Realizing the potential of ${\mathrm{MoS}}_2$ and other two-dimensional semiconductors in spintronic devices requires a fundamental understanding of their interaction with ferromagnetic metals. We have investigated the transport and magnetic properties of thin Ni films grown on single-crystal ${\mathrm{MoS}}_2$ substrates. The Ni is found to have a granular morphology with an average grain size of approximately 2 nm. Like previously investigated granular Ni/insulator composites below the percolation threshold, transport is found to proceed via hopping. However, our $\mathrm{Ni}\text{/}{\mathrm{MoS}}_2$ sample exhibits a dual crossover: activated hopping to $T^{-1/2}$ variable-range hopping at a higher temperature and then a transition to power-law behavior at a lower temperature. The magnetoresistance was used as a novel probe for uncovering superspin-glass-like behavior above room temperature in the $\mathrm{Ni}\text{/}{\mathrm{MoS}}_2$ nanostructure, which also exhibits a collective ferromagnetic state at very low temperatures, reminiscent of superferromagnetism.

The discovery of high-temperature superconductivity (HTSC) in strongly correlated cuprates opened a new chapter in condensed matter physics, breaking existing stereotypes of what is a material base for a good superconductor (“Matthias rules”), at the same time emphasizing the richness and challenge of strongly correlated physics, personified by the most strongly correlated $3d$ ion, ${\mathrm{Cu}}^{2+}$. A recently reported new compound, $\mathrm{CuAg}{\left({\mathrm{SO}}_4\right)}_2$, combines in a fascinating way the same ion with the most strongly correlated $4d$ one, ${\mathrm{Ag}}^{2+}$. In this Letter, we present a detailed analysis of electronic and magnetic properties of this material, and show that it is very different from the HTSC cuprates in several different ways, and opens a door into further research of superconductivity and magnetism, in particular altermagnetism, in strongly correlated materials.

Nuclear spins in solids offer a promising avenue for developing scalable quantum hardware. Leveraging nearby single-color centers, these spins can be efficiently addressed at the single-site level through spin resonance. However, characterizing individual nuclear spins is quite cumbersome since the characterization protocols may differ depending on the strength of the hyperfine coupling, necessitating tailored approaches and experimental conditions. While modified electron spin Hahn echoes like Carr-Purcell-Meiboom-Gill (CPMG) and phase cycled CPMG (XY8) pulse sequences are commonly employed, they encounter significant limitations in scenarios involving spin-1/2 systems, strongly coupled spins, or nuclear spin baths comprising distinct isotopes. Here, we present a more straightforward approach for determining the hyperfine interactions among each nuclear spin and the electron spin. This method holds promise across diverse platforms, especially for emerging $S=1/2$ group IV defects in diamond (e.g., SiV, GeV, SnV, PbV) and silicon (T center, P donors). We provide a theoretical framework and adapt it for color centers exhibiting various spins. Through simulations conducted on nuclear spin clusters, we evaluate different protocols and compare their performance using the Fisher information matrix and Cramér-Rao bounds.

Understanding the nature of the transition from the delocalized to the many-body localized (MBL) phase is an important unresolved issue. To probe the nature of the MBL transition, we investigate the universal properties of single-particle excitations produced in highly excited many-body eigenstates of a disordered interacting quantum many-body system. In a class of one-dimensional spinless fermionic models with random disorder, we study the finite-size scaling of the ratio of typical to average values of the single-particle local density of states and the scattering rates across the MBL transition. Our results indicate that the MBL transition in this class of one-dimensional models of spinless fermions is continuous in nature. For various ranges of interactions in the system, the critical exponent $\nu$ with which the correlation length $\xi$ diverges at the transition point $W_c, \xi \sim |W-W_c{|}^{-\nu }$, satisfies the Chayes-Chayes-Fisher-Spencer (CCFS) bound $\nu \ge 2/d$ where $d$ is the physical dimension of the system. We also discuss why the critical exponent obtained from finite-size scaling of the conventional diagnostic of many-body localization, the level spacing ratio, strongly violates the CCFS bound while the single-particle density of states and scattering rates are consistent with the CCFS criterion.

Level statistics is an important quantity for exploring and understanding localized physics. The level-spacing distribution (LSD) of the disordered localized phase follows Poisson statistics, and many studies naturally apply it to the quasiperiodic localized phase. Here, we analytically obtain the LSD of the quasiperiodic localized phase, and find that it deviates from Poisson statistics. Moreover, based on this level statistics, we derive the ratio of adjacent gaps and find that for a single sample, it is a $\delta$ function, which is in excellent agreement with numerical studies. Additionally, unlike disordered systems, in quasiperiodic systems, there are variations in the LSD across different regions of the spectrum, and the presence of spectral correlations results in nonequivalence between increasing the size and increasing the sample. Our findings carry significant implications for the reevaluation of level statistics in quasiperiodic systems and a profound understanding of the distinct effects of quasiperiodic potential-induced and disorder-induced localization.

The optical-conductivity profile carries information on electronic dynamics in interacting quantum many-body systems. Its computation is a formidable task that is usually approached by invoking the single-particle (bubble) approximation and neglecting vertex corrections, the importance of which remains elusive even in model Hamiltonian calculations. Here, we combine analytical arguments with our recent breakthroughs in numerically exact and approximate calculations of finite-temperature real-time correlation functions to thoroughly assess the importance of vertex corrections in the one-dimensional Holstein polaron model. We find, both analytically and numerically, vanishing vertex corrections to optical conductivity in the limits of zero electron-phonon interaction, zero electronic bandwidth, and infinite temperature. Furthermore, our numerical results show that vertex corrections to the electron mobility also vanish in many parameter regimes between these limits. In some of these cases, the vertex corrections still introduce important qualitative changes to the optical-conductivity profile in comparison to the bubble approximation even though the self-energy remains approximately local. We trace these changes back to the bubble approximation not fully capturing a time-limited slowdown of the electron on intermediate timescales between ballistic and diffusive transport. We find that the vertex corrections are overall most pronounced for intermediate electron-phonon interaction and may increase or decrease the bubble-approximation mobility depending on the values of the model parameters.

Using local quantum fidelity distances, we study the dynamical quantum phase transition in integrable and nonintegrable one-dimensional Ising chains. Unlike the Loschmidt amplitude, the standard measure for distinguishing between two quantum states to describe the dynamical quantum phase transition, the local fidelity requires only a part of the system to characterize it. The nonanalyticities in the quantum distance between two subsystem density matrices identify the critical time and the corresponding critical exponent reasonably well in a finite-size system. Moreover, we propose a distance measure from the upper bound of the local quantum fidelity for certain quench protocols where the entanglement entropy features oscillatory growth in time. This local distance encodes the difference between the eigenvalue distribution of the initial and quenched subsystem density matrices and quantifies the critical properties. The alternative distance measure could be employed to examine the dynamical quantum phase transitions in a broader range of models, with implications for gaining insights into the transition from the entanglement perspective.

We theoretically investigate the acoustic analogs of high-angular-momentum rotating black holes in exciton-polariton condensates. Performing numerical simulations of a long-lived ring-shaped condensate configuration with an acoustic horizon and ergoregion for high-angular-momentum states, we observed a quasistable state near critical angular momentum where the acoustic black hole horizon disappears. Our findings offer an insight into the quantum nature of the instability of naked singularity.

Given the strong electron-phonon coupling observed during both ferroelectric (FE) and superconducting (SC) transitions, there is significant interest in investigating SC arising from FE instability. $\mathrm{S}{\mathrm{n}}_2{\mathrm{P}}_2\mathrm{S}{\mathrm{e}}_6$ has garnered considerable attention due to its unique FE properties. Here, we report on the electronic phase transitions and SC in this compound based on high-pressure electrical transport measurement, optical absorption spectroscopy, and Raman-based structural analysis. Upon compression, the conductivity of $\mathrm{S}{\mathrm{n}}_2{\mathrm{P}}_2\mathrm{S}{\mathrm{e}}_6$ increased monotonously, with an electronic phase transition occurring ∼5.4 GPa, as evidenced by optical absorption spectroscopy. The insulating state was fully suppressed ∼15 GPa, coinciding with the onset of SC ∼15.3 GPa. A zero-resistance state was achieved from 19.4 GPa onwards, with SC exhibiting continuous enhancement under pressure. The SC behavior was further confirmed by the magnetic field effect, and it exhibited a critical temperature ($T_c$) of 5.4 K at 41.8 GPa and a zero-temperature upper critical field of 6.55 T. Raman spectra supported the structural origin of the electronic transition ∼5.4 GPa, indicative of a transition from the paraelectric (PE) phase to the incommensurate phase—a distorted PE phase without symmetry change. Furthermore, a possible first-order phase transition was suggested during the semiconductor-metal transition ∼15 GPa. Comparison with the high-pressure behavior of sister compounds $\mathrm{S}{\mathrm{n}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ and $\mathrm{P}{\mathrm{b}}_2{\mathrm{P}}_2{\mathrm{S}}_6$, along with the low-temperature behavior of $\mathrm{S}{\mathrm{n}}_2{\mathrm{P}}_2\mathrm{S}{\mathrm{e}}_6$ at ambient pressure, suggests that SC in $\mathrm{S}{\mathrm{n}}_2{\mathrm{P}}_2\mathrm{S}{\mathrm{e}}_6$ likely emerges in a FE or polar metal state. In this paper, we highlight the versatile physical properties of FEs and motivate further investigation into the correlation between FE instability and SC in the $M_2{\mathrm{P}}_2{X}_6$ family.

The exploration of topology in natural materials and metamaterials has garnered significant attention. Notably, the one-dimensional (1D) and two-dimensional (2D) Su-Schrieffer-Heeger (SSH) models, assessed through tight-binding approximations, have been extensively investigated in both quantum and classical systems, encompassing general and higher-order topology. Despite these advancements, a comprehensive examination of these models from the perspective of wave physics, particularly the scattering view, remains underexplored. In this study, we systematically unveil the origin of the 1D and 2D Zak phases stemming from the zero-reflection point, termed the scattering singularity in momentum space. Employing an expanded plane wave expansion, we accurately compute the reflective spectrum of an infinite 2D photonic crystal (2D-PhC). Analyzing the reflective spectrum reveals the presence of a zero-reflection line in the 2D-PhC, considered the topological origin of the nontrivial Zak phase. Two distinct models, representing omnidirectional nontrivial cases and directional nontrivial cases, are employed to substantiate these findings. Our work introduces a perspective for characterizing the nature of nontrivial topological phases. The identification of the zero-reflection line not only enhances our understanding of the underlying physics but also provides valuable insights for the design of innovative devices.

Winding number and Zak phase, as topological invariants, play crucial roles in characterizing the topological phases of one-dimensional Floquet topological insulators. It is difficult to directly detect Floquet topological invariants with a unified scheme, especially for low driving frequencies. Here, by defining the dynamic winding number based on the time-averaged spin textures, we propose to use it to characterize one-dimensional Floquet topological phases with arbitrarily varying frequencies. In the high-frequency regime, both the conventional winding number and Zak phase can be fully determined by measuring the dynamic winding number of time-averaged spin textures, regardless of initial states and initial phases. In the low-frequency regime, only conventional winding number can accurately describe the system's topological phase due to the presence of large topological numbers, the Zak phase only captures the parity of the winding number and fails to characterize the number of topological edge states. However, one can still correctly characterize the Floquet topological phases through our dynamic winding number approach, in which stroboscopic measurement of spin textures is needed and the initial phases should guarantee chiral symmetry of the time-evolution operator. Our paper not only clarifies the necessity of stroboscopic measurements, but also provides a general dynamic approach for the detection of Floquet topological insulators.

It is shown that in spin injection experiments the interplay between external magnetic field and alternating current can be observed already on a single ferromagnet/normal metal interface. The interface resistance is predicted to exhibit prominent features whenever the frequency of spin precession in the applied field becomes equal to the frequency of the driving current. Using these features, material-specific g-factors of electrons in normal metals can be measured with less effort, albeit also with less precision.

Spontaneous symmetry breaking of a U(1) symmetry in interacting systems leads to superfluidity of a corresponding conserved charge. We generalize the superfluidity to systems with U(1) symmetries acting on both matter fields and 2D spatial coordinates. Such systems can be effectively realized in easy-plane ferromagnetic systems with spin-orbit coupling where the conserved charge is a total angular momentum. We clarify that under a steady injection of spin angular momentum, the superfluid of the total angular momentum shows spacetime oscillations of the spin density and geometry-dependent spin hydrodynamics. We also demonstrate that the steady spin injection destabilizes the superfluid of total angular momentum, causing a dissipation effect in its spin hydrodynamic properties. Although a stability analysis shows that the superfluid under the spin injection is nonideal, the unique spin-transport features persist with weak dissipation of the spin angular momentum. Our study broadens the comprehension of superfluidity and sheds new light on the interplay between symmetries and phases of matter.

Highly mismatched alloys are a class of semiconductor alloys with large electronegativity differences between the alloying elements. We predict that the absorption spectrum due to transitions between the split bands of a doped highly mismatched alloy with a conduction band anticrossing shows qualitative features revealing the fractional distribution of states in the split bands and providing valuable insight into their electronic structure. Our prediction is based on the analysis of the joint densities of states for both direct and indirect transitions between the split bands. In particular, we predict a peak near the absorption edge, which arises due to the suppression of direct transitions at large momenta. As a result of the suppression of direct transitions, indirect transitions dominate the spectrum away from the edge of absorption. We present analytic forms of the near-absorption-edge and large-energy behaviors of the spectra, comparing them with the asymptotic forms of absorption from a single deep impurity level.

We model the equilibrium properties of a two-dimensional electron gas in a square lateral superlattice of quantum dots in a GaAs heterostructure subject to an external homogeneous perpendicular magnetic field and a far-infrared circular cylindrical photon cavity with one quantized mode, the TE011 mode. In a truncated linear basis constructed by a tensor product of the single-electron states of the noninteracting system and the eigenstates of the photon number operator, a local spin density approximation of density functional theory is used to compute the electron-photon states of the two-dimensional electron gas in the cavity. The common spatial symmetry of the vector fields for the external magnetic field and the cavity photon field in the long wavelength approximation enhances higher order magnetic single- and multiphoton processes for both the para- and the diamagnetic electron-photon interactions. The electron-photon coupling introduces explicit photon replicas into the band structure and all subbands gain a photon content, constant for each subband, that can deviate from an integer value as the coupling is increased or the photon energy is varied. The subbands show a complex Rabi anticrossing behavior when the photon energy and the coupling bring subbands into resonances. The complicated energy subband structure leads to photon density variations in reciprocal space when resonances occur in the spectrum. The electron-photon coupling polarizes the charge density and tends to reduce the Coulomb exchange effects as the coupling strength increases.

A quantum battery (QB) is an energy storage and extraction device that is governed by the principles of quantum mechanics. Here we propose a three-level Dicke QB and investigate its charging process by considering three quantum optical states: a Fock state, a coherent state, and a squeezed state. The performance of the QB in a coherent state is significantly superior to that in a Fock or squeezed state. We show that the locked energy is positively related to the entanglement between the charger and the battery, and diminishing the entanglement increases the ergotropy. The role of quantum phase transitions in the charging process of the QB system is analyzed and we find that the behavior of maximum stored energy is jointly determined by ground and excited-state quantum phase transitions. We further demonstrate that, no matter what the initial state is, the stored energy can be completely extracted when $N$ is small ($N=10$), far below the thermodynamic limit, and the charging power follows a behavior consistent with that of the stored energy.

We report studies of the electron capture process in self-assembled quantum dots (SAQDs) far from equilibrium with their environment and at large distance to the reservoirs. Deep level transient spectroscopy is used to determine the capacitance transients in response to bias voltage pulses, from which the capture rates are obtained as a function of the temperature and the quantum dot occupancy. The observed activated character of the capture suggests that the dominant electron source is the back contact. A model is developed based on electrons diffusing from the reservoir across the flat band region and getting captured in the SAQDs after overcoming the barrier formed by the space charge region between its onset and the quantum dots. For small barriers, we identify a distinct tunneling contribution to the capture current.

We study an anisotropic cubic Dirac semimetal subjected to a constant magnetic field. In the case of an isotropic dispersion in the $x\text{−}y$ plane, with parameters $v_x=v_y$, it is possible to find exact Landau levels, indexed by the quantum number $n$, using the typical ladder operator approach. Interestingly, we find that the lowest energy level (the zero-energy state in the case of $k_z=0$) has a degeneracy that is 3 times that of other states. This degeneracy manifests in the Hall conductivity as a step at a zero chemical potential 3/2 the size of other steps. Moreover, as $n\to \infty$, we find energies $E_n\propto n^{3/2}$, which means the $n\mathrm{th}$ step as a function of the chemical potential roughly occurs at a value $\mu \propto n^{3/2}$. We propose that these exciting features could be used to experimentally identify cubic Dirac semimetals. Subsequently, we analyze the anisotropic case $v_y=\lambda v_x$, with $\lambda \ne 1$. First, we consider a perturbative treatment around $\lambda \approx 1$ and find that energies $E_n\propto n^{3/2}$ still hold as $n\to \infty$. To gain further insight into the Landau level structure for a maximum anisotropy, we turn to a semiclassical treatment that reveals interesting star-shaped orbits in phase space that close at infinity. This property is a manifestation of weakly localized states. Despite being infinite in length, these orbits enclose a finite phase space volume and permit finding a simple semiclassical formula for the energy, which has the same form as above. Our findings suggest that both isotropic and anisotropic cubic Dirac semimetals should leave similar experimental imprints.

We report on measurements of the electrical resistivity (ρ) and thermoelectric power ($S$) in a thin film of strained single-crystalline α-Sn grown by molecular beam epitaxy on an insulating substrate. The temperature ($T$) dependence of the resistivity of α-Sn can be divided into two regions: below $T*\approx 135$ K ρ($T$) shows metalliclike behavior, while above this temperature, an increasing contribution from thermally excited holes to electrical transport is observed. However, it is still dominated by highly mobile electrons, resulting in a negative sign of the Seebeck coefficient above $T=47$ K. In the presence of the magnetic field ($B$) applied along an electric field or thermal gradient, we note negative magnetoresistance or a negative slope of $S$($B$), respectively. The theoretical prediction for the former (calculated using density functional theory) agrees well with the experiment. However, these characteristics quickly disappear when the magnetic field is deviated from an orientation parallel to the electrical field or the thermal gradient. We indicate that the behavior of the electrical resistivity and thermoelectric power can be explained in terms of the chiral current arising from the topologically nontrivial electronic structure of α-Sn. Its decay at high temperature is a consequence of the decreasing ratio between the intervalley Weyl relaxation time to the Drude scattering time.