We combined optical spectroscopy and first-principles electronic structure calculations to reveal the charge gap in the polar magnet ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$. Iron occupation on the octahedral site draws the gap strongly downward compared to the Zn parent compound, and subsequent occupation of the tetrahedral site creates a narrow resonance near the Fermi energy that draws the gap downward even further. This resonance is a many-body effect that emanates from the flat valence band in a Mott-like state due to screening of the local moment—similar to expectations for a Zhang-Rice singlet, except that here it appears in a semiconductor. We discuss the unusual hybridization in terms of orbital occupation and character as well as the structure-property relationships that can be unveiled in various metal-substituted systems (Ni, Mn, Co, Zn).

Attraction of like charges in a localized system implies that, upon increasing the Fermi energy, the occupation of the system changes as $n\to (n+2)$, while the occupation, $(n+1)$, is skipped. In this way, the attraction translates into the bunching of electrons. For a localized system of $N=4$ sites, attraction of electrons manifests itself in skipping of $n=2$ occupation. The origin of the attraction is the rearrangement of the occupations of the surrounding sites, which plays the role of a polaronic effect. We consider a Coulomb cluster with $N=5$ sites, and we demonstrate that, with screened Coulomb repulsion, three-electron bunching becomes possible, i.e., the change of occupation $n=1\to n=4$ with $n=2$ and 3 occupations skipped.

The Wiener chaos expansion method provides a direct approach to accurately compute the thermal radiation from arbitrary structures. Here, we further extend this method to the structures with a permittivity or temperature inhomogeneity. The inhomogeneous Wiener chaos expansion method treats the two kinds of inhomogeneities with the same governing equation and thus provides a systematic solution to the radiative spectra. Compared to the traditional approach that approximates the thermal radiation from an inhomogeneous structure by the superposition of contributions from its quasihomogeneous subparts, our method calculates the thermal radiation from the entire inhomogeneous structure simultaneously and thus improves the time efficiency. We validate the method against the traditional approach and apply it to study the near-field radiative heat transfer between two gold nanorods with different temperature profiles, from which a four-time tunability is demonstrated at the plasmonic resonance.

After the experimental realization, the Berry curvature dipole (BCD) induced nonlinear Hall effect (NLHE) has attracted tremendous interest to the condensed matter community. Here, we investigate another family of Hall effect, namely, chiral anomaly induced nonlinear Hall effect (CNHE) in multi-Weyl semimetal (mWSM). In contrast to the BCD-induced NLHE, CNHE appears because of the combination of both chiral anomaly and anomalous velocity due to nontrivial Berry curvature. Using the semiclassical Boltzmann theory within the relaxation time approximation, we show that, in contrast to the chiral anomaly induced linear Hall effect, the magnitude of CNHE decreases with the topological charge $n$. Interestingly, we find that the CNHE has different behaviors in different planes for single and triple Weyl semimetals. Our prediction on the behavior of CNHE in mWSM can directly be checked in experiments.

To confirm the Majorana signatures, significant effort has been devoted to distinguishing between Majorana zero modes (MZMs) and spatially separated quasi-Majorana modes (QMMs). Because both MZMs and QMMs cause a quantized zero-bias peak in the conductance measurement, their verification task is thought to be very difficult. Here, we proposed a simple device with a single nanowire where the device could develop clear evidence of the topological Kondo effect in the topologically trivial phase with four QMMs. On the other hand, in the topological superconducting phase with MZMs, the transport signatures are significantly different. Therefore, our scheme provides a simple way to distinguish Majorana and quasi-Majorana modes.

The electronic, magnetic, and optical properties of the double perovskite ${\mathrm{Sr}}_2{\mathrm{CoIrO}}_6$ (SCIO) under biaxial strain are explored in the framework of density functional theory, including a Hubbard $U$ term and spin-orbit coupling in combination with absorption spectroscopy measurements on epitaxial thin films. While the end member ${\mathrm{SrIrO}}_3$ is a semimetal with a quenched spin and orbital moment and bulk ${\mathrm{SrCoO}}_3$ is a ferromagnetic (FM) metal with spin and orbital moment of 2.50 and 0.13 ${\mu }_B$, respectively, the double perovskite SCIO emerges as an antiferromagnetic Mott insulator with antiparallel alignment of Co, Ir planes along the [110] direction. Co exhibits a spin and enhanced orbital moment of $\sim 2.35–2.45$ and $0.31–0.46{\mu }_B$, respectively. Most remarkably, Ir acquires a significant spin and orbital moment of 1.21–1.25 and 0.13 ${\mu }_B$, respectively. Analysis of the orbital occupation indicates an electronic reconstruction due to a substantial charge transfer from minority to majority spin states in Ir and from Ir to Co, signaling an ${\mathrm{Ir}}^{4+\delta }$, ${\mathrm{Co}}^{4-\delta }$ configuration. Biaxial strain, varied from $-1.02\%$ $\left(a_{{\mathrm{NdGaO}}_3}\right)$ through 0% $\left(a_{{\mathrm{SrTiO}}_3}\right)$ to $1.53\%$ $\left(a_{{\mathrm{GdScO}}_3}\right)$, affects the orbital polarization of the $t_{2g}$ states and leads to a nonmonotonic change of the band gap between 163 and 235 meV. The absorption coefficient reveals a two-plateau feature due to transitions from the valence to the lower-lying narrow $t_{2g}$ and the higher-lying broader $e_g$ bands. Inclusion of many-body effects, in particular, excitonic effects by solving the Bethe-Salpeter equation, increases the band gap by $\sim 0.2\mathrm{eV}$ and improves the agreement with the measured spectrum concerning the position of the second peak at $\sim 2.6\mathrm{eV}$.

The spectral and temporal response of a set of $N$ quantum emitters embedded in a photonic cavity is studied. Quantum mechanically, such systems can be described by the Tavis-Cummings (TC) model of $N$ two-level systems coupled to a single light mode. Here we compare the full quantum solution of the TC model for different numbers of quantum emitters with its semiclassical limit after a pulsed excitation of the cavity mode. Considering different pulse amplitudes, we find that the spectra obtained from the TC model approach the semiclassical one for an increasing number of emitters $N$. Furthermore, they match very well for small pulse amplitudes. While we observe a very good agreement in the temporal dynamics for photon numbers much smaller than $N$, considerable deviations occur in the regime of photon numbers similar to or larger than $N$, which are linked to collapse and revival phenomena. Wigner functions of the light mode are calculated for different scenarios to analyze the quantum state of the light field. We find strong deviations from a coherent state even if the dynamics of the expectation values are still well described by the semiclassical limit. For higher pulse amplitudes, Wigner functions similar to those of Schrödinger cat states between two or more quasicoherent contributions build up.

Electron transport in a Dirac semimetal (DSM) quantum wire without/with an external electric field is investigated by using the Green-function-based Landauer-Büttiker formula. Normal-topological phase transition is certificated by the different characteristics between the DSM quantum wire without and with the electric field, including the orbital decomposition for the energy bands, transversally local charge and spin polarization density distributions, topological Chern number, transport behaviors, and disorder effects. Topologically trivial surface states are found in the former system and their transport can be localized by disorders. However, topologically nontrivial surface states are generated in the latter one and their transport is robust against strong disorders when the electric field strength is larger than a critical value. In addition, switching effect of the electron transport in the DSM quantum wire can be achieved by varying the electric field strength. Further studies indicate that the switching effect shows a strong robustness against disorder, displaying the feasibility of designing a topological field effect transistor based on the considered system.

Surface adsorption of ${\mathrm{C}}_{60}$ affects its chemical and electronic properties. Numerous studies have reported observation of bright and dark fullerenes on metal surfaces that suggest extensive surface reconstruction; however, the underpinning mechanism of the reconstruction remains under debate. Here we report tip-functionalized noncontact atomic force microscope measurements which unambiguously reveal that ${\mathrm{C}}_{60}$ fullerenes adsorb with three well-defined adsorption heights on the Cu(111) surface, consistent with theoretical reports of top-layer hollow sites, single-atom vacancies, and surface nanopits. Using single-molecule resolution $\mathrm{\Delta }f\left(z\right)$ measurements we identify well-defined adsorption heights specific to each site, confirming the presence of a complex vacancy model for ${\mathrm{C}}_{60}$ monolayers on metal surfaces.

Two-dimensional ferromagnetic (FM) materials are promising for designing quantum anomalous Hall (QAH) states and spintronics devices. Here, electronic structures, magnetic properties, and topological properties of monolayer Zirconium trihalide $\mathrm{Zr}{X}_3$ $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I})$ are investigated by using first-principles calculations with hybrid functional HSE06. Monolayer ${\mathrm{ZrCl}}_3$, ${\mathrm{ZrBr}}_3$, and ${\mathrm{ZrI}}_3$ are all in the FM state with the Curie temperature $\left(T_C\right)$ of 332 K, 268 K, and 233 K, respectively. The obtained intrinsic FM states in monolayer $\mathrm{Zr}{X}_3$ are robust against strain, and the $T_C$ can be tuned effectively by strain. Before the spin-orbit coupling (SOC) is included, a nearly flat band and a quadratic non-Dirac band near the Fermi level in monolayer ${\mathrm{ZrCl}}_3$ are degenerate at the $\mathrm{\Gamma }$ point, and the two spin-up bands around the non-Dirac degenerate $\mathrm{\Gamma }$ point are dominated by Zr $d_{xy}/d_{x^2-y^2}$ orbitals. A band gap of 28.6 meV is opened around the non-Dirac $\mathrm{\Gamma }$ point by including the SOC, indicating the realization of a room-temperature non-Dirac QAH state in monolayer ${\mathrm{ZrCl}}_3$. The physical mechanism of the non-Dirac QAH state is analyzed through a $k·p$ model. For monolayer ${\mathrm{ZrCl}}_3$, the half metal and half semiconductor can be achieved by applying compressive and tensile strain, respectively. Monolayer ${\mathrm{ZrBr}}_3$ and ${\mathrm{ZrI}}_3$ are half semiconductors and can be tuned to half metals with a compressive strain. The room-temperature half metals can be obtained in monolayer ${\mathrm{ZrCl}}_3$ and ${\mathrm{ZrBr}}_3$ by applying a compressive strain larger than $0.75\%$ and $2.0\%$, respectively. In addition, the non-Dirac QAH states can be achieved in monolayer ${\mathrm{ZrBr}}_3$ and ${\mathrm{ZrI}}_3$ by electron doping. Our paper demonstrates that the monolayers $\mathrm{Zr}{X}_3$ $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I})$ are promising candidates for the applications of room-temperature QAH and spintronics devices.

We use laser-based spin- and angle-resolved photoemission spectroscopy (laser-SARPES) with high resolution and experimentally determine, for the first time, the Rashba parameters of quantum-well states (QWSs) systematically changing with the film thickness and the quantum numbers, through the observation of the Ag films grown on an Au(111) substrate. The data are very well reproduced by the theoretical calculations based on the density functional theory. Most importantly, we find a scaling law for the Rashba parameter (${\alpha }_{\mathrm{R}}$) that the magnitude of ${\alpha }_{\mathrm{R}}$ is scaled by the charge density at the interface and the spin-orbit coupling ratio between the film and the substrate, and it is expressed by a single straight line regardless of the film thickness and the quantum numbers. The new finding not only is crucial to understand the Rashba effect in QWSs but also gives a foundation of film growth engineering to fine-tune the spin splitting in 2D heterostructure systems.

We measure temperature-dependent quasiparticle fluctuations in a small Al volume, embedded in a NbTiN superconducting microwave resonator. The resonator design allows for readout close to equilibrium. By placing the Al film on a membrane, we enhance the fluctuation level and separate quasiparticle effects from phonon effects. When lowering the temperature, the recombination time saturates and the fluctuation level reduces by a factor $\sim 100$. From this we deduce that the number of free quasiparticles is still thermal. Therefore, the theoretical, inverse relation between the quasiparticle number and recombination time is invalid in this experiment. This is consistent with quasiparticle trapping, where on-trap recombination limits the observed quasiparticle lifetime.

Manipulation of spin-polarized electronic states of two-dimensional (2D) materials under ambient conditions is necessary for developing new quantum devices with small physical dimensions. Here, we explore spin-dependent electronic structures of ultra-thin films of recently introduced 2D synthetic materials $M{\mathrm{Si}}_2{Z}_4$ $(M=\mathrm{Mo} \mathrm{or} \mathrm{W} \mathrm{and} Z=\mathrm{N} \mathrm{or} \mathrm{As})$ using first-principles modeling. Stacking of $M{\mathrm{Si}}_2{Z}_4$ monolayers is found to generate dynamically stable bilayer and bulk materials with thickness-dependent properties. When spin-orbit coupling (SOC) is included in the computations, $M{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayers display indirect band gaps and large spin-split states at the $K$ and $K^{\prime }$ symmetry points at the corners of the Brillouin zone with nearly 100% spin polarization. The spins are locked in opposite directions along an out-of-the-plane direction at $K$ and $K^{\prime }$, leading to spin-valley coupling effects. As expected, spin polarization is absent in the pristine bilayers due to the presence of inversion symmetry, but it can be induced via an external out-of-plane electric field much like the case of $\mathrm{Mo}\left(\mathrm{W}\right){\mathrm{S}}_2$ bilayers. A transition from an indirect to a direct band gap can be driven by replacing N by As in $M{\mathrm{Si}}_2{(\mathrm{N},\mathrm{As})}_4$ monolayers. Our study indicates that the $M{\mathrm{Si}}_2{Z}_4$ materials can provide a viable alternative to the ${\mathrm{MoS}}_2$ class of 2D materials for valleytronics and optoelectronics applications.

We analytically calculate the electronic friction tensor for a molecule near a metal surface in the case where the electronic Hamiltonian is complex valued, e.g., the case where there is spin-orbit coupling and/or an external magnetic field. In such a case, even at equilibrium, we show that the friction tensor is not symmetric. Instead, the tensor is the real-valued sum of one positive definite tensor (corresponding to dissipation) plus one antisymmetric tensor (corresponding to a Berry pseudomagnetic force). Moreover, we find that this Berry force can be much larger than the dissipational force, suggesting the possibility of strongly spin-polarized chemicurrents or strongly spin-dependent rate constants for systems with spin-orbit coupling.

We present a numerical method for the evaluation of the mass gap and the low-lying energy gaps of a large family of free-fermionic and free-parafermionic quantum chains. The method is suitable for some generalizations of the quantum Ising and XY models with multispin interactions. We illustrate the method by considering the Ising quantum chains with uniform and random coupling constants. The mass gaps of these quantum chains are obtained from the largest root of a characteristic polynomial. We also show that the Laguerre bound, for the largest root of a polynomial, used as an initial guess for the largest root in the method, gives us estimates for the mass gaps sharing the same leading finite-size behavior as the exact results. This opens an interesting possibility of obtaining precise critical properties very efficiently, which we explore by studying the critical point and the paramagnetic Griffiths phase of the quantum Ising chain with random couplings. In this last phase, we obtain the effective dynamical critical exponent as a function of the distance to criticality. Finally, we compare the mass gap estimates derived from the Laguerre bound and the strong-disorder renormalization-group method. Both estimates require comparable computational efforts, with the former having the advantage of being more accurate and also being applicable away from infinite-randomness fixed points. We believe this method is a relevant tool for tackling critical quantum chains with and without quenched disorder.

The trigonal compound ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ was recently discovered to host Dirac surface states within the bulk band gap and orders antiferromagnetically below the Néel temperature $T_{\mathrm{N}}=23.5\left(2\right)$ K from our neutron-diffraction measurements. Here the magnetic ground state of single-crystal ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ and the evolution of its properties versus temperature $T$ and applied magnetic field $H$ are reported. Included are the zero-field single-crystal neutron diffraction measurements versus $T$, magnetization $M(H,T)$, magnetic susceptibility $\chi (H,T)=M(H,T)/H$, heat capacity $C_{\mathrm{p}}(H,T)$, and electrical resistivity $\rho (H,T)$ measurements. The neutron-diffraction and $\chi \left(T\right)$ measurements both indicate a collinear A-type antiferromagnetic (AFM) structure below $T_{\mathrm{N}}$, where the ${\mathrm{Eu}}^{2+}$ spins $S=7/2$ in a triangular $ab$-plane layer (hexagonal unit cell) are aligned ferromagnetically in the $ab$ plane, whereas the spins in adjacent Eu planes along the $c$ axis are aligned antiferromagnetically. The $\chi (H_{ab},T)$ and $\chi (H_c,T)$ data together indicate a smooth crossover between the collinear AFM alignment and an unknown magnetic structure at $H\approx 0.12$ T. Dynamic spin fluctuations up to 60 K are evident in the $\chi \left(T\right)$, $C_{\mathrm{p}}\left(T\right)$ and $\rho (H,T)$ measurements, a temperature that is more than twice $T_{\mathrm{N}}$. The $\rho (H,T)$ is consistent with a low-carrier-density metal with strong magnetic scattering and does not reflect a contribution of the topological state of the material as reported earlier by ARPES measurements. This observation is consistent with previous ones for other topological insulators where the chemical potential is above the Dirac point so that ARPES readily detects the surface states, whereas resistivity measurements do not. The magnetic phase diagrams for both $H\parallel c$ and $H\parallel ab$ in the $H\text{−}T$ plane are constructed from the $T_{\mathrm{N}}\left(H\right)$, $\chi (H,T)$, $C_{\mathrm{p}}(H,T)$, and $\rho (H,T)$ data.

Using first-principles calculations, we investigated the magnetic, electronic, and structural properties of oxygen-substituted ${\mathrm{NaMnF}}_3$ $\left({\mathrm{NaMnF}}_{1.5}{\mathrm{O}}_{1.5}\right)$ with in-plane biaxial strain. For simplicity, a structure containing an oxygen octahedron is used to explore the underlying physical mechanism. We found that the oxygen octahedron induces a transition from an insulating antiferromagnet to a high-temperature half-metallic ferromagnet. More importantly, the Curie temperature can be significantly enhanced and even might reach room temperature by applying tensile strain. The changing trends of exchange coupling constants with the increasing biaxial tensile strain can be attributed to the cooperative effects of Jahn-Teller distortion and rotation distortion. It is expected that these findings can enrich the versatility of ${\mathrm{NaMnF}}_3$ and make it a promising candidate for spintronic applications.

The mechanocaloric effect is the temperature change of a material upon application or removal of an external stress. Beyond its fundamental interest, this caloric response represents a promising and ecofriendly alternative to current cooling technologies. To obtain large mechanocaloric effects, we need materials whose elastic properties (e.g., strain, elastic compliance) are strongly temperature dependent. This is the case of ferroelectric perovskite oxides, where the development of the spontaneous electric polarization is accompanied by significant strains and lattice softening. Thus, in this work we study the mechanocaloric properties of model ferroelectric ${\mathrm{PbTiO}}_3$ by means of predictive atomistic (“second-principles”) simulations and a perturbative formalism introduced here. Our calculations reveal relatively large effects (up to $-4$ K for relatively small applied compressions of $-0.1$ GPa) and several striking features. In particular, we find that the mechanocaloric response is highly anisotropic in the ferroelectric phase, as it can be either conventional (temperature increases upon compression) or inverse (temperature decreases) depending on the direction of the applied stress. We discuss and explain these surprising results, which compare well with existing experimental information. Our analysis suggests that the coexistence of conventional and inverse mechanocaloric responses is probably common among ferroelectrics and materials displaying a negative thermal expansion.

The century-old Vegard‘s law has been remarkably accurate in describing the evolution of the lattice parameters of almost all solid solutions. Contractions or expansions of lattice parameters of such systems depend on the size of the guest atom being smaller or larger than the host atom it replaces to form the solid solution. This has given rise to the concept of “chemical pressure” in analogy to the physical pressure. We have investigated using EXAFS the evolution of the local structure in terms of atom-pair distances extending up to the third-nearest neighbors in the family of compounds, $\mathrm{Zn}{\mathrm{Se}}_x{\mathrm{S}}_{1-x}$ as an example of an anionic solid solution, in contrast to all previous studies focusing on cationic solid solutions. Our results establish several common features between these two types of solid solutions, while strongly suggesting that the concept of a chemical pressure is inaccurate and misleading. Most interestingly, we also find a qualitative difference between the cationic solid solutions, reported earlier, and the anionic solid solution.