We report the analysis of magnetic susceptibility $\chi \left(T\right)$ of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ single crystal in the paramagnetic phase. We formulate the theoretical susceptibility based on isotropic Heisenberg antiferromagnetism incorporating the Dzyaloshinsky-Moriya interaction exactly, and include the interlayer couplings in a mean-field approximation. $\chi \left(T\right)$ above $T_{\mathrm{N}}$ was found to be well described by the model, indicating the predominant Heisenberg exchange consistent with the microscopic theory. The analysis points to a competition of nearest and next-nearest-neighbor interlayer couplings, which results in the up-up-down-down configuration of the in-plane canting moments identified by the diffraction experiments.

The sequences of ground states in frustrated antiferromagnetic rings with odd number of local spins characterized by a single bond defect or by arbitrary uniform couplings to an additional spin located at the center are determined. The sequences provide firm constraints on the total ground-state quantum numbers, which are more stringent than those arising from the Lieb-Mattis theorem for bipartite quantum spin systems. Apart from their theoretical importance, they suggest the possibility of tailoring a given class of the molecular nanomagnets with desired ground-state properties by tuning the relevant couplings. In particular, they predict the spin $S=1/2$ ground state for the centered rings composed of the half-integer spins with approximately uniform interactions. They confirm the applicability of the recent classification of spin frustration in both types of molecular nanomagnets. The classification is also discussed in the classical limit for the first class of the rings, providing a direct picture of frustration types. The Lieb-Mattis energy-level ordering and an analog of the Landé band, i.e., the energy spectra properties simplifying the characterization of the rings using the bulk magnetic or NMR measurements, are briefly discussed.

Antiferromagnetic molecular rings are widely studied both for fundamental quantum-mechanical issues and for technological applications, particularly in the field of quantum information processing. Here we present a detailed first-principles study of two families—purple and green—of $\left\{{\mathrm{Cr}}_{7}M\right\}$ antiferromagnetic rings, where $M$ is a divalent transition metal ion ($M={\mathrm{Ni}}^{2+}, {\mathrm{Mn}}^{2+}$, and ${\mathrm{Zn}}^{2+}$). We employ a recently developed flexible and efficient scheme to build ab initio system-specific Hubbard models. From such many-body models we systematically derive the low-energy effective spin Hamiltonian for the rings. Our approach allows us to calculate isotropic as well as anisotropic terms of the spin Hamiltonian, without any a priori assumption on its form. For each compound we calculate magnetic exchange couplings, zero-field splitting tensors, and gyromagnetic tensors, finding good agreement with experimental results.

Thin films of perovskite oxides offer the possibility of combining emerging concepts of strongly correlated electron phenomena and spin current in magnetic devices. However, spin transport and magnetization dynamics in these complex oxide materials are not well understood. Here, we experimentally quantify spin transport parameters and magnetization damping in epitaxial perovskite ferromagnet/paramagnet bilayers of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3/{\mathrm{SrRuO}}_3$ (LSMO/SRO) by broadband ferromagnetic resonance spectroscopy. From the SRO thickness dependence of the Gilbert damping, we estimate a short spin diffusion length of $\lesssim 1$ nm in SRO and an interfacial spin-mixing conductance comparable to other ferromagnet/paramagnetic-metal bilayers. Moreover, we find that anisotropic non-Gilbert damping due to two-magnon scattering also increases with the addition of SRO. Our results demonstrate LSMO/SRO as a spin-source/spin-sink system that may be a foundation for examining spin-current transport in various perovskite heterostructures.

We study the crossover in a zero-temperature superconducting nanofilm from a single to a double superconducting slab induced by a barrier in the middle. We use the Bogoliubov–de Gennes (BdG) equations in the Anderson approximation to show that the single-phase superconducting ground state of this heterostructure is intrinsically multigapped and has a new type of resonance caused by the strength of the barrier, thus distinct from the Thompson–Blatt shape resonance which is caused by tuning the thickness of the film. The simplest theoretical framework able to describe a finite height and very thin tunable insulating potential barrier in the middle is provided by a $\delta$-function potential. In this framework, the even single-particle states are affected by the insulating barrier, whereas the odd ones are not. The new type of resonance, hereafter called barrier-driven resonance, is caused by the crossing of the even single-particle states through the Fermi surface. The lift of the even-odd degeneracy at the barrier reconfigures the pairing interaction and leads to a multigapped superconducting state with barrier-driven resonances.

We study numerically the spin-1/2 XXZ model in a field on an infinite kagome lattice. We use different algorithms based on infinite projected entangled pair states (iPEPSs) for this, namely, (i) an approach with simplex tensors and a 9-site unit cell, and (ii) an approach based on coarse-graining three spins in the kagome lattice and mapping it to a square-lattice model with local and nearest-neighbor interactions, with the usual PEPS tensors, 6- and 12-site unit cells. Similarly to our previous calculation at the SU(2)-symmetric point (Heisenberg Hamiltonian), for any anisotropy from the Ising limit to the XY limit, we also observe the emergence of magnetization plateaus as a function of the magnetic field, at $m_z=\frac{1}{3}$ using 6-, 9-, and 12-site PEPS unit cells, and at $m_z=\frac{1}{9},\frac{5}{9}$, and $\frac{7}{9}$ using a 9-site PEPS unit cell, the latter setup being able to accommodate $\sqrt{3}\times \sqrt{3}$ solid order. We also find that, at $m_z=\frac{1}{3}$, (lattice) nematic and $\sqrt{3}\times \sqrt{3}$ VBC-order states are degenerate within the accuracy of the nine-site simplex method, for all anisotropy. The 6- and 12-site coarse-grained PEPS methods produce almost-degenerate nematic and $1\times 2$ VBC-solid orders. We also find that, within our accuracy, the six-site coarse-grained PEPS method gives slightly lower energies, which can be explained by the larger amount of entanglement this approach can handle, even in cases where the PEPS unit cell is not commensurate with the expected ground-state unit cell. Furthermore, we do not observe chiral spin liquid behaviors at and close to the XY point, as has been recently proposed. Our results are the first tensor network investigations of the XXZ model in a field and reveal the subtle competition between nearby magnetic orders in numerical simulations of frustrated quantum antiferromagnets, as well as the delicate interplay between energy optimization and symmetry in tensor network numerical simulations.

We use angle-resolved photoemission (ARPES) to study the three-dimensional (3D) electronic structure of Co pnictides $A{\mathrm{Co}}_2{\mathrm{As}}_2$ with $A=\text{Ba}$, Sr, Ca or a mixture of Sr and Ca. These compounds are isostructural to Fe based superconductors but have one more electron in the Co $3d$ orbitals. Going from Ba to Ca, they become more and more 3D, eventually forming a “collapsed” tetragonal phase, where the distance between CoAs layers is markedly reduced. We observe with ARPES the periodicity of the electronic structure as a function of $k_z$ (i.e., perpendicularly to CoAs layers) and find that it matches in each case that expected from the distance between the planes in the bulk. However, the electronic structure is better fitted by a calculation corresponding to a slab with two CoAs layers than to the bulk structure. We attribute this to subtle modifications of the 2D electronic structure induced by the truncation of the 3D dispersion at the surface in the ARPES measurement. We further study how this affects the electronic properties. We show that, despite this distortion, the electronic structure of ${\mathrm{CaCo}}_2{\mathrm{As}}_2$ is essentially that expected for a collapsed phase. Electronic correlations produce a renormalization of the electronic structure by a factor 1.4, which is not affected by the transition to the collapsed state. On the other hand, a small shift of the Fermi level reduces the density of states in the $e_g$ bands and suppresses the magnetic transition expected in ${\mathrm{CaCo}}_2{\mathrm{As}}_2$. Our study evidences that observing the 3D bulk periodicity is not sufficient to ensure bulk sensitivity. It further gives direct information on the role of 3D interactions, mostly governed by Co-As hybridization, among $e_g$ and $t_{2g}$ orbitals. It is also useful to better understand the electronic structure of Fe superconductors and the range of validity of ARPES measurements.

Pump-probe spectroscopy is the most common time-resolved technique for investigation of electronic dynamics, and the results provide the incoherent population decay time $T_1$. Here we use a modified pump-probe experiment to investigate coherent dynamics, and we demonstrate this with a measurement of the inhomogeneous dephasing time $T_2^{*}$ for phosphorus impurities in silicon. The pulse sequence produces the same information as previous coherent all-optical (photon-echo-based) techniques but is simpler. The probe signal strength is first order in the pulse area but its effect on the target state is only second order, meaning that it does not demolish the quantum information. We propose simple extensions to the technique to measure the homogeneous dephasing time $T_2$, or to perform tomography of the target qubit.

We propose a Berry phase effect on the chiral degrees of freedom of a triangular magnetic molecule. The phase is induced by adiabatically varying an external electric field in the plane of the molecule via a spin-electric coupling mechanism present in these frustrated magnetic molecules. The Berry phase effect depends on spin-orbit interaction splitting and on the electric dipole moment. By varying the amplitude of the applied electric field, the Berry phase difference between the two spin states can take any arbitrary value between zero and $\pi$, which can be measured as a phase shift between the two chiral states by using spin-echo techniques. Our result can be used to realize an electric-field-induced geometric phase-shift gate acting on a chiral qubit encoded in the ground-state manifold of the triangular magnetic molecule.

The electronic structure and transport properties of monolayer ${\mathrm{MoS}}_2$ are studied using a tight-binding approach coupled with the nonequilibrium Green's function method. A zigzag nanoribbon of ${\mathrm{MoS}}_2$ is conducting due to the intersection of the edge states with the Fermi level that is located within the bulk gap. We show that applying a transverse electric field results in the disappearance of this intersection and turns the material into a semiconductor. By increasing the electric field the band gap undergoes a two stage linear increase after which it decreases and ultimately closes. It is shown that in the presence of a uniform exchange field, this electric field tuning of the gap can be exploited to open low energy domains where only one of the spin states contributes to the electronic conductance. This introduces possibilities in designing spin filters for spintronic applications.

We study a lattice model of interacting Dirac fermions in $(2+1)$ dimensions space-time with an SU(4) symmetry. While increasing the interaction strength, this model undergoes a continuous quantum phase transition from a weakly interacting Dirac semimetal to a fully gapped and nondegenerate phase without condensing any Dirac fermion bilinear mass operator. This unusual mechanism for mass generation is consistent with recent studies of interacting topological insulators/superconductors, and also consistent with recent progress in the lattice QCD community.

We derive the stress-tensor formula within the $\text{LSDA}+U$ scheme by differentiating analytically the $\text{LSDA}+U$ total-energy function with respect to the strain tensor. The rotationally invariant form of the $\text{LSDA}+U$ functional is employed and the double-counting correction is considered in the fully localized limit and around mean field. The electronic wave functions are expanded with either pseudoatomic orbitals (PAOs) or plane waves. In the PAO-basis case, the orthogonality stress term is included. Our $\text{LSDA}+U$ stress-tensor formula is numerically tested with antiferromagnetic NiO and reproduces successfully the stress values obtained from numerical derivatives of the total-energy values. As an application, we study elastic constants, bulk moduli, and sound velocities of NiO and MnO, obtaining results in good agreement with experimental data.

There have been tremendous experimental and theoretical efforts toward the discovery of a quantum spin-liquid phase in honeycomb-based-lattice materials with strong spin-orbit coupling. Here the bond-dependent Kitaev interaction between local moments provides strong magnetic frustration and, if it is the only interaction present in the system, it will lead to an exactly solvable quantum spin-liquid ground state. In all of these materials, however, the ground state is in a magnetically ordered phase due to additional interactions between local moments. Recently, it has been reported that the magnetic order in the hyperhoneycomb material, $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$, is suppressed upon applying hydrostatic pressure and the resulting state becomes a quantum paramagnet or possibly a quantum spin liquid. Using ab initio computations and strong-coupling expansion, we investigate the lattice structure and resulting local moment model in pressurized $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$. Remarkably, the dominant interaction under high pressure is not the Kitaev interaction or further neighbor interactions, but a different kind of bond-dependent interaction. This leads to strong magnetic frustration and may provide a platform for discovery of a new kind of quantum spin-liquid ground state.

Time-periodic (Floquet) topological phases of matter exhibit bulk-edge relationships that are more complex than static topological insulators and superconductors. Finding the edge modes unique to driven systems usually requires numerics. Here we present a minimal two-band model of Floquet topological insulators and semimetals in two dimensions where all the bulk and edge properties can be obtained analytically. It is based on the extended Harper model of quantum Hall effect at flux one-half. We show that periodical driving gives rise to a series of phases characterized by a pair of integers. The model has a most striking feature: the spectrum of the edge modes is always given by a single cosine function, $\omega \left(k_y\right)\propto cos{k}_y$ where $k_y$ is the wave number along the edge, as if it is freely dispersing and completely decoupled from the bulk. The cosine mode is robust against the change in driving parameters. It also persists in the semimetallic phases with Dirac points.

We propose a multilayer device comprising a thin-film-based ferromagnetic heterostructure (FMH) deposited on a diamond layer doped with nitrogen vacancy centers (NVC's). We find that when the NVC's are in close proximity (1–2 nm) to the FMH, the exchange energy is comparable to, and may even surpass, the magnetostatic interaction energy. This calls forth the need to consider and utilize both effects in magnetometry based on NVC's in diamond. As the distance between the FMH and NVC is decreased to the subnanometer scale, the exponential increase in the exchange energy suggests spintronic applications of NVC's beyond magnetometry, such as detection of spin Hall effect or spin currents.

We study theoretically metallization and polarization enhancement effects of H adsorption at the $\mathrm{Ti}{\mathrm{O}}_2$-terminated (001) $\mathrm{BaTi}{\mathrm{O}}_3$ (BTO) surface. The clean BTO slab does not polarize because the slab is too thin to support a ferroelectric state. We find that the metallization of the surface is a strong function of hydrogen coverage and that the insulating surfaces do not support a rumpling or polarization in BTO films, whereas all metallic surfaces do so. The donated charge from the adsorbed H transforms the $\mathrm{Ti}{\mathrm{O}}_2$ into a metallic capping layer, stabilizing the ferroelectric state.

For NiTi-based alloys, the shape memory effect is governed by a transition from a low-temperature martensite phase to a high-temperature austenite phase. Despite considerable experimental and computational work, basic questions regarding the stability of the phases and the martensitic phase transition remain unclear even for the simple case of binary, equiatomic NiTi. We perform ab initio molecular dynamics simulations to describe the temperature-dependent behavior of NiTi and resolve several of these outstanding issues. Structural correlation functions and finite temperature phonon spectra are evaluated to determine phase stability. We show that finite temperature, entropic effects stabilize the experimentally observed martensite (B19') and austenite (B2) phases while destabilizing the theoretically predicted (B33) phase. Free energy computations based on ab initio thermodynamic integration confirm these results and permit estimates of the transition temperature between the phases. In addition to the martensitic phase transition, we predict a new transition between the B33 and B19' phases. The role of defects in suppressing phase transformation temperatures is discussed.

We investigate the photovoltaic (PV) effects in ferroelectrics based on the polar structure in domains and the intrinsic symmetry breaking of ferroelastic domain walls (DWs). A comprehensive analysis for $\mathrm{BiFe}{\mathrm{O}}_3$ films with the single-domain and 71° domain structures reveals a major contribution of the DWs superimposed on the bulk PV effect, which can explain the enhanced PV response in the multidomain film exhibiting photovoltages greater than the band gap. Our methodology can reveal the contributions of the bulk PV and the DW-PV effects quantitatively and uncover a design strategy for enhancing ferroelectric photovoltaics.

We present a self-contained operator-based approach to derive the spectrum of trapped ions. This approach provides the complete normal form of the low-energy quadratic Hamiltonian in terms of bosonic phonons, as well as an effective free-particle degree of freedom for each spontaneously broken spatial symmetry. We demonstrate how this formalism can directly be used to characterize an ion chain both in the linear and the zigzag regimes. In particular, we compute, both for the ground state and finite temperature states, spatial correlations, heat capacity, and dynamical susceptibility. Last, for the ground state, which has quantum correlations, we analyze the amount of energy reduction compared to an uncorrelated state with minimum energy, thus highlighting how the system can lower its energy by correlations.

We have studied the generation of spin-orbit torques in Pt/Co/Pt trilayers fabricated on ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (GGG) and commonly used ${\mathrm{SiO}}_2$ substrates. By fabricating Pt/Co/Pt trilayers with symmetric thicknesses of the top and bottom Pt layers, it has been confirmed that complete cancellation of both fieldlike and dampinglike torques can be obtained on the GGG substrate. However, the spin-orbit torques cannot be completely canceled out on the ${\mathrm{SiO}}_2$ substrate, which suggests that a GGG substrate is more suitable for precise measurements of the spin-orbit torques. We have also fabricated asymmetric Pt/Co/Pt trilayers on a GGG substrate to generate nonzero spin-orbit torques, and a relatively strong dampinglike torque has been measured accompanied with a fieldlike torque which is about four times smaller. Furthermore, by conducting the vector measurements of the spin-orbit torques from 300 to 10 K, it has been found that the dampinglike torque only slightly varies with temperatures.