A quasi-one-dimensional layered material CuTe undergoes a charge density wave (CDW) transition in Te chains with a modulation vector of $q_{\mathrm{CDW}}=(0.4,0.0,0.5)$. Despite the clear experimental evidence for the CDW, the theoretical understanding, especially the role of the electron-electron correlation in the CDW, has not been fully explored. Here, using first-principles calculations, we demonstrate that the correlation effect of Cu is critical to stabilize the $5\times 1\times 2$ modulation of Te chains. We find that the phonon calculation with the strong Coulomb correlation exhibits the imaginary phonon frequency, i.e., the so-called phonon soft mode, at $q_{ph0}=(0.4,0.0,0.5)$, indicating the structural instability. The corresponding lattice distortion of the soft mode agrees well with the experimental modulation. These results demonstrate that the CDW transition in CuTe originates from the interplay of the Coulomb correlation and electron-phonon interaction.

The time-resolved magneto-optical Kerr effect technique has been exploited to study the magnetization dynamics over femtosecond to nanosecond timescale for Gd-Fe thin films with different thicknesses. The precessional magnetization dynamics is studied by varying the magnitude and orientation of the bias magnetic field from nearly in-plane to nearly out-of-plane (OOP) direction. The analysis of bias magnetic field dependence of precessional frequency has revealed the development of OOP anisotropy when the film thickness was increased from 20 to 100 nm and this is in good agreement with the static magnetic properties of the films. Moreover, the 100-nm-thick film has shown a signature of perpendicular standing spin wave modes along with the uniform Kittel mode. The evolution of their frequency, decay time, and damping with the magnitude and orientation of bias magnetic field is analyzed in detail, showing possible energy transfer between the perpendicular standing spin wave modes and the uniform Kittel mode. Interestingly, the effective damping could be tuned over a broader range of 0.02 to 0.15, which shows the potential for applications in miniaturized and fast magnetic storage, magnetic memory, and magnonic devices.

Dangling edge spins of dimerized two-dimensional spin-1 Heisenberg antiferromagnets are shown to exhibit nonordinary quantum critical correlations, akin to the scaling behavior observed in recently explored spin-1/2 systems. Based on large-scale quantum Monte Carlo simulations, we observe remarkable similarities between these two cases and examine the crossover to the fundamentally distinct behavior in the one-dimensional limit of strongly coupled edge spins. We complement our numerical analysis by a cluster mean-field theory that encompasses the qualitatively similar behavior for the spin-1 and the spin-1/2 cases and its dependence on the spatial edge spin configuration in a generic way.

$\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_2\mathrm{Se}$ is a novel layer-structured material with high electron mobility, while its efficiency could be greatly improved by doping different elements to introduce a magnetic spin order. We investigated the electronic and magnetic properties of various transition metal (TM) ($\mathrm{TM}=\mathrm{Mn}$, Cr, Fe, Co, and Ni) doped $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_2\mathrm{Se}$ within a framework of density functional theory (DFT), and discovered that $\mathrm{B}{\mathrm{i}}_{2-n}{\mathrm{X}}_n{\mathrm{O}}_2\mathrm{Se}$ exhibits long-range magnetic ordered structure via competition among double-exchange, p-d exchange, and superexchange interaction. The magnetic order of the bulk phase in which the magnetic atoms form interlayer coupling would vary with the type and concentration of doped atoms, but all the layered phases in which the magnetic atoms are in-plane coupled show ferromagnetic order. By combing DFT calculations with the Monte Carlo scheme, we solve the exchange interaction constants for the Heisenberg model and further evaluate the Curie temperatures of $\mathrm{B}{\mathrm{i}}_{2-n}{\mathrm{X}}_n{\mathrm{O}}_2\mathrm{Se}$. Ferromagnetic order for most doped systems exhibit to be robust with high Curie temperature, some of which overcomes room temperature (for 12.5% Co-doped layer $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_2\mathrm{Se}$). It is also worth mentioning that the appearance of impurity energy levels narrows the band gap and enhances the spin-orbit coupling of $d$ orbitals and therefore increase large magnetic anisotropy energy. Our study demonstrates a potential pathway to design new dilute magnetic semiconductors through doping of $\mathrm{B}{\mathrm{i}}_{2-n}{\mathrm{X}}_n{\mathrm{O}}_2\mathrm{Se}$ by magnetic transitional elements.

We discuss theoretically phase transitions in frustrated antiferromagnets with biaxial anisotropy or dipolar forces in magnetic field applied along the easy axis at $T=0$. There are well-known sequences of phase transitions on the field increasing: the conventional spin-flop transition and the flop of the spiral plane at strong and weak easy-axis anisotropy, respectively. We argue that much less studied scenarios can appear at moderate anisotropy in which the magnetic field induces transitions of the first order from the collinear state to phases with spiral orderings. Critical fields of these transitions are derived in the mean-field approximation and the necessary conditions are found for the realization of these scenarios. We show that one of the considered sequences of phase transitions was found in multiferroic ${\mathrm{MnWO}}_4$ both experimentally and numerically (in a relevant model) and our theory reproduces quantitatively the numerical findings.

The performance of skyrmion-based spintronic devices can be greatly affected by the presence of defects in the materials. Here we develop an analytic model for describing the interaction between skyrmions and defects in ultrathin films focusing on the case of extended defects. The dynamics of skyrmions under the driving of in-plane polarized current is studied, considering several types of torques. The system is modeled with Thiele's equation in the complex plane and described with a reduced number of parameters. We start by considering a Gaussian-like interaction with point defects and extend the treatment to segment, line, and grid defects. Conditions for dynamic regimes of pinning, guiding, accelerating, or arranging skyrmions by defects are established and discussed. In particular, expressions for the threshold driven current density to pin or depin skyrmions in such defects, the position of the critical points, as well as the guiding conditions along long defects are found, analytically in some cases. This enables a deeper understanding of the parameter dependence of the skyrmion dynamics under the influence of defects. Micromagnetic simulations show qualitative agreement with the analytical treatment.

Pyrochlore ${\mathrm{FeF}}_3$ (pyr-${\mathrm{FeF}}_3$) is a Heisenberg antiferromagnetic (AF) with a magnetic susceptibility deviating from the Curie-Weiss law, even at the room temperature. This compound shows a transition to a long-range ordered state with all-in all-out (AIAO) spin configuration. The critical properties of this transition have remained a matter of dispute. In this paper, to gain more insight into the critical properties of pyr-${\mathrm{FeF}}_3$, using ab initio density-functional theory, we obtain spin Hamiltonian of this material under the relative volume change with respect to the experimental volume ($\frac{\mathrm{\Delta }V}{V_0}$) from $-0.2$ to 0.2. We show that the relevant terms in the spin Hamiltonians are the AF exchange up to third neighbors, the nearest-neighbor biquadratic and the direct Dyzaloshinski-Moriya interactions and find how these coupling constants vary under the volume change. Then we study the effect of volume change on the finite temperature critical behavior, using classical Monte Carlo simulation. We show that the spin system undergoes a weakly first-order transition to AIAO at small volumes which turns to a second-order transition close to the experimental structure. However, increasing $\frac{\mathrm{\Delta }V}{V_0}$ to $\sim 0.2$, systems shows a transition to an incommensurate spin structure. This finding suggests the existence of a Lifshitz point in pyr-${\mathrm{FeF}}_3$ and may explain the unusual critical exponents observed for this compound.

Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite ${\mathrm{SrFe}}_4{\mathrm{As}}_{12}$ [Q.-P. Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in the newly discovered isostructural superconductor ${\mathrm{SrOs}}_4{\mathrm{As}}_{12}$ with a superconducting transition temperature of $T_{\mathrm{c}}\sim 4.8\mathrm{K}$. Knight shift $K$ determined by the NQR spectrum under a small magnetic field ($\le 0.5$ T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, $1/T_{1}T$, is nearly independent of temperature above $\sim 50$ K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in ${\mathrm{SrFe}}_4{\mathrm{As}}_{12}$, we found that the values of $\left|K\right|$ and $1/T_{1}T$ in ${\mathrm{SrOs}}_4{\mathrm{As}}_{12}$ are smaller than those in ${\mathrm{SrFe}}_4{\mathrm{As}}_{12}$, indicating no obvious ferromagnetic spin correlations in ${\mathrm{SrOs}}_4{\mathrm{As}}_{12}$. From the temperature dependence of $1/T_1$ in the superconducting state, an $s$-wave superconductivity is realized.

Light enables the ultrafast, direct, and nonthermal control of the exchange and Dzyaloshinskii-Moriya interactions. We consider two-dimensional honeycomb lattices described by the Kane-Mele-Hubbard model at half filling and in the strongly correlated regime, i.e., an antiferromagnetic spin-orbit Mott insulator. Based on the Floquet theory, we demonstrate that by changing the amplitude and frequency of polarized laser pulses, one can tune the amplitudes and signs of and even the ratio between the exchange and Dzyaloshinskii-Moriya spin interactions. Furthermore, the renormalizations of the spin interactions are independent of the helicity. Our results pave the way for ultrafast optical spin manipulation in recently discovered two-dimensional magnetic materials.

With the discovery of strong-coupling physics and superconductivity in moiré superlattices, it is essential to have an understanding of strong-coupling driven superconductivity in systems with trigonal symmetry. The simplest lattice model with trigonal symmetry is the triangular lattice Hubbard model. Although the triangular lattice spin model is a heavily studied model in the context of frustration, studies of the hole-doped triangular lattice Hubbard model are rare. Here, we use density matrix renormalization group to investigate the dominant superconducting channels in the hole-doped triangular lattice Hubbard model over a range of repulsive interaction strengths. We find a clear transition from $p$-wave superconductivity at a moderate on-site repulsion strength ($U/t=2$) at filling above 1/4 ($n\sim 0.65$) to $d$-wave superconductivity at a strong on-site repulsion strength ($U/t=10$) at filling below 1/4 ($n\sim 0.4$). The unusual tunability that moiré superlattices offer in controlling $U/t$ would open up the opportunity to realize this transition between $d$-wave and $p$-wave superconductivity.

Andreev reflection (AR) in ferromagnet/superconductor junctions is an indispensable spectroscopic tool for measuring spin polarization. We study theoretically how the presence of a thin semiconducting interface in such junctions, inducing Rashba and Dresselhaus spin-orbit coupling, modifies AR processes. The interface gives rise to a momentum- and spin-dependent scattering potential, making the AR probability strongly asymmetric with respect to the sign of the incident electrons' transverse momenta. This skew AR creates spatial charge carrier imbalances and transverse Hall currents in the ferromagnet. We show that the effect is giant, compared to the normal regime. We provide a quantitative analysis and a qualitative picture of this phenomenon, and finally show that skew AR also leads to a widely tunable transverse supercurrent response in the superconductor.

A classic example of a quantum quench concerns the release of an interacting Bose gas from an optical lattice. The local properties of quenches such as this have been extensively studied; however, the global properties of these nonequilibrium quantum systems have received far less attention. Here we study several aspects of global nonequilibrium behavior by calculating the amount of work done by the quench as measured through the work distribution function. Using Bethe ansatz techniques, we determine the Loschmidt amplitude and work distribution function of the Lieb-Liniger gas after it is released from an optical lattice. We find the average work and its universal edge exponents from which we determine the long-time decay of the Loschmidt echo and highlight striking differences caused by the interactions as well as changes in the geometry of the system. We extend our calculation to the attractive regime of the model and show that the system exhibits properties similar to the super-Tonks-Girardeau gas. Finally, we examine the prominent role played by bound states in the work distribution and show that, with low probability, they allow for work to be extracted from the quench.

We study tunnel magnetoresistance (TMR) ratio of self-assembled $2d$ magnetic nanoparticle (MNP) arrays by modeling them as interacting dipoles on a triangular lattice, which is representative of experimentally obtained assemblies. Low-temperature Monte Carlo simulations are performed to understand the effects of dipolar interactions and uniaxial anisotropy on TMR behavior. In magnetic tunnel junctions with a perfect antiparallel state, the TMR amplitude reaches the ideal value $2P^2/(1-P^2)$, where $P$ is the spin polarization. We show that, for MNPs with their anisotropy axes perpendicular to the array, the TMR amplitude can reach a value as large as $2400\%$, which represents an order of magnitude improvement over previous studies. We evaluate the conditions for which these large amplitudes could be obtained experimentally at room temperature and find that spherical MNPs with large magnetocrystalline anisotropy or nanorods are suitable candidates. Our theoretical results should stimulate experimental groups into elaborating the samples proposed in this work, which could display large TMR amplitudes.

We theoretically investigate the Goos-Hänchen (GH) effect of spin-wave beams reflected from the interface between two ferromagnetic films with different Dzyaloshinskii-Moriya interactions (DMIs). The formula of the GH shift as functions of the incident angle and material parameters is derived analytically. We show that the GH effect occurs only when spin waves are totally reflected at the interface and vanishes otherwise. We further explore the GH shift of spin waves by narrow DMI strips of different widths. It is found that the induced shift is independent of the strip width down to 10 nm, offering an approach to measure the DMI strength of ultranarrow magnetic strips out of the scope of current technology. Full micromagnetic simulations compare well with our theoretical findings. Strong distortion of edge magnetizations for narrower strips, however, generates a width dependence of the GH shift. The results presented in this paper are helpful for understanding the GH effect in chiral magnets and for quantifying the DMI parameter in magnetic strips of sub-50-nm scales.

Antiferromagnetic materials hold promising prospects in novel types of spintronics applications. Assessing the stability of antiferromagnetic nanostructures against thermal excitations is a crucial aspect of designing devices with a high information density. Here we use theoretical calculations and numerical simulations to determine the mean switching time of antiferromagnetic nanoparticles in the superparamagnetic limit. It is demonstrated that the thermal stability is drastically reduced compared to ferromagnetic particles in the limit of low Gilbert damping, attributed to the exchange enhancement of the attempt frequencies. It is discussed how the system parameters have to be engineered in order to optimize the switching rates in antiferromagnetic nanoparticles.

Perovskite iridates of $6H$ hexagonal structure present a plethora of possibilities in terms of the variety of ground states resulting from a competition between spin-orbit coupling (SOC), hopping, noncubic crystal field (${\mathrm{\Delta }}_{\text{CFE}}^{\text{NC}}$), and superexchange energy scales within the ${\mathrm{Ir}}_2{\mathrm{O}}_9$ dimers. Here we investigate one such compound, ${\mathrm{Ba}}_3{\mathrm{CdIr}}_2{\mathrm{O}}_9$, by x-ray diffraction, dc magnetic susceptibility ($\chi$), heat capacity ($C_p$), and also ${}^{113}\mathrm{Cd}$ nuclear magnetic resonance (NMR) spectroscopy. We have established that the magnetic ground state has a small but finite magnetic moment on $\mathrm{Ir}^{5+}$ in this system, which likely arises from intradimer Ir-Ir hopping and local crystal distortions. Our heat capacity, NMR, and dc magnetic susceptibility measurements further rule out any kind of magnetic long- or short-range ordering among the Ir moments down to at least 2 K. In addition, the magnetic heat capacity data show linear temperature dependence at low temperatures under applied high fields ($>30$ kOe), suggesting a gapless spin density of states in the compound.

All-optical switching refers to the magnetization reversal induced by ultrashort laser pulses. Both single-pulse all-optical helicity-independent switching and multiple pulses all-optical helicity-dependent switching have been reported for GdFeCo thin films. In this paper, we demonstrate that for two GdFeCo compositions, the transition between these two behaviors can appear by increasing the thickness of the Pt capping or by tuning the laser fluence. The two types of switching cannot be observed for the same set of parameters, which supports that their mechanisms are uncorrelated. Our calculations indicate that the transition may be induced by a strong inhomogeneity of the electronic temperature within the thickness of the GdFeCo layer in the subpicosecond timescale.

Gate-tunable semiconductor-superconductor nanowires with superconducting leads form exotic Josephson junctions that are a highly desirable platform for two types of qubits: Those with topological superconductivity (Majorana qubits) and those based on tunable anharmonicity (gatemon qubits). Controlling their behavior, however, requires understanding their electrostatic environment and electronic structure. Here we study gated InAs nanowires with epitaxial aluminum shells. By measuring current-phase relations and comparing them with analytical and numerical calculations, we show that we can tune the number of modes, determine the transparency of each mode, and tune into regimes in which electron-electron interactions are apparent, indicating the presence of a quantum dot. To take into account electrostatic and geometrical effects, we perform microscopic self-consistent Schrodinger-Poisson numerical simulations, revealing the energy spectrum of Andreev states in the junction as well as their spatial distribution. Our work systematically demonstrates the effect of device geometry, gate voltage, and phase bias on mode behavior, providing insights into ongoing experimental efforts and predictive device design.

We study $1s$ and $2p$ hard x-ray photoemission spectra (XPS) in a series of late transition metal oxides: ${\mathrm{Fe}}_2{\mathrm{O}}_3$ ($3d^5$), ${\mathrm{FeTiO}}_3$ ($3d^6$), CoO ($3d^7$), and NiO ($3d^8$). The experimental spectra are analyzed with two theoretical approaches: ${\mathrm{MO}}_6$ cluster model and local density approximation (LDA) $+$ dynamical mean-field theory (DMFT). Owing to the absence of the core-valence multiplets and spin-orbit coupling, $1s$ XPS is found to be a sensitive probe of chemical bonding and nonlocal charge-transfer screening, providing complementary information to $2p$ XPS. The $1s$ XPS spectra are used to assess the accuracy of the ab initio $\mathrm{LDA}+\mathrm{DMFT}$ approach, developed recently to study the material-specific charge-transfer effects in core-level XPS.