Poor man's Majorana bound states (MBS) arise in minimal Kitaev chains when the parameters are fine-tuned to a sweet spot. We consider an interacting two-site Kitaev chain coupled to a single-mode cavity and show that the sweet spot condition can be controlled with the cavity frequency and the hopping between sites. Furthermore, we demonstrate that photon-mediated effective interactions can be used to screen intrinsic interactions, improving the original quality of the MBS. We describe experimental signatures in the cavity transmission to detect their presence and quality. Our work proposes a new way to tune poor man's MBS in a quantum dot array coupled to a cavity.

Nonreciprocal charge transport (NCT) characterized by unequal resistances for opposite current direction enriches the fundamental physics of electron transport and is promising for potential device applications. Here, we report on a study on the magnetic control of the NCT in two-dimensional systems with different types of spin-orbit coupling (SOC). Based on the Boltzmann transport theory, we first study this effect using a general Hamiltonian model and show that the nonlinear conductivity can be efficiently tuned by the exchange field. In particular, the exchange-field orientation dependence of the nonlinear conductivity reveals significantly distinct relations for different SOCs. We further derive the analytical expressions with the weak-field approximation, which are in good accordance with numerical results. Then, we exemplify this phenomenon in the ${\mathrm{Ag}}_2\mathrm{Te}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ surface based on density functional theory calculations. We predict that the nonlinear conductivity in such a system strongly depends on the surface magnetization orientation of the ${\mathrm{Cr}}_2{\mathrm{O}}_3$ layer. Those findings also suggest that measuring the NCT as a function of the exchange-field orientation provides a simple electrical means to quantify the SOC type. Our results are expected to provide a valuable insight into the NCT physics and open avenues for future rectifying devices.

This study presents an ab initio investigation of the XANES spectra at the aluminum $K$ edge for three compounds: ${\mathrm{Al}}_2{\mathrm{O}}_3, {\mathrm{AlF}}_3$, and ${\mathrm{AlCl}}_3$, where the Al atoms share the same oxidation state (III) and are coordinated in an octahedral symmetry. The XANES spectra calculated within the independent-particle approximation (IPA) reveal significant differences, including shifts in the spectrum onset, variations in the spectral shapes, and the presence of a prepeak in the case of ${\mathrm{AlCl}}_3$, all in correspondence with the behavior of the projected density of states of the absorbing atom in the different materials. The origin of those features can, therefore, be identified in the specific band structure of each compound. When electron-hole interactions are taken into account through the solution of the Bethe-Salpeter equation, a series of dark and bright excitons with large binding energies and Frenkel character is obtained. The strong excitonic effects lead to the suppression of the prepeak in ${\mathrm{AlCl}}_3$ and further accentuate the differences among the three Al $K$-edge spectra, which shows that drawing conclusions solely on the basis of IPA spectra may be misleading.

We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$. Below the $\approx 40$ K Kondo coherence temperature, a narrow Drude-like peak forms, as a result of the $f$-orbital–conduction-electron hybridization and the formation of the heavy-fermion state. The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Via the extended Drude model analysis, we measure the frequency-dependent scattering rate ($1/\tau$) and effective mass ($m^{*}/m_b$). This scattering rate shows a linear dependence on temperature, which matches the dependence of the resistivity as expected. Nevertheless, the width of the low-frequency Drude peak itself that is set by the renormalized quasiparticle scattering rate ($1/{\tau }^{*}=m_b/m^{*}\tau$) shows a $T^2$ dependence. This is the scattering rate that characterizes the relaxation time of the renormalized quasiparticles. This gives evidence for a Fermi liquid state, which in conventional transport experiments is hidden by the strong temperature dependent mass.

Unlike Hermitian systems, non-Hermitian energy spectra under periodic boundary conditions can form closed loops in the complex energy plane, a phenomenon known as the point-gap topology. In this paper, we investigate the self-intersection points of such non-Hermitian energy spectra and reveal their geometric origins. We rigorously demonstrate that these self-intersection points result from the intersection of the auxiliary generalized Brillouin zone and the Brillouin zone in one-band systems, as confirmed by an extended Hatano-Nelson model. This finding is further generalized to multiband systems, illustrated through a non-Hermitian Su-Schrieffer-Heeger model. Moreover, we address multiple self-intersection points and derive the geometric conditions for general $n$-fold self-intersection points. Our results enhance the fundamental understanding of generic non-Hermitian quantum systems and provide theoretical support for further experimental investigations of energy self-intersection points.

The discovery of the kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ sparked broad interest, due to the coexistence of a charge density wave (CDW) phase and possible unconventional superconductivity in the material. In this Letter, we use low-energy electron diffraction (LEED) with a $\mu \mathrm{m}$-sized electron beam to explore the periodic lattice distortion at the antimony-terminated surface in the CDW phase. We recorded high-quality backscattering diffraction patterns in ultrahigh vacuum from multiple cleaved samples. Unexpectedly, we did not find superstructure reflexes at intensity levels predicted from dynamical LEED calculations for the reported $2\times 2\times 2$ bulk structure. Our results suggest that in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ the periodic lattice distortion accompanying the CDW is less pronounced at Sb-terminated surfaces than in the bulk.

We predict the giant ferroelectric control of interfacial properties of $\mathrm{Ni}/{\mathrm{HfO}}_2$, namely, (i) the magnetocrystalline anisotropy and (ii) the inverse spin and orbital Rashba effects. The reversible control of magnetic properties using electric gating is a promising route to low-energy consumption magnetic devices, including memories and logic gates. Synthetic multiferroics, composed of a ferroelectric in proximity to a magnet, stand out as a promising platform for such devices. Using a combination of ab initio simulations and transport calculations, we demonstrate that reversing the electric polarization modulates the interface magnetocrystalline anisotropy from in-plane to out-of-plane. This modulation compares favorably with recent reports obtained upon electromigration induced by ionic gating. In addition, we find that the current-driven spin and orbital densities at the interface can be modulated by about 50% and 30%, respectively. This giant modulation of the spin-charge and orbit-charge conversion efficiencies opens appealing avenues for voltage-controlled spin- and orbitronics devices.

Dirac semimetals are renowned for the host of singular symmetry-protected band degeneracies which can give rise to other exotic phases. In this work, we consider a two-dimensional Dirac semimetal stabilized by $\mathcal{PT}$ symmetry and nonsymmorphic symmetries. We find that an out-of-plane Zeeman field can lift the Dirac points and transform the system into a Chern insulator with chiral edge states. By placing the nonsymmorphic Dirac semimetal in proximity to an $s$-wave superconductor, we uncover that chiral topological superconductors with large Chern numbers can be achieved. In addition, we find that topologically protected Bogoliubov Fermi surfaces can also emerge in this system, due to the coexistence of inversion symmetry and particle-hole symmetry. Notably, we find that the chiral Majorana edge state persists even when the Chern number becomes ill defined due to the appearance of Bogoliubov Fermi surfaces. The impact of these Bogoliubov Fermi surfaces on the thermal Hall effects is also investigated. Our study not only identifies a class of materials capable of realizing topological Bogoliubov Fermi surfaces through conventional $s$-wave superconductivity, but also uncovers an exotic phase where chiral Majorana edge states and Bogoliubov Fermi surfaces coexist.

We study surface states and domain wall bound states in altermagnets using a rutile-lattice tight-binding model of electrons coupled to a Néel order. We discover that two symmetry-protected Weyl nodal lines in the bulk band structure can give rise to unconventional antichiral surface states—surface states from opposite surfaces propagate in a parallel manner, as opposed to the antiparallel manner for the more conventional chiral surface states. We also find that the antichiral surface states can be turned into chiral surface states upon changing the surface termination. The origin of the surface states, the dependence on the surface termination, and key features of domain wall bound states are explained using a map from the altermagnet to a family of a modified Su-Schrieffer-Heeger chain and the associated bulk-boundary correspondence. Our work reveals rutile altermagnets as a promising candidate among very few quantum materials that can support antichiral surface states.

Magnetic skyrmions are topological objects that have been observed in helimagnets. Mostly treated as classical magnetic textures, they are studied mainly for spintronics applications due to their topological stability. Recent experimental observations of skyrmions with length scales comparable to the interatomic lattice spacing have led to a stronger focus on their quantum nature. Despite experimental and theoretical evidence showing nanoscale quantum skyrmions, it remains unclear what physical phenomena differentiate a classical from a quantum skyrmion. We present numerical evidence for the existence of a quantum skyrmion liquid (SkL) phase in quasi-one-dimensional lattices which has no classical counterpart. The transition from a conventional quantum skyrmion crystal (SkX) to a field-polarized phase (FP) is found to be of second order while the analogous classical transition near zero temperature is first-order due to a missing SkL phase. As an indicator of the quantum mechanical origin of the SkL phase, we find concentrated entanglement (indicated by the concurrence) around the skyrmion center, which we attribute to the uncertainty in the skyrmion position resulting from the noncommutativity of the skyrmion coordinate operators. The latter also gives rise to a nontrivial kinetic energy in the presence of an atomic lattice. The SkL phase emerges when the kinetic energy dominates over the skyrmion-skyrmion interaction energy. It is tied to the breaking of discrete translational invariance of the skyrmion crystal and occurs when the skyrmion radius is comparable with the size of the magnetic unit cell. In contrast to the long-range order present in the SkX phase, spin-spin correlations in the SkL phase exponentially decay with distance, indicating the fluid-like behavior of uncorrelated skyrmions. The emergence of kinetic energy-induced quantum SkL phase serves as a strong indication of the possible Bose-Einstein condensation of skyrmions in higher-dimensional systems. Our findings are effectively explained by effective models based on collective coordinate formalism and trial wave functions, effectively enhancing our understanding of the numerical findings.

Large scale atomistic simulations involving spin-lattice coupling are at the forefront of research in condensed matter physics and materials science. A key challenge lies in determining the ground state configurations of spins and lattices given this strong coupling, complicated by their evolution adherence to distinct governing equations. Unlike first-principles calculations, where the magnetic configuration is naturally determined by minimizing the electronic energy, no existing algorithm can simultaneously optimize both lattice and spin degrees of freedom at the atomic scale. This paper introduces a synergistic optimization framework for lattice and spin optimization in magnetic materials, overcoming the conventional separation approach. Employing a “pseudoatomic” method, the simultaneous optimization of atomic and spin configurations enhances accuracy and speed, enabling both atomic scale precision and large scale calculations. Applied to face-centered-cubic iron and ${\mathrm{BiFeO}}_3$, this approach demonstrates effective convergence, offering a robust tool for complex spin-lattice coupling problems, which paves a path for addressing high-dimensional optimization problems.

Theoretically, Josephson junction (JJ) arrays can exhibit either a superconducting or insulating state, separated by a quantum phase transition (QPT). In this work, we analyze published data in three one-dimensional arrays and two two-dimensional arrays using a recently developed phenomenological model of QPTs. The model is based on the insight that the scaled experimental data depend in a universal way on two characteristic length scales of the system: the microscopic length scale $L_0$ from which the renormalization group flow starts, and the dephasing length $L_{\phi }\left(T\right)$ as given by the distance traveled by system-specific elementary excitations over the Planckian time. Our analysis reveals that the data for all five arrays (both 1D and 2D) can be quantitatively and self-consistently explained within the framework of interacting superconducting plasmons. In this picture, $L_{\phi }=v_p\hslash /k_{B}T$ and $L_0\approx \mathrm{\Lambda }$, where $v_p$ is the speed of the plasmons and $\mathrm{\Lambda }$ is the Coulomb screening length of the Cooper pairs. We also discuss similarities and differences with recent microwave studies of extremely long JJ chains, as well as with the pair-breaking QPT observed in superconducting nanowires and films.

Transition-metal dichalcogenides (TMDs) are characterized by their unique layered structures and diverse electronic properties. Recent studies have highlighted the distinctive superconducting and charge density wave (CDW) behaviors of $4H_b\text{−}{\mathrm{TaS}}_2$ and $4H_b\text{−}{\mathrm{TaSe}}_2$. To explore the intriguing characteristics of these structurally modulated compounds, we synthesized $4H_b\text{−}\mathrm{TaSeS}$, which alternates between $1H$ and $1T$ layers, and investigated its structural and superconducting properties. At ambient pressure, $4H_b\text{−}\mathrm{TaSeS}$ displays superconductivity with a transition temperature ($T_{\text{c}}$) of approximately 3.8 K. Under increasing pressure, $T_{\text{c}}$ decreases to a minimum of around 2.3 K at 37 GPa, followed by a gradual recovery that forms an unusual valley in the $T_{\text{c}}$-pressure curve. High-pressure synchrotron x-ray diffraction measurements show that $4H_b\text{−}\mathrm{TaSeS}$ maintains its hexagonal symmetry up to 82 GPa without undergoing any structural transitions. Resistivity measurements also indicate a transition from non-Fermi liquid to Fermi liquid behavior induced by pressure. Further theoretical calculations shed light on the pressure-dependent superconducting mechanism, demonstrating that superconductivity is primarily influenced by Ta atoms, with contributions predominantly from $H$-layer Ta at lower pressures, gradually shifting to $T$-layer Ta as pressure increases. These findings offer valuable insights into pressure-induced superconductivity in TMDs and other complex layered systems.

Quantum Hall effects provide intuitive ways of revealing the topology in crystals, i.e., each quantized “step” represents a distinct topological state. Here, we seek a counterpart for “visualizing” quantum geometry, which is a broader concept. We show how geometry emerges in quantum as an intrinsic consequence of unitary evolution, composing a framework compatible with quantum metric and independent of specific details or approximations, suggesting quantum geometry may have widespread applicability. Indeed, we exemplify geometric observables, such as oscillation, dephasing, in magnetic resonance or band driving scenarios. Anomalies, supported by both analytic and numerical solutions, underscore the advantages of adopting a geometric perspective, potentially yielding distinguishable experimental signatures.

Recent advancements in exchange-correlation functionals within density functional theory highlight the need for rigorous validation across diverse types of materials properties. In this study, we assess the performance of the newly developed meta-GGA ${\mathrm{r}}^2\mathrm{SCAN}$ and its predecessor, SCAN, in predicting the Néel transition temperature of antiferromagnetic materials. Our analysis includes 48 magnetic materials, spanning both simple and complex systems. Using DFT, we compute the energies of various magnetic configurations and extract exchange interaction parameters through a least-squares fitting approach. These parameters are then used in classical Monte Carlo simulations to estimate the transition temperatures. Our results demonstrate that both SCAN and ${\mathrm{r}}^2\mathrm{SCAN}$ greatly outperform standard GGA and $\text{GGA}+U$ methods, yielding predictions that closely align with experimental values. The Pearson correlation coefficients for SCAN and ${\mathrm{r}}^2\mathrm{SCAN}$ are 0.97 and 0.98, respectively, when compared to experimental transition temperatures. Additionally, we calculate the energy differences between antiferromagnetic and ferromagnetic configurations to assess the performance of the hybrid HSE06 functional. We found that the HSE06 functional underestimates transition temperatures compared to the meta-GGA functionals and experimental values.

We introduce the Aleppo spin ice geometry, another variation of decimated square ice patterns, which in contrast to similar systems previously studied, does not exhibit vertex frustration. Using synchrotron-based photoemission electron microscopy, we directly visualize low-energy states achieved after thermal annealing, in addition to temperature-dependent moment fluctuations. The results reveal the observation of ground state patterns and the absence of ergodicity-breaking dynamics. Our observations further confirm vertex frustration to be an important criterion for the emergence of ergodicity transitions.

Since the discovery of femtosecond laser pulse induced magnetization switching, all-optical helicity-dependent switching (AO-HDS) has emerged as a powerful technique in the manipulation of magnetization across various magnetic materials, notably without any applied magnetic field. Nevertheless, the underlying mechanisms of AO-HDS remain a subject of intensive debate. Here, we investigate the AO-HDS phenomenon through a complementary approach, employing anomalous Hall effect and microscope optical Kerr effect analyses on a 10 × 10 µm² square Co/Pt multilayer with strong perpendicular magnetic anisotropy. Our focus is on the dynamics of magnetization switching induced by multiple picosecond circularly polarized laser pulses. We meticulously record the evolution of the anomalous Hall voltage as a function of time, revealing distinct “jump” and “relaxation” phases in the Hall signal. These phases are intricately linked to the processes of magnetic domain nucleation and domain wall propagation. Furthermore, we consider the influence of helicity-independent laser heating, domain pressure, and temperature gradients to understand the domain dynamics during the magnetization switching process. This comprehensive analysis not only sheds light on the role of micromagnetic evolution but also constitutes a pivotal step toward unraveling the complex mechanics of AO-HDS.