling regime, where light-induced modifications of the exciton wave function become essential and the approximate coupled oscillator description breaks down. Our results show excellent agreement with recent experimental measurements of the diamagnetic shift of the ground and excited exciton states in ${\mathrm{WS}}_2$, ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, and ${\mathrm{MoTe}}_2$ monolayers. For polaritons, we consider experimentally relevant system parameters and demonstrate that the diamagnetic shifts of both the ground and excited states at high magnetic fields exhibit clear signatures of the very strong coupling regime, highlighting the necessity of our microscopic and numerically exact treatment over perturbative approaches. Furthermore, our microscopic approach allows us to evaluate the exciton-exciton and polariton-polariton interaction strengths. Comparing results specific to TMD monolayers with those applicable to quantum wells, we find that variational approaches overestimate the TMD excitons' interaction strength. We also observe that magnetic fields weaken the interaction strength for both excitons and polaritons, with a less pronounced effect in TMDs than in quantum wells, and that light-induced modifications to the matter component in TMD polaritons can enhance interaction strengths beyond those of purely excitonic interactions.