We implement and benchmark tensor network algorithms with $SU\left(2\right)$ symmetry for systems in two spatial dimensions and in the thermodynamic limit. Specifically, we implement $SU\left(2\right)$-invariant versions of the infinite projected entangled pair states and infinite projected entangled simplex states methods. Our implementation of $SU\left(2\right)$ symmetry follows the formalism based on fusion trees from Schmoll et al. [Ann. Phys. 419, 168232 (2020)]. In order to assess the utility of implementing $SU\left(2\right)$ symmetry, the algorithms are benchmarked for three models with different local spin: the spin-1 bilinear-biquadratic model on the square lattice, and the kagome Heisenberg antiferromagnets (KHAFs) for spin-$1/2$ and spin-2. We observe that the implementation of $SU\left(2\right)$ symmetry provides better energies in general than nonsymmetric simulations, with smooth scalings with respect to the number of parameters in the ansatz, and with the actual improvement depending on the specifics of the model. In particular, for the spin-2 KHAF model, our $SU\left(2\right)$ simulations are compatible with a quantum spin-liquid ground state.

Higher-order topological insulators (HOTIs) are a newly discovered class of topological insulators which exhibit unconventional bulk-boundary correspondence. Very recently, the concept of HOTIs has been extended to aperiodic quasicrystalline systems, where the topological band theory fails to describe topological phases. More importantly, a novel HOTI phase protected by an eightfold rotational symmetry, not found in crystalline materials, was identified in the Ammann-Beenker tiling quasicrystals. Here we report the discovery of a quasicrystalline HOTI in a dodecagonal quasicrystal. The quasicrystalline HOTI supports twelve in-gap zero-energy modes symmetrically distributed at the corners of a quasicrystal dodecagon. These zero-energy corner modes are protected by a combination of the twelvefold rotational symmetry and mirror symmetry, as well as particle-hole symmetry, which has no crystalline counterpart.

A space-time dependent node separation in Weyl semimetals acts as an axial vector field. Coupled with domain wall motion in magnetic Weyl semimetals, this induces axial electric and magnetic fields localized at the domain wall. We show how these fields can activate the axial (chiral) anomaly and provide a direct experimental signature of it. Specifically, a domain wall provides a spatially dependent Weyl node separation and an axial magnetic field ${\mathit{B}}_5$, and domain wall movement, driven by an external magnetic field, gives the Weyl node separation a time dependence, inducing an axial electric field ${\mathit{E}}_5$. At magnetic fields beyond the Walker breakdown, ${\mathit{E}}_5·{\mathit{B}}_5$ becomes nonzero and activates the axial anomaly that induces a finite axial charge density—imbalance in the number of left- and right-handed fermions—moving with the domain wall. This axial density in turn produces, via the chiral magnetic effect, an oscillating current flowing along the domain wall plane, resulting in a characteristic radiation of electromagnetic waves emanating from the domain wall. A detection of this radiation would constitute a direct measurement of the axial anomaly induced by axial electromagnetic fields.

Organic halide perovskites have attracted much attention due to their potential applications in optoelectronic devices. Since the generally higher flexibility compared to their inorganic counterparts, their structures are prone to be more sensitive toward external effects, where the fundamental understanding of their band structure evolutions is still inconclusive. In this study, different electronic structure evolutions of perovskite single crystals are found via angle-resolved photoelectron spectroscopy: (i) Unchanged top valence band (VB) dispersions under different temperatures can be found in the $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$. (ii) Phase transitions induced the evolution of top VB dispersions, and even a top VB splitting with Rashba effects can be observed in the $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{Br}}_3$. Combined with low-energy electron diffraction, metastable atom electron spectroscopy, and density functional theory calculation, we confirm that different band structure evolutions observed in these two perovskite single crystals originated from the cleaved top surface layers, where the different surface geometries with $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{{\mathrm{H}}_3}^{+}\text{−}\mathrm{I}$ in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ and Pb-Br in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{Br}}_3$ are responsible for finding band dispersion change and appearance of the Rashba-type splitting. Such findings suggest that the top surface layer in organic halide perovskites should be carefully considered to create functional interfaces for developing perovskite devices.

Dipolar-octupolar pyrochlore magnets in a strong external magnet field applied in the [110] direction are known to form a “chain” state, with subextensive degeneracy. Magnetic moments are correlated along one-dimensional chains carrying effective Ising degrees of freedom which are noninteracting on the mean-field level. Here we investigate this phenomenon in detail, including the effects of quantum fluctuations. We identify two distinct types of chain phases, both featuring distinct subextensive, classical ground-state degeneracy. Focusing on one of the two kinds, we discuss lifting of the classical degeneracy by quantum fluctuations. We map out the ground-state phase diagram as a function of the exchange couplings, using linear spin wave theory and real-space perturbation theory. We find a hierarchy of energy scales in the ground-state selection, with the effective dimensionality of the system varying in an intricate way as the hierarchy is descended. We derive an effective two-dimensional anisotropic triangular lattice Ising model with only three free parameters which accounts for the observed behavior. Connecting our results to experiment, they are consistent with the observation of a disordered chain state in ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$. We also show that the presence of two distinct types of chain phases has consequences for the field-induced breakdown of the apparent $U\left(1\right)$ octupolar quantum liquid phase recently observed in ${\mathrm{Ce}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$.

The mechanisms governing the formation of Schottky barriers at graphene/hydrogen-passivated silicon interfaces where the graphene plays the role of a two-dimensional (2D) metal electrode have been investigated by means of x-ray photoemission spectroscopy and density functional theory (DFT) calculations. To control the graphene work function without altering either the structure or the band dispersion of graphene we used a method that consists in depositing small amounts of gold forming clusters on the graphene/hydrogen-passivated silicon system under an ultra-high-vacuum environment. We observe from experimental measurements that the Fermi level is mainly free from pinning at the graphene/hydrogen-silicon interface whereas for a semi-infinite metal on silicon the Fermi level is almost fully pinned. This alleviation of the Fermi level pinning observed with the graphene layer is explained by DFT calculations showing that the graphene and the semiconductor are decoupled and that the metal-induced gap states (MIGS) density at the silicon midgap at the interface is very low ($<5\times {10}^{10}\mathrm{states}/\left(\mathrm{eV}\mathrm{c}{\mathrm{m}}^2\right)$]. The important conclusion that stems from the DFT results analysis is that the low MIGS density at the semiconductor midgap is related to the 2D nature of the graphene layer. More precisely, the MIGS density is low owing to the lack of propagating states perpendicular to the graphene layer. This finding brings important information to understand the mechanisms that govern the formation and the electronic properties of Schottky barriers at 2D-metal/three-dimensional-semiconductor interfaces.

Calorimetric measurements are experimentally realizable methods to assess thermodynamics relations in quantum devices. With this motivation in mind, we consider a resonant level coupled to a Fermion reservoir. We consider a transient process in which the interaction between the level and the reservoir is initially switched on and then switched off again. We find the time dependence of the energy of the reservoir, of the energy of the level, and of the interaction energy between them at weak, intermediate, strong, and ultrastrong coupling. We also determine the statistical distributions of these energies.

We theoretically examine the transport through an Aharonov-Bohm ring with an embedded quantum dot (QD), the so-called QD interferometer, to address two controversial issues regarding the shape of the Coulomb peaks and measurement of the transmission phase shift through a QD. We extend a previous model [B. R. Bulka and P. Stefański, Phys. Rev. Lett. 86, 5128 (2001); W. Hofstetter, J. König, and H. Schoeller, ibid. 87, 156803 (2001)] to consider multiple conduction channels in two external leads, $L$ and $R$. We introduce a parameter $p_{\alpha }$ ($|p_{\alpha }|\le 1$) to characterize a connection between the two arms of the ring through lead $\alpha$ ($=L$, $R$), which is the overlap integral between the conduction modes coupled to the two arms. First, we study the shape of a conductance peak as a function of energy level in the QD, in the absence of electron-electron interaction $U$. We show an asymmetric Fano resonance for $|p_{L,R}|=1$ in the case of single conduction channel in the leads and an almost symmetric Breit-Wigner resonance for $|p_{L,R}|<0.5$ in the case of multiple channels. Second, the Kondo effect is taken into account by the Bethe ansatz exact solution in the presence of $U$. We precisely evaluate the conductance at temperature $T=0$ and show a crossover from an asymmetric Fano-Kondo resonance to the Kondo plateau with changing $p_{L,R}$. Our model is also applicable to the multiterminal geometry of the QD interferometer. We discuss the measurement of the transmission phase shift through the QD in a three-terminal geometry by a “double-slit experiment.” We derive an analytical expression for the relation between the measured value and the intrinsic value of the phase shift.

Charged-defect calculation using a periodic supercell is a significant class of problems in solid state physics. However, the finite supercell size induces an undesirable long-range image charge Coulomb interaction. Although a variety of methods have been proposed to eliminate such image Coulomb interaction, most of the previous schemes are based on a rough approximation of the defect charge screening. In this work, we present a rigorous derivation of the image charge interaction with a defect screening model where the use of a bulk macroscopic dielectric constant can be avoided. We have verified this approach in comparison with a widely used approach for 12 different defects. Our correction scheme offers a much faster convergence concerning the supercell size for cases with considerable image charge interactions. In those cases, we also found that the nonlinear dielectric screening might play an important role. Our proposed defect screening model will also shed new light on understanding the defect screening properties and can be applied to other defect systems.

We present a generalization of the small set of ordered structures (SSOS) method that is especially suitable for high-throughput density-functional theory (DFT) calculations of high-entropy alloys in solid-solution phase. Using equiatomic quinary high-entropy alloys with fcc lattice structure, we show that the solution of the existing SSOS method is not unique. In fact, there are six solutions each containing three special ordered structures (SOS) when the range of atomistic pair correlations is up to second-nearest neighbors (NN). We also demonstrate that the SSOS method can be extended to consider atomic pair correlations up to the third NN. In this case, there are 24 solutions each containing six SOS and a negative weight. By increasing the number of SOS to seven to get rid of the negative weight, we show that there are 444 solutions. We further propose an averaging scheme to deal with the nonuniqueness of the generalized SSOS method. The generalized SSOS method allows fast, accurate calculations of ground-state energies of equiatomic high-entropy alloys, thus ideally suitable for high-throughput DFT calculations.

The influence of Ho substitution for Tm atoms on the magnetic properties and magnetocaloric effect (MCE) of TmGa compound was systematically investigated according to magnetic measurements and neutron powder diffraction (NPD) experiments. The magnetic transitions of ${\mathrm{Tm}}_{1-x}{\mathrm{Ho}}_x\mathrm{Ga}$ compounds show different types by Ho substitution due to the variation of spin and orbital angular momentum quantum number and the complete magnetic diagram of ${\mathrm{Tm}}_{1-x}{\mathrm{Ho}}_x\mathrm{Ga}$ compounds was obtained. The spin reorientation (SR) transition of ${\mathrm{Tm}}_{0.1}{\mathrm{Ho}}_{0.9}\mathrm{Ga}$ compound was directly confirmed by variable-temperature NPD experiments. Results show that the magnetic moment orders along the $c$ axis at the temperatures between $T_{\mathrm{SR}}$ and $T_{\mathrm{C}}$ and it cants away from the $c$ axis towards the $ab$ plane upon cooling below $T_{\mathrm{SR}}$. Furthermore, Ho substitution plays a dominant role in MCE of ${\mathrm{Tm}}_{1-x}{\mathrm{Ho}}_x\mathrm{Ga}$ compounds. When $x=0.15$, the peak value of magnetic entropy change reaches the maximum value of 18.0 J/kg K under field change of 0–2 T. The refrigerant temperature span ($\delta T_{\mathrm{FWHM}}$) and refrigeration capacity of ${\mathrm{Tm}}_{0.85}{\mathrm{Ho}}_{0.15}\mathrm{Ga}$ compound show enhancement of 23.0 and 21.6%, correspondingly, compared with TmGa compound. The giant MCE of ${\mathrm{Tm}}_{0.85}{\mathrm{Ho}}_{0.15}\mathrm{Ga}$ compound results from the optimization of spin and total angular momentum quantum number by Ho substitution.

In-plane magnetization reversal of a permalloy/platinum bilayer was detected using the spin rectification effect. Using a subgigahertz microwave frequency to excite spin-torque ferromagnetic resonance (ST-FMR) in the bilayer induces two discrete DC voltages around an external static magnetic field of 0 mT. These discrete voltages depend on the magnetization directions of the permalloy and enable detection of the in-plane magnetization reversal. The threshold current density for the magnetization reversal is around $10–20\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2$, the same order as for known spin-orbit torque switching with in-plane magnetization materials. The magnitude of the signal is the same or larger than that of the typical ST-FMR signal; that is, detection of magnetization switching is highly sensitive in spite of deviation from the optimal ST-FMR condition. The proposed method is applicable to a simple device structure even for a small ferromagnetic electrode with a width of 100 nm.

Through an analysis of single-crystal neutron diffraction data, we present the magnetic structure and magnetoelectric properties of ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ under various magnetic fields. In zero field, neutron diffraction experiments below $T_N=27\mathrm{K}$ reveal that the ${\mathrm{Co}}^{2+}$ moments order in the (ab) plane without any spin canting along the $c$ axis, manifested by the magnetic symmetry $C2/c^{\prime }$. Along each Co chain parallel to the $c$ axis, the moments of nearest-neighbor Co atoms order ferromagnetically with a small cant away from the next-nearest-neighbor Co moments. Under the applied magnetic field $H\parallel a$, three magnetic domains were aligned with their major magnetic moments perpendicular to the magnetic field with no other observable magnetic transitions. The influences of magnetic fields on the magnetic structures associated with the observed magnetoelectric coupling are discussed.

Noncollinear antiferromagnetic (AFM) materials have drawn research interest because they exhibit large anomalous Hall effect at room temperature (RT) due to large Berry curvature. ${\mathrm{Mn}}_3\mathrm{Ga}$ is a noncollinear AFM in which the order of Mn magnetic moments is arranged in inverse triangular configuration on a kagome lattice. It makes ${\mathrm{Mn}}_3\mathrm{Ga}$ a promising candidate for inverse spin Hall effect (ISHE) study which has not been studied before. In this work, investigation of ISHE and spin pumping in polycrystalline ${\mathrm{Mn}}_3\mathrm{Ga}$/CoFeB heterostructures at RT has been performed. Angle-dependent measurements of ISHE have been performed in order to disentangle various spin-rectification effects. Spin-mixing conductance ($g_{\mathrm{eff}}^{\uparrow \downarrow })$, spin Hall angle $\left({\theta }_{SH}\right)$, and spin Hall conductivity (${\sigma }_{SH}$) are evaluated to be $(5.0\pm 1.8)\times {10}^{18}{\mathrm{m}}^{-2}, 0.31\pm 0.01$, and $7.5\times {10}^5(\hslash /2e){\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$, respectively. The observed value of ${\theta }_{SH}$ is higher than ${\mathrm{Mn}}_3\mathrm{Sn}$ and comparable to the ${\mathrm{IrMn}}_3$, which is also a noncollinear AFM. Large spin Hall angle makes ${\mathrm{Mn}}_3\mathrm{Ga}$ a promising candidate for future spintronics devices.

We formulate a theoretical description of antiferromagnetic magnons and their transport in terms of an associated pseudospin. The need and strength of this formulation emerges from the antiferromagnetic eigenmodes being formed from superpositions of spin-up and -down magnons, depending on the material anisotropies. Consequently, a description analogous to that of spin-$\frac{1}{2}$ electrons is demonstrated while accounting for the bosonic nature of the antiferromagnetic eigenmodes. Introducing the concepts of a pseudospin chemical potential together with a pseudofield and relating magnon spin to pseudospin allows a consistent description of diffusive spin transport in antiferromagnetic insulators with any given anisotropies and interactions. Employing the formalism developed, we elucidate the general features of recent nonlocal spin transport experiments in antiferromagnetic insulators hosting magnons with different polarizations. The pseudospin formalism developed herein is valid for any pair of coupled bosons and is likely to be useful in other systems comprising interacting bosonic modes.

We theoretically investigate the carrier-free exchange interaction between magnetic impurities in ultrathin ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ topological insulator films by taking into account the low-energy states produced by the impurities. To match with experimental observations of magnetism on the surface of ultrathin topological insulator films, we restrict the calculations to having the chemical potential within the energy gap, with then interband processes mediating the exchange interaction. We find that the locally induced impurity resonances strongly influence the exchange interaction between magnetic moments. In particular, we find a noncollinear alignment to be more favorable than the collinear ferromagnetic alignment preferred when impurity states are ignored and only the pristine topological insulator band structure is considered. As a result, chiral ferromagnetism can easily become favorable over the ferromagnetic phase in these materials. Moreover, we show that by applying an electric field perpendicular to the ultrathin film, the exchange interaction can be drastically enhanced. This generates the possibility of highly tunable magnetism by electric field.

Investigation of the recently synthesized series of isostructural compounds ${\mathrm{Ba}}_4{\mathrm{NbTM}}_3{\mathrm{O}}_{12}$ $(\mathrm{TM}=\mathrm{Mn}, \mathrm{Rh}, \mathrm{and} \mathrm{Ir})$ with transition-metal trimers in a face-sharing geometry makes it possible to examine a tendency to the molecular orbitals (MO) formation going from $3d$ to $5d$ transition metal ions. Our ab initio calculations of electronic and magnetic properties describe the experimental findings and demonstrate gradual transition from the picture of localized electrons for Mn to the MO picture for Rh and especially for Ir. We also show that the often used criterion, according to which the metal-metal distance in a compound shorter than in a respective metal always gives MO, may break down in some cases.

We explore the dependence of the magnetic memory effect on demagnetizing interactions by considering two differently organized ensembles of anisotropic ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$ nanoparticles: a compact ensemble (CE) and a hollow core ensemble (HCE). The dynamic magnetic behavior is extensively investigated by performing ac susceptibility, magnetic memory measurements, and dc magnetic aging protocols. The frequency dependence of the freezing temperature is analyzed with the help of two dynamic scaling models: the Vogel-Fulcher (VF) model and the power-law model. Both the systems exhibit a cluster spin-glass phase, as realized from the Mydosh parameter, VF temperature, relaxation time for single spin flip, and critical exponent. The progressive spin freezing is ensured by multiple intermediate metastable states and significant memory effects in both the systems. The presence of hollow core geometry with highly interacting surface nanoparticles and partially aligned magnetic easy axes leads to a complex anisotropic energy landscape in HCE. As a consequence, prominent magnetic memory effect is observed in HCE along with higher activation energy, reduced blocking temperature, and enhanced coercivity compared to CE.

We present studies of Abrikosov vortex motion across the magnetically charged edges of long thin ferromagnetic stripes placed above or under a thin superconducting film. The magnetic charges at the stripe edges form attractive or repulsive potential wells for vortices. Using a relatively small in-plane magnetic field to polarize the stripe edges and normal-to-plane magnetic field to induce up or down polarized vortices, we tune the attractive or repulsive stripe-vortex interactions. Imaging of the vortex dynamics reveals that the repulsive magnetic potential ${U_m}^{+}$ acts as a robust vortex pinning barrier, while the attractive ${U_m}^{-}$ has practically no effect on the vortex motion irrespective of the position of the stripes located above or underneath the superconducting film. We analyze the observed asymmetry using equations of the overdamped vortex motion. The formal analytical solution yields an asymmetry, but the numerical modeling with and without noise terms does not confirm it. Instead, we find that the asymmetry is caused by the creation of spontaneous vortices at the maxima of $U_m$, which depends on the edge polarity and suppress ${U_m}^{-}$. Furthermore, our experiment and time-dependent Ginzburg-Landau simulations demonstrate that magnetic pinning dominates over vortex pinning due to corrugations of the superconducting layer deposited on top of the ferromagnetic stripes.