Entanglement of spin and orbital degrees of freedom, via relativistic spin-orbit coupling, in $4d$ transition metal oxides can give rise to a variety of unique quantum phases. A previous study of mixed $3d\text{−}4d$ quasi-1D spin-chain oxide ${\mathrm{Sr}}_3{\mathrm{NiRhO}}_6$ using the magnetization measurements by Mohapatra et al. [Phys. Rev. B 75, 214422 (2007)] revealed a partially disordered antiferromagnetic structure below 50 K. We here report the magnetic ground state and spin-wave excitations in ${\mathrm{Sr}}_3{\mathrm{NiRhO}}_6$ using muon spin rotation and relaxation $\left(\mu \mathrm{SR}\right)$, and neutron (elastic and inelastic) scattering techniques. Our neutron diffraction study reveals that in the magnetic structure of ${\mathrm{Sr}}_3{\mathrm{NiRhO}}_6$ ${\mathrm{Rh}}^{4+}$ and ${\mathrm{Ni}}^{2+}$ spins are aligned ferromagnetically in a spin chain, with moments along the crystallographic $c$ axis. However, spin chains are coupled antiferromanetically in the $ab$ plane. $\mu \mathrm{SR}$ reveals the presence of oscillations in the asymmetry-time spectra below 50 K, supporting the long-range magnetically ordered ground state. Our inelastic neutron scattering study reveals gapped quasi-1D magnetic excitations with a large ratio of gap to exchange interaction. The observed spin-wave spectrum could be well fitted with a ferromagnetic isotropic exchange model (with $J=3.7$ meV) and single ion anisotropy $(D=10 \mathrm{meV})$ on the ${\mathrm{Ni}}^{2+}$ site. The magnetic excitations survive up to 85 K, well above the magnetic ordering temperature of $\sim 50$ K, also indicating a quasi-1D nature of the magnetic interactions in ${\mathrm{Sr}}_3{\mathrm{NiRhO}}_6$.

We study the phase diagram of the antiferromagnetic $J_1\text{−}{J}_2$ Heisenberg model on the pyrochlore lattice with $S=1$ spins at zero and finite temperatures. We use a combination of complementary state-of-the-art quantum many-body approaches such as density matrix renormalization group (DMRG), density-matrix purification, and pseudo-Majorana functional renormalization group (PMFRG). We present an efficient approach to preserve the applicability of the PMFRG for spin-1 systems at finite temperatures despite the inevitable presence of unphysical spin states. The good performance of our methods is first demonstrated for the nearest-neighbor pyrochlore Heisenberg model where the finite-temperature behavior of the specific heat and uniform susceptibility show excellent agreement within PMFRG and density-matrix purification. Including an antiferromagnetic second-neighbor coupling we find that the nonmagnetic ground-state phase of the nearest-neighbor model extents up to $J_2/J_1\sim 0.02$ within DMRG, beyond which magnetic $\mathit{k}=0$ long-range order sets in. Our PMFRG calculations find the phase transition in a similar regime $J_2/J_1\sim 0.035\left(8\right)$ which, together with the DMRG result, provides a strong argument for the existence of a small but finite nonmagnetic ground-state phase in the spin-1 pyrochlore Heisenberg model. We also discuss the origin of discrepancies between different versions of the functional renormalization group concerning the location of this phase transition.

We study the effect of diffuse surface scattering on the edge modes in two-dimensional chiral superconductors with time-reversal symmetry-breaking Cooper pairs, each carrying angular momentum $m\hslash$ ($m=1,2,3,\cdots$). To elucidate the diffuse scattering effect, we formulate the inverse lifetime $\mathrm{\Gamma }=\hslash /\tau$ corresponding to the broadening of the surface density of states (SDOS) for the edge mode. This derivation uses random $S$-matrix theory, which allows us to describe the surface effect in a unified way from the specular to the diffuse limit within the quasiclassical theory framework of superconductivity. We find that $\mathrm{\Gamma }$ in the chiral states with $m\ge 2$ is larger than that in the chiral $p$-wave state ($m=1$) because of the multiple edge mode branches in former superconducting states (the number of which equals $m$). Diffuse scattering between the different branches causes a significant broadening of SDOS. In contrast, the edge mode in the chiral $p$-wave state is robust to diffuse scattering because only a single edge mode branch exists. We also discuss SDOS at the diffuse limit, where the description in terms of $\mathrm{\Gamma }$ is not useful. The diffuse scattering effect on SDOS can be understood qualitatively in terms of destructive interference analogous to that caused by impurity scattering in unconventional superconductors.

Time-dependent spin-density functional theory (TD-SDFT) allows the theoretical description of spin and magnetization dynamics in electronic systems from first principles. TD-SDFT accounts for electronic interaction effects via exchange-correlation scalar potentials and magnetic fields, which have to be suitably approximated in practice. We consider here an approach that was recently proposed for the ground state by Sharma et al. [J. Chem. Theory Comput. 14, 1247 (2018)], which enforces the so-called “source-free” condition by eliminating monopole contributions contained within a given approximate exchange-correlation magnetic field. This procedure was shown to give good results for the structure of magnetic materials. We analyze the source-free construction in the linear-response regime, considering spin waves in paramagnetic and ferromagnetic electron gases. We observe a violation of Larmor's theorem in the paramagnetic case and a wrong long-wavelength behavior of ferromagnetic magnons. The practical implications of these violations are discussed.

High-quality single-crystalline hexagonal Ruthenium thin films were epitaxially grown on (110)-oriented ${\mathrm{LaAlO}}_3$ substrates. The hexagonal Ruthenium is found to be a semimetal demonstrating nonsaturating and near-quadratic field-dependent magnetoresistance (MR). Remarkably, while exhibiting a linear Hall effect at temperatures above 50 K, the Hall resistivity of hexagonal Ruthenium undergoes an unexpectedly sign reversal at magnetic field strengths exceeding 10 T, specifically within the low-temperatures range of 2–30 K. First-principles calculations well reproduce these characteristics, including both the nonsaturating MR and the sign-reversal Hall effect. Based on the two-band model and semiclassical equation of motion, we found a general principle that at low temperatures or in high fields, the sign of Hall effect is governed by charge carriers with higher density, whereas at higher temperatures or in lower fields, it is dominated by carriers with higher mobility. Consequently, materials where higher-density charge carriers possess lower mobility are expected to exhibit a sign reversal in Hall resistivity. This framework offers fresh insights and expands our understanding on the Hall effect of nonmagnetic semimetals.

In response to the growing demand for efficient and cost-effective optically active materials for magneto-optoelectronic devices, we investigated the electronic structure and optical properties of La-substituted ${\mathrm{Ba}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{Fe}}_{12}{\mathrm{O}}_{19}$ (BLM) $M$-type hexaferrite, a $3d$ magnetic transition metal oxide with a high absorption coefficient and strong perpendicular magnetic anisotropy (PMA). Theoretical calculations using generalized gradient approximation (GGA)$+\text{U}$ revealed localized charge confinement at $3d_{z^2}$ of the $\mathrm{Fe}\left(2a\right)$ octahedral sites which also leads to the conversion of ${\mathrm{Fe}}^{3+}$ to ${\mathrm{Fe}}^{2+}$. The optical calculations revealed optical anisotropy where light polarization along ${\alpha }_{xx}\left(\omega \right)(\vec{E}\parallel a)$ is observed to be greater than that for light polarization ${\alpha }_{zz}\left(\omega \right)(\vec{E}\parallel c)$ and a high optical absorption coefficient. The presence of optical absorption in the low-energy range (0.0–2.0 eV) in BLM is likely due to a $3d\text{−}3d$ optical transition, attributed to ${\mathrm{Fe}}^{2+}$ at $\mathrm{Fe}\left(2a\right)$ octahedral site. Furthermore, BLM thin films are fabricated on n-type Si and quartz substrates to study magnetic and photoresponse characteristics. It is revealed that PMA with a coercivity ($H_c$) of $2403.86\pm 0.25$ Oe and a magnetic anisotropy constant ($K_{\mathrm{eff}}$) of $2.2\times {10}^6{\mathrm{erg}/\mathrm{cm}}^3$. The $M_r/M_s$ ratio changes significantly with the applied field direction, suggesting weak exchange interaction due to ${\mathrm{Fe}}^{2+}$ ion. BLM also exhibits optically active absorption in the deep UV region, which is consistent with the obtained high absorption coefficient, highlighting its optical responsiveness. To evaluate the photoresponse characteristics, we have fabricated prototype photodetectors using a BLM film, which achieved a responsivity ($R$) of 5 A/W and a detectivity ($D$*) of $5.72\times {10}^{11}$ Jones. These photodetectors exhibited rapid response times, with a rise time of $60\mu \mathrm{s}$ and a fall time of $7\mu \mathrm{s}$ under 405-nm laser illumination. These findings highlight the potential of BLM thin films in magneto-optoelectronic devices, paving the way for innovative applications.

The thermal resistance of a spin-polarized hydrodynamic Dirac plasma in graphene is considered. A mechanism for the coupling of heat and spin flows is discussed, demonstrating that spin diffusion and spin thermocurrent modify viscous dissipation, leading to a significant enhancement of thermal resistance. Practical calculations are then presented for graphene devices in the Corbino geometry.

One of the important and fascinating issues in condensed matter physics is the transport phenomena relevant to massless Dirac fermions (MDFs). The $\alpha \text{−}\text{(}\mathrm{BEDT}\text{−}\mathrm{TTF}{\text{)}}_2{\mathrm{I}}_3$ salt is a two-dimensional organic material with tilted and anisotropic Dirac cones, and these tilt and anisotropy affect transport properties of MDFs. In this paper, by using a minimal Weyl Hamiltonian, we study the transport properties of MDFs in a $\alpha \text{−}\text{(}\mathrm{BEDT}\text{−}\mathrm{TTF}{\text{)}}_2{\mathrm{I}}_3$ based $n\text{−}p\text{−}n$ junction. The Klein tunneling is investigated and discussed in detail and we find that the Klein tunneling depends on valley and it can occur in a wide range of electron incident angles. These properties are in contrast to graphene in which the Klein tunneling is independent of valley and occurs only at zero angle (normal incident). We also show that a $\alpha \text{−}\text{(}\mathrm{BEDT}\text{−}\mathrm{TTF}{\text{)}}_2{\mathrm{I}}_3$-based $n\text{−}p\text{−}n$ junction can work as a perfect valley filter. This valley filter is controllable by tuning the junction parameters such as the Fermi energy and the top gate voltage. The results of this paper provide deeper insight into the transport phenomena relevant to MDFs with tilted and anisotropic cones and also can be useful in valleytronic devices.

${\mathrm{VSi}}_2{\mathrm{N}}_4$ is a newly reported magnetic semiconductor and is promising for being turned into a half metal. For this purpose, we construct a ${\mathrm{VSi}}_2{\mathrm{N}}_4$/${\mathrm{In}}_2{\mathrm{Te}}_3$ van der Waals multiferroic heterostructure based on first-principles calculations. It is found that the inversion of ferroelectric polarization of monolayer ${\mathrm{In}}_2{\mathrm{Te}}_3$ can efficiently modulate the electronic states of monolayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$. A phase transition of ${\mathrm{VSi}}_2{\mathrm{N}}_4$ from semiconductor to half metal can be effectually realized, leading to distinct electronic transport properties. Next, we design a magnetic tunnel junction (MTJ) (using the ${\mathrm{VSi}}_2{\mathrm{N}}_4$ monolayer as the tunneling barrier and the multiferroic heterostructure as the electrode) and investigate its transport properties in various magnetic configurations under different polarization directions of the ferroelectric ${\mathrm{In}}_2{\mathrm{Te}}_3$ layer by combining the nonequilibrium Green's function with density functional theory. The results show that the junction exhibits half-metallic transport in the parallel magnetic configuration and near-zero transport in the antiparallel magnetic configuration, resulting in a giant tunneling magnetoresistance ratio of $\sim 1\times {10}^{12}\%$. Moreover, a $\sim 1\times {10}^{17}\%$ tunneling electroresistance ratio is achieved in the parallel magnetic configuration accompanying the polarization reversal. The findings suggest that MTJs based on the ${\mathrm{VSi}}_2{\mathrm{N}}_4$/${\mathrm{In}}_2{\mathrm{Te}}_3$ heterojunction have great potential for applications in multifunctional spintronic devices.

The computation of Casimir forces, in principle, requires knowledge about the permittivity of the materials over a very wide range of frequencies. In particular, the permittivity at the high frequency limit should approach unity for the numerical integration to converge. In practice, however, the permittivities at high frequencies beyond a certain frequency cutoff, especially for many emerging materials, are not yet known experimentally or accounted for in existing models, which presents a challenge for the computation of Casimir forces for these material systems. Here we consider a scheme to overcome such a challenge by artificially adding extra Lorentz-Drude poles at frequencies beyond the frequency cutoff from measurements or existing models. We provide a systematic evaluation of the range of the validity of such a scheme by calculations of Casimir forces between parallel plates. We show that there is a distance scale inversely proportional to the frequency cutoff. The scheme of adding extra poles gives reliable results if the distances between the plates are larger than the distance scale. We validate our results by considering the cases of plasmonic materials and magnetic Weyl semimetals.

Due to the linearly vanishing density of states, emergent massless Dirac quasiparticles resulting from the free fermion motion in a family of two-dimensional half-filled bipartite hyperbolic lattices feature dynamic mass generation through quantum phase transitions only for sufficiently strong finite-range Coulomb repulsion. As such, strong nearest-neighbor Coulomb repulsion $V$ favors the nucleation of a charge-density-wave (CDW) order with a staggered pattern of average fermionic density between two sublattices of bipartite hyperbolic lattices. Considering a collection of spinless fermions (for simplicity), here we show that application of strong external magnetic fields by virtue of producing a finite density of states near the zero energy triggers the condensation of the CDW order even for infinitesimal $V$. The proposed curved space magnetic catalysis mechanism is operative for uniform and inhomogeneous (bell-shaped) magnetic fields. We present scaling of the CDW order with the total flux enclosed by hyperbolic Dirac materials for a wide range of (especially subcritical) $V$.

Transition metal dichalcogenide ${\mathrm{IrTe}}_2$ undergoes structural transitions together with charge ordering due to formation of Ir dimers. Various low temperature phases of ${\mathrm{IrTe}}_2$ have been reported. However, the true ground state still remains controversial. We present three kinds of coherent oscillations in bulk ${\mathrm{IrTe}}_2$ at low temperatures probed by near infrared pump-probe spectroscopy. The temperature dependent evolution suggests that the charge ordered phase with (1/6 0 1/6) modulation vector develops in a bulk at the lowest temperature, which changes to a (1/8 0 1/8) modulation state around 235 K during warming up. The observed phonon spectra should provide a key to confirm the low temperature structures.

Photodoped states in strongly correlated charge-transfer insulators are characterized by $d\text{−}d$ and $d\text{−}p$ interactions and the resulting intertwined dynamics of charge excitations and local multiplets. Here, we use femtosecond x-ray absorption spectroscopy in combination with dynamical mean-field theory to disentangle these contributions in NiO. Upon resonant optical excitation across the charge-transfer gap, the Ni $L_3$ and O $K$ absorption edges red-shift for $>10$ ps, which we associate with photoinduced changes in the screening environment. An additional signature below the Ni $L_3$ edge is identified for $<1$ ps, reflecting a transient nonthermal population of local many-body multiplets. We employ a nonthermal generalization of the multiplet ligand field theory and argue that this feature originates from $d\text{−}d$ transitions. Overall, the photodoped state differs significantly from a chemically doped state. Our results demonstrate that x-ray spectroscopies are helpful in revealing excitation pathways in correlated materials.

In this study, we investigate the construction of efficient model potentials for perovskite heterostructures by transferring relevant dynamic quantities from bulk systems and short-period superlattices. Through first-principles calculations, we provide a comprehensive analysis and comparison of the dynamical properties of different perovskite-based superlattice structures, including ${\left(\mathrm{PbTi}{\mathrm{O}}_3\right)}_n/{\left(\mathrm{SrTi}{\mathrm{O}}_3\right)}_m$ superlattices, ${\left(A\mathrm{Ti}{\mathrm{O}}_3\right)}_n/{\left(A\mathrm{O}\right)}_2$ superlattices, and $A_{n+1}\mathrm{T}{\mathrm{i}}_n{\mathrm{O}}_{3n+1}$ Ruddlesden-Popper phases, where $A$ can be Sr, Ba, or Pb. We demonstrate that as the thickness of these structures increases, the dynamical properties of the perovskite blocks converge rapidly towards their bulk counterparts. We discuss this convergence in terms of phonon density of states in reciprocal space and interatomic force constants in real space. Our findings suggest a straightforward strategy for developing transferable model potentials that accurately capture the dynamics of layered oxide compounds with progressively thicker layers.

We investigate the spin-coherence decay of ${\mathrm{NV}}^{-}$ spins interacting with the strongly coupled bath of nitrogen defects in diamond layers. For thin diamond layers, we demonstrate that the spin-coherence times exceed those of bulk diamond, thus allowing to surpass the limit imposed by high-defect concentrations in bulk. We show that the stretched-exponential parameter for the short-time spin-coherence decay is governed by the hyperfine interaction in the bath, thereby constraining random-noise models. We introduce a method based on the cluster-correlation expansion applied to strongly interacting bath partitions instead of individual spins. Our results facilitate material development for quantum-technology devices.

We predict a general local spin-flip transition mechanism caused by magnetic quantum impurities in (partially) polarized phases of quantum magnets in the absence of conservation laws. This transition arises when a magnon bound-state crosses zero energy as a function of the magnetic field. As an application, we study two-dimensional (2D) van der Waals magnets described by the Kitaev-Heisenberg honeycomb model which applies to the transition metal trihalides ${\mathrm{CrI}}_3$ and $\alpha \text{−}{\mathrm{RuCl}}_3$. We consider the adatom and substitutional impurity positions, and show how spin-flip transitions can be detected in scanning tunneling spectroscopy.

We calculate analytically the charge density ${\rho }_{\text{ind}}\left(\mathit{r}\right)$ at small distances induced by a Coulomb impurity in two-dimensional transition-metal dichalcogenides. The calculations were performed exactly in the coupling constant. We found the first three leading terms of the charge-density asymptotics at small distances. Using this result, we demonstrate that the impurity with charge number $Z=1$ remains subcritical at any value of the coupling constant, whereas impurities with higher charge number could become supercritical at certain coupling constant values. Such a behavior is similar to that in graphene.