The Jahn-Teller distortion and the resulting orbital order usually cause some fascinating correlated electronic behaviors, and generally lead to antiferromagnetism in perovskite bulks. Here we demonstrate that the Jahn-Teller distortion present in the perovskite fluoride ${\mathrm{KCrF}}_3$ bulk can be retained to the two-dimensional limit, resulting in a staggered orbital order and ferromagnetism in the perovskite monolayer. Octahedral tilt and rotation distortion also appear in the ground-state structure of the perovskite monolayer, which have minor effects on the electronic and magnetic properties with respect to the Jahn-Teller distortion. In addition, in the prototype phase without structural distortion, the partial occupation of the $e_g$ orbitals leads to a ferromagnetic metallic state. This work facilitates the design of two-dimensional ferromagnets and functional properties based on Jahn-Teller distortion and orbital order.

Motivated by the recent discovery of a semiclassical nematic spin liquid state in a Heisenberg kagomé antiferromagnet ${\mathrm{Li}}_9{\mathrm{Fe}}_3{\left({\mathrm{P}}_2{\mathrm{O}}_7\right)}_3{\left({\mathrm{PO}}_4\right)}_2$ (LFPO) with $S=5/2$ [Kermarrec $etal$. Phys. Rev. Lett. 127, 157202 (2021)], we now investigate the impact of spin quantum number $S$ on the ground-state properties by studying the isostructural kagomé magnet ${\mathrm{Li}}_9{\mathrm{Cr}}_3{\left({\mathrm{P}}_2{\mathrm{O}}_7\right)}_3{\left({\mathrm{PO}}_4\right)}_2$ (LCPO) with active $t_{2g}$ orbitals and $S=3/2$. Thermodynamic measurements reveal that the ground-state properties of LCPO is dominated by the ferromagnetic interactions with a mean-field temperature $\mathrm{\Theta }\sim 3\text{K}(J<1\text{K})$ and the ordering temperature, $T_c$ ∼2.7 K, and the size of the ordered moment ∼$1.05\pm 0.25{\mu }_{\mathrm{B}}$ is significantly reduced from that of a fully ordered moment. The ab initio electronic structure calculations nicely corroborate the thermodynamic results and suggest the presence of additional in- and out-of-plane further-neighbor antiferromagnetic couplings, although significantly weaker in comparison to the dominant first-nearest-neighbor ferromagnetic coupling. The spin-lattice relaxation rate measured with fields larger than the saturation field shows a magnetic field induced gap $(\mathrm{\Delta }\propto$ B) in the excitation spectrum, and in $B\to 0$ limit the gap has a finite intercept ∼3 K, equivalent to the mean-field scale. We interpret the origin of this gap is associated with the magnetic interactions inherent to the material. With our experimental results, we establish the stabilization of a ferromagnetic like ground state and the persistence of magnetic correlations above the ordering temperature in LCPO.

We study the characterization of the non-Hermitian skin effect (NHSE) in non-Hermitian systems with on-site disorders. We extend the applications of generalized Brillouin zone (GBZ) theory to these systems. By proposing a modified GBZ theory, we give a faithful description of the NHSE. For applications, we obtain a unified $\beta$ for system with long-range hopping and explain the GBZ irrelevance of the magnetic suppression of the NHSE in the previous study.

A scheme is presented to calculate on a first-principles level the spin-lattice coupling (SLC) parameters needed to perform combined molecular-spin dynamics (MSD) simulations. By treating changes to the spin configuration and atomic positions on the same level, closed expressions for the atomic SLC parameters could be derived in a coherent way up to any order. The properties of the SLC parameters are discussed considering separately the symmetric and antisymmetric parts of the SLC tensor. The changes due to atomic displacements of the spin-spin exchange coupling (SSC) parameters estimated using the SLC parameters are compared with the SSC parameters calculated for an embedded cluster with the central atom displaced, demonstrating good agreement of these results. Moreover, this allows to study the impact of different SLC contributions, linear and quadratic with respect to displacements, on the properties of the modified SSC parameters. In addition, we represent an approach to calculate the site-diagonal SLC parameters characterizing local magnetic anisotropy induced by a lattice distortion, which is a counterpart of the approach based on magnetic torque used for the investigations of magneto-crystalline anisotropy (MCA) as well as for calculations of the MCA constants. In particular, the dependence of the induced magnetic torque on different types of atomic displacements is analyzed.

Quantum simulations of the Bose-Hubbard model (BHM) at commensurate filling can follow spreading of correlations after a sudden quench for times long enough to estimate their propagation velocities. In this work we perform tensor network simulation of the quantum Kibble-Zurek (KZ) ramp from the Mott towards the superfluid phase in the square lattice BHM and demonstrate that even relatively short ramp/quench times allow one to test the power laws predicted by the KZ mechanism (KZM). They can be verified for the correlation length and the excitation energy but the most reliable test is based on the KZM scaling hypothesis for the single-particle correlation function: scaled correlation functions for different quench times evaluated at the same scaled time collapse to the same scaling function of the scaled distance. The scaling of the space and time variables is done according to the KZ power laws.

Symmetry is usually defined via transformations described by a (higher) group. But a symmetry really corresponds to an algebra of local symmetric operators, which directly constrains the properties of the system. In this paper, we point out that the algebra of local symmetric operators contains a special class of extended operators—transparent patch operators, which reveal the selection sectors and hence the corresponding symmetry. The algebra of those transparent patch operators in $n$-dimensional space gives rise to a nondegenerate braided fusion $n$-category, which happens to describe a topological order in one higher dimension (for finite symmetry). Such a holographic theory not only describes (higher) symmetries, it also describes anomalous (higher) symmetries, noninvertible (higher) symmetries (also known as algebraic higher symmetries), and noninvertible gravitational anomalies. Thus, topological order in one higher dimension, replacing group, provides a unified and systematic description of the above generalized symmetries. This is referred to as symmetry/topological-order (Symm/TO) correspondence. Our approach also leads to a derivation of topological holographic principle: boundary uniquely determines the bulk, or more precisely, the algebra of local boundary operators uniquely determines the bulk topological order. As an application of the Symm/TO correspondence, we show the equivalence between ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry with mixed anomaly and ${\mathbb{Z}}_4$ symmetry, as well as between many other symmetries, in 1-dimensional space.

Despite the chiral topological semimetal CoSi being known as bulk diamagnetic, it shows unusual surface ferromagnetism of debatable origin. The ferromagnetic ordering has been attributed to the distorted bonds, the superlattice of ordered vacancies, or even to topological surface textures due to the spin polarization in the neighboring Fermi arcs. We experimentally compare magnetization reversal curves for initially oxidized CoSi single crystals and cleaved samples with a fresh, oxide-free surface. While the oxidized CoSi samples do not show sizable ferromagnetism, the fresh CoSi surface gives a strong ferromagnetic response, which is accompanied by the pronounced modulation of the angle dependence of magnetization, as it can be expected for easy and hard axes in a ferromagnet. In addition to the first order reversal curves analysis, this observation allows us to distinguish between different mechanisms of the ferromagnetic ordering in CoSi single crystals. We conclude that the surface states-induced Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between distorted bonds near the sample surface is responsible for the strong ferromagnetic multidomain behavior for freshly cleaved samples.

As a fundamental physical quantity of thermal phonons, temporal coherence participates in a broad range of thermal and phononic processes, while a clear methodology for the measurement of phonon coherence is still lacking. In this paper, we derive a theoretical model for the experimental exploration of phonon coherence based on spectroscopy, which is then validated by comparing Brillouin light scattering data and direct molecular dynamic simulations of confined modes in nanostructures. The proposed model highlights that confined modes exhibit a pronounced wavelike behavior characterized by a higher ratio of coherence time to lifetime. The dependence of phonon coherence on system size is also demonstrated from the spectroscopy data. The proposed theory allows for reassessing the data of conventional spectroscopy to yield coherence times, which are essential for the understanding and the estimation of phonon characteristics and heat transport in solids in general.

We report that the layered semiconductors ${\mathrm{CeTe}}_{2-x}{\mathrm{Sb}}_x$ with low charge carrier density $\le 2\times {10}^{18}{\mathrm{cm}}^{-3}$ exhibit giant negative magnetoresistance exceeding two digits at 2 K. The drastic decrease in electrical resistivity is induced by the transition from the antiferromagnetic to the forced-ferromagnetic state with a wide range of electrical resistivities [approximately 20 to $(3\times {10}^7)\mathrm{\Omega }\mathrm{cm}$] in zero magnetic field. In contrast, the negative magnetoresistance almost disappears in the low-resistivity sample with much higher carrier density ($\sim 2.4\times {10}^{20}{\mathrm{cm}}^{-3}$). These results indicate that the observed giant magnetoresistance is driven by a substantial change in the mobility of the magnetic polarons. In ${\mathrm{CeTe}}_{2-x}{\mathrm{Sb}}_x$, the charge carrier density is variable by several digits by the substitution of Te with Sb. Consequently, the giant negative magnetoresistance is preserved within a wide range of electrical resistivity as long as the magnetic polaron density is low enough that the magnetic polarons rarely overlap each other. The magnetic polaron diameter is discussed using the samples' carrier density.

We classify the dissipative topological insulators (TIs) with edge dark states (EDSs) by using the 38-fold way of non-Hermitian systems in this paper. The dissipative dynamics of these quadratic open fermionic systems is captured by a non-Hermitian single-particle matrix which contains both the internal dynamics and the dissipation, referred to as the damping matrix $X$. The dark states in these systems are the eigenmodes of $X$ in which the eigenvalues' imaginary part vanishes. However, there is a constraint on $X$, namely, that the modes in which the eigenvalues' imaginary parts are positive are forbidden. In other words, the imaginary line-gap of $X$ is ill-defined, so the topological band theory classifying the dark states cannot be applied to $X$. To reveal the topological protection of the EDSs, we propose the double damping matrix $\tilde{X}=\text{diag}(X,X^{*})$, where the imaginary line-gap is well-defined. Thus, the 38-fold way can be applied to $\tilde{X}$, and the topological protection of the EDS is uncovered. Different from previous studies of EDSs in purely dissipative dynamics, the EDSs in the dissipative TIs are robust against the inclusion of Hamiltonians. Furthermore, the topological classification of $\tilde{X}$ not only reflects the topological protection of EDSs in dissipative TIs but also provides a paradigm to predict the appearance of EDSs in other open free fermionic systems.

Studies of structure-property relationships in spintronics are essential for the design of materials that can fill specific roles in devices. For example, materials with low symmetry allow unconventional configurations of charge-to-spin conversion, which can be used to generate efficient spin-orbit torques. Here, we explore the relationship between crystal symmetry and geometry of the Rashba-Edelstein effect (REE) that causes spin accumulation in response to an applied electric current. Based on a symmetry analysis performed for 230 crystallographic space groups, we identify classes of materials that can host conventional or collinear REE. Although transverse spin accumulation is commonly associated with the so-called “Rashba materials”, we show that the presence of specific spin texture does not easily translate to the configuration of REE. More specifically, bulk crystals may simultaneously host different types of spin-orbit fields, depending on the crystallographic point group and the symmetry of the specific $k$ vector, which, averaged over the Brillouin zone, determine the direction and magnitude of the induced spin accumulation. To explore the connection between crystal symmetry, spin texture, and the magnitude of REE, we perform first-principles calculations for representative materials with different symmetries. We believe that our results will be helpful for further computational and experimental studies, as well as the design of spintronics devices.

Bound states in the continuum (BICs) are peculiar discrete states embedded in the continuous spectrum. It has been reported that BICs still exist and exhibit new features when parity-time (PT)-symmetric perturbation is applied to the system supporting BICs. Here, we further study the PT unbalanced system, especially a purely passive one, and find that a BIC with a divergent radiative Q factor also exists when differential loss is introduced. Meanwhile, merging of two BICs is observed when varying the strength of the differential loss. Different from the ordinary BIC, this loss-induced BIC can be excited by an external plane wave, although it will not radiate to infinity. On the contrary, another mode at the same frequency but opposite wavevector can radiate, but cannot be excited by an external plane wave, manifesting the asymmetric behavior in momentum space. As the gain is introduced, the net loss can even be compensated precisely, giving rise to a PT-BICs. The Q-factor divergence rate of the PT-BICs is anisotropic in the parameter space. These results can be extended to other systems favorable for experimental implementation and may facilitate applications in light trapping and lasing.

The optical injection of charge and spin currents is investigated in ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ semiconductors as a function of Sn content. These emerging silicon-compatible materials enable the modulation of these processes across the entire mid-infrared range. Under the independent-particle approximation, the one- and two-photon interband absorption processes are elucidated, and the evolution of the coherent control is discussed for three different polarization configurations. To evaluate the contribution of high-energy transitions, a full-zone 30-band $\mathrm{k}·\mathrm{p}$ is employed in the calculations. It was found that, besides the anticipated narrowing of the direct gap and the associated shift of the absorption to longer wavelengths, incorporating Sn in Ge also increases the one-photon degree of spin polarization (DSP) at the $E_1$ resonance. Moreover, as the Sn content increases, the magnitude of the response tensors near the band edge exhibits an exponential enhancement. This behavior can be attributed to the Sn-incorporation-induced decrease in the carrier effective masses. This trend appears to hold also at the $E_1$ resonance for pure spin current injection, at least at low Sn compositions. The two-photon DSP at the band edge exceeds the value in Ge to reach 60% at a Sn content above 14%. These results demonstrate that ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ semiconductors can be exploited to achieve the quantum coherent manipulation in the molecular fingerprint region relevant to quantum sensing.

III-nitride materials including AlN, GaN, and InN are promising for semiconductor industry applications; however, the material growth process gives rise to high defect densities in the epilayer, which can affect the device performance. A systemical understanding of the defect physics is necessary for realistic applications. Among the defects in III-nitride materials, the stacking mismatch boundary (SMB) is a kind of extended defect generated due to the presence of a stacking fault, whose structure-function relationship is still not well understood. Here, we report on a first-principles investigation of the growth and electronic properties of the SMB in III-nitride materials. Based on the wurtzite crystal symmetry, it is found that the SMBs can be categorized into three basic types, depending on the terrace edge of the coalescent normal and stacking-fault regions on the (0001) surface, and the corresponding edge type is controllable by varying the chemical potential and initial nucleation size during the material growth process. Additionally, it is revealed that SMBs produce in-gap states in III-nitride materials with various properties, including itinerant magnetism with high Curie temperature and optical transition correlated with the experimentally observed sub-band-gap spectrum. It is worth noting that one type of SMB is a possible source of the yellow luminescence that is widely observed in GaN. Our findings add comprehensive insight into the SMB in III-nitride materials; the unique growth controllable property of an SMB is also a possible routine to broaden the applications of III-nitride materials.

Preparing finite-temperature states in quantum simulators of spin systems, such as trapped ions or Rydberg atoms in optical tweezers, is challenging due to their almost perfect isolation from the environment. Here, we show how finite-temperature observables can be obtained with an algorithm motivated from the Jarzynski equality and equivalent to the one in Lu et al., PRX Quantum 2, 020321 (2021). It consists of classical importance sampling of initial states and a measurement of the Loschmidt echo with a quantum simulator. We use the method as a quantum-inspired classical algorithm and simulate the protocol with matrix product states to analyze the requirements on a quantum simulator. This way, we show that a finite-temperature phase transition in the long-range transverse-field Ising model can be characterized in trapped ion quantum simulators. We propose a concrete measurement protocol for the Loschmidt echo and discuss the influence of measurement noise, dephasing, as well as state preparation and measurement errors. We argue that the algorithm is robust against those imperfections under realistic conditions.

A number of polycrystalline films feature distinct sharp peaks in the angular variations of their in-plane major-loop remanent magnetization and coercivity, centered ${90}^{\circ }$ off the easy-axis position, a property referred to as hard-axis collapse. In such systems a striking phenomenon, recoil-curve overshoot (RCO), recoil magnetization branches that lie outside the major loop, was recently reported, resulting in a significantly greater recoil loop area compared with that of the respective major loop. In the present work pieces of polycrystalline magnetron-sputtered Co films were subjected to ${\mathrm{Ne}}^{+}$ bombardment in vacuum at different ion fluences in the presence of magnetic field $H_{ib}$ applied along different in-plane directions. Ion irradiation at a certain fluence range results in films with easy-magnetization directions parallel to that of ${\mathbf{H}}_{\mathbf{ib}}$. Moreover, the same holds for posterior sequential irradiations. The films' hard-magnetization axes are always collapsed, accompanied by significant RCO as well. The analysis and interpretation of our data strongly indicate that both hard-axis collapse and RCO result predominantly from a domain splitting when the measurement magnetic field is nearly perpendicular to the grain's easy axis during demagnetization.

We report the growth and characterization of ${\mathrm{MnPd}}_5\mathrm{P}$, a rare-earth-free ferromagnet, with $T_C\approx 295\mathrm{K}$ and planar anisotropy, and conduct a substitutional study with its antiferromagnetic analog ${\mathrm{MnPt}}_5\mathrm{P}$. All compounds in the family adopt the layered anti-${\mathrm{CeCoIn}}_5$-type structure with the space group $P4/mmm$, and EDS and x-ray diffraction results indicate that ${\mathrm{MnPt}}_5\mathrm{P}$ and ${\mathrm{MnPd}}_5\mathrm{P}$ form a complete solid solution. Based on measurements of the temperature- and field-dependent magnetization and resistance, we construct a temperature-composition ($T\text{−}x$) phase diagram for $\mathrm{Mn}{\left({\mathrm{Pt}}_{1-x}{\mathrm{Pd}}_x\right)}_5\mathrm{P}$ and demonstrate that the essentially antiferromagnetic order found in ${\mathrm{MnPt}}_5\mathrm{P}$ is extraordinarily sensitive to Pd substitution. At low Pd fractions ($x<0.010$), the single antiferromagnetic-like transition in pure ${\mathrm{MnPt}}_5\mathrm{P}$ splits into a higher-temperature ferromagnetic transition followed first, upon cooling, by a lower temperature ferromagnetic to antiferromagnetic transition and then by a re-entrant antiferromagnetic to ferromagnetic transition at even lower temperatures. The antiferromagnetic region makes up a bubble phase that persists up to $x\approx 0.008$–0.009 for $T\approx 150\mathrm{K}$, with all samples $0\le x<0.008$ recovering their initial ferromagnetic state upon further cooling to base temperature. Once $x>0.010$, $\mathrm{Mn}{\left({\mathrm{Pt}}_{1-x}{\mathrm{Pd}}_x\right)}_5\mathrm{P}$ undergoes a only single transition into the ferromagnetic phase. The Curie temperature initially increases rapidly with $x$, rising from $T_C\approx 197\mathrm{K}$ at $x$ = 0.013 to a maximum of $T_C\approx 312\mathrm{K}$ for $x\approx 0.62$, and then falling back to $T_C\approx 295\mathrm{K}$ for pure ${\mathrm{MnPd}}_5\mathrm{P}$ $(x=1.00)$. Given that Pt and Pd are isoelectronic, this work raises questions as to the origin of the extreme sensitivity of the magnetic ground state and the nature of the re-entrant ferromagnetism at dilute Pd levels.