The atomic and electronic structures of ${\mathrm{Cu}}_2\mathrm{H}$ and CuH have been investigated by high-pressure nuclear magnetic resonance spectroscopy up to 96 GPa, X-ray diffraction up to 160 GPa, and density functional theory-based calculations. Metallic ${\mathrm{Cu}}_2\mathrm{H}$ was synthesized at a pressure of 40 GPa, and semimetallic CuH at 90 GPa, found stable up to 160 GPa. For ${\mathrm{Cu}}_2\mathrm{H}$, experiments and computations show an anomalous increase in the electronic density of state at the Fermi level for the hydrogen $1s$ states and the formation of a hydrogen network in the pressure range 43–58 GPa, together with high ${}^1\mathrm{H}$ mobility of $\sim {10}^{-7}\mathrm{c}{\mathrm{m}}^2/\mathrm{s}$. A comparison of these observations with results on FeH suggests that they could be common features in metal hydrides.

Nodal-line semimetals (NLSMs) are typically characterized by a nodal ring, a closed line of Dirac points in the Brillouin zone, which features a torus-shaped Fermi surface, as well as a quantized Berry phase $\pi$. By investigating the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in NLSMs, a close relationship between the magnetic interaction and the radius $k_0$ of the nodal ring is revealed. For impurities deposited in the direction normal to the nodal-ring plane, when the long-range interaction at half-band filling follows the decaying law $R^{-3}$ with impurity distance, which is a result of anisotropic dispersion of semi-Dirac semimetal, the decaying rate for the relatively short-range impurity distance would be prolonged by nodal-ring radius $k_0$, which is prominent for the intermediate radius. This phenomenon originates from the competition between the contribution from the electron states inside and outside the nodal ring. In contrast, for impurities distributed in the nodal-ring plane, the RKKY interaction $J\propto k_0^2\cos \left(2k_{0}R\right)R^{-4}$ shows an oscillation, from whose period one can easily determine the parameter $k_0$ of the nodal ring. Furthermore, at finite Fermi energy, we find a distinct signature of the RKKY interaction to characterize the topological trivial and topological nontrivial phases in NLSMs.

The nonlocal nature of unpaired Majorana bound states (MBSs) in topological superconductors can be exploited to create topologically protected qubits and perform gate operations fault-tolerantly via braidings. However, the time-dependent noises induced by coupling to an environment which is inevitable in any realistic system could spoil the topological protection. In this work, we study the effects of various dynamical noises such as Lorentzian, thermal, and quantum point contact on the MBSs in the recently proposed one-dimensional topological superconductors. We begin by investigating the Kitaev $p$-wave superconductors and examine the effects of long-range hopping and pairing on the transition rate of MBSs. We found that, especially, the long-range pairings significantly reduce the transition rate of bound states. Then, we consider the recently discovered topological superconducting nanowires and magnetic chains. Our findings are consequential for the recent attempts to manipulate MBSs. In particular, for the latter two experimentally realized systems we argue how low magnetic/Zeeman fields and strong spin-orbit coupling make the MBSs more robust to noises.

We study networks of Coulomb-blockaded integer quantum Hall islands with even fillings $\nu =2k$ ($k$ being an integer), including cases with $2k$ layers each of $\nu =1$ fillings. Allowing only spin-current interactions between the islands (i.e., without any charge transfer), we obtain solvable models leading to a rich set of insulating $SU{\left(2\right)}_k$ topologically ordered phases. The case with $k=1$ is dual to the Kalmeyer-Laughlin phase, $k=2$ to Kitaev's chiral spin liquid and the Moore-Read state, and $k=3$ contains a Fibonacci anyon that may be utilized for universal topological quantum computation. Additionally, we show how the $SU{\left(2\right)}_k$ topological phases may be obtained also in an array of islands with $\nu =2k$ integer quantum Hall states and critical spin chains in a checkerboard pattern. The array and checkerboard constructions gap out the charge mode and additional “flavor” modes by virtue of their geometry. Furthermore, we find that a fine tuning of the system parameter is not needed in the checkerboard configuration and the $\nu =2$ case. We also discuss their bulk excitations, and show that their thermal Hall conductance is universal, reflecting the central charge $c=3k/(k+2)$ of the chiral edge modes.

The electronic and optical features of some potential single-photon sources in two-dimensional silicon carbide monolayers is studied via ab initio calculations and group theory analyses. A few point defects in three charge states (negative, positive, and neutral) are considered. By applying performance criteria, Stone-Wales defects without and with combination of antisite defects are studied in detail. The formation energy calculations reveal that neutral and positive charge states of these defects are stable. We compute the zero-phonon-line energy, the Huang-Rhys (HR) factor, and the photoluminescence spectrum for the available transitions in different charge states. The calculated HR values and the related Debye-Waller factors guarantee that the Stone-Wales defects have a high potential of performing as a promising single-photon emitter.

Magnetization measurements on single-crystal cubic $\mathrm{Sr}\mathrm{Cu}{\mathrm{Te}}_2{\mathrm{O}}_6$ with an applied magnetic field along three inequivalent high symmetry directions [100], [110], and [111] reveal weak magnetic anisotropy. The fits of the magnetic susceptibility to the result from a quantum Monte Carlo simulation on the Heisenberg spin-chain model, where the chain is formed via the dominant third-nearest-neighbor exchange interaction $J_3$, yield the intrachain interaction $(J_3/k_B)$ between 50.12(7) K for the applied field along [110] and 52.5(2) K along [100] with about the same $g$ factor of 2.2. Single-crystal neutron diffraction unveils the transition to the magnetic ordered state as evidenced by the onset of the magnetic Bragg intensity at $T_{\text{N1}}=5.25\left(9\right)\mathrm{K}$ with no anomaly of the second transition at $T_{\text{N2}}$ reported previously. Based on irreducible representation theory and magnetic space group analysis of powder and single-crystal neutron diffraction data, the magnetic structure in the Shubnikov space group $P{4}_{1}32$, where the ${\mathrm{Cu}}^{2+}S=1/2$ spins antiferromagnetically align in the direction perpendicular to the spin chain, is proposed. The measured ordered moment of $0.52\left(6\right){\mu }_B$, which represents 48% reduction from the expected value of $1{\mu }_B$, suggests the remaining influence of frustration resulting from the $J_1$ and $J_2$ bonds.

Intriguing novel phenomena in lead films have inspired a new understanding of quantum physics, such as the quantum size effect and quantum phase transitions, etc. The improvement of the sample quality makes it even more promising to explore the intrinsic properties in two-dimensional systems. In this Rapid Communication, we show that the crystalline interfacial striped incommensurate layer can increase the quality of the lead films and significantly enhance the magnitude of magnetoresistance. By performing systematic transport measurements, a predominant anomalous linear magnetoresistance is revealed, and the widely used Parish-Littlewood model and Abrikosov's explanation fail to describe the observation. Instead, we propose another model of linear magnetoresistance based on linear band structure, which shows good agreement with the experimental results. Our studies reveal a different origin of linear magnetoresistance which may also be helpful to understand the linear magnetoresistance in other materials with a linear dispersion of the electronic structure.

Determination of the energy of the interface Dzyaloshinskii-Moriya interaction (DMI) along with a definition of the basic magnetic characteristics in ferromagnetic/nonmagnetic multilayered systems are both required for the construction of a magnetic skyrmion recording medium. A method for estimating the energy of the effective DMI, which compares the periods of micromagnetically simulated and experimentally measured demagnetized domain structures, is widely used for investigations of heavy metal/ferromagnetic superlattices. In the present paper, the applicability of this method was tested on a series of symmetric epitaxial ${\left[\mathrm{Co}\text{/}\mathrm{Pd}\left(111\right)\right]}_5$ superlattices with Co layers of varying thickness. We examined the problem of ambiguous determination of the DMI energy, which is related to the dependence of the periodicity of the domain structure, experimentally measured in a sample on the direction of a demagnetizing field. The Co thickness dependencies of the effective and surface DMI constants, obtained using in-plane and out-of-plane demagnetized structures and fixed micromagnetic parameters, were qualitatively and quantitatively different. Having analyzed the Co thickness dependencies of the DMI energies in the entire series of samples, we found that the DMI energies obtained using in-plane rather than out-of-plane demagnetized domain structures are more acceptable. Of greatest importance for the accurate determination of the DMI energy in an investigated system is the knowledge of the exchange interaction constant. The applicability of the method in the case of variable micromagnetic parameters and the reliability of the results obtained from the samples with different magnetic parameters are discussed.

We introduce the notion of non-Abelian tensors and use them to construct a general non-Abelian time evolving block decimation (NA-TEBD) scheme that uses an arbitrary number of Abelian and non-Abelian symmetries. Our approach increases the speed and memory storage efficiency of matrix product state based computations by several orders of magnitude and makes large bond dimensions accessible even on simple desktop architectures. We use it to study post-quench dynamics in the repulsive SU(3) Hubbard model and to determine the time evolution of various local operators and correlation functions efficiently. Interactions turn algebraic charge relaxation into exponential and suppress coherent quantum oscillations rapidly.

Magnetic topological materials, in which the time-reversal symmetry is broken, host various exotic quantum phenomena, including the quantum anomalous Hall effect, axion insulator states, and Majorana fermions. The study of magnetic topological materials is at the forefront of condensed matter physics. Recently, a variety of magnetic topological materials have been reported, such as ${\mathrm{Mn}}_3\mathrm{Sn}, {\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2, {\mathrm{Fe}}_3{\mathrm{Sn}}_2$, and ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Here, we report the observation of a topological electronic structure in an antiferromagnet, HoSbTe, a member of the ZrSiS family of materials, by angle-resolved photoemission spectroscopy (ARPES) measurements and first-principles calculations. We demonstrate that HoSbTe is a Dirac nodal line semimetal when spin-orbit coupling (SOC) is neglected. However, our theoretical calculations show that the strong SOC in HoSbTe fully gaps out the nodal lines and drives the system to a weak topological insulator state, with each layer being a two-dimensional topological insulator. Because of the strong SOC in HoSbTe, the gap is as large as hundreds of meV along specific directions, which is directly observed by our ARPES measurements. The existence of magnetic order and topological properties in HoSbTe makes it a promising material for the realization of exotic quantum devices.

Topological superconductors, characterized by topologically nontrivial states residing in a superconducting gap, are a recently discovered class of materials having Majorana fermions. The interplay of superconductivity and topological states gives rise to opportunities for achieving such topological superconductors in condensed matter systems. Up to now, several single-material topological superconductors in this form have been theoretically predicted and experimentally confirmed. Here, using the first-principles calculations, we study the superconducting single-layer $\beta -{\mathrm{Bi}}_2\mathrm{Pd}$. The electronic density of states near Fermi level of this monolayer are dominated by the Bi-$p$ and Pd-$d$ orbitals, forming a two-band Fermi surface with multiclass sheets. The presence of soft phonon bands, in cooperation with the electron susceptibility, accounts for electron-phonon superconductivity of single-layer $\beta -{\mathrm{Bi}}_2\mathrm{Pd}$. With the centrosymmetric structure, single-layer $\beta -{\mathrm{Bi}}_2\mathrm{Pd}$ possesses a continuous gap over the whole Brillouin zone and topological Dirac-like states at its one-dimensional boundary. The present findings would lead to the expectation of one-dimensional topological superconductivity and Majorana bound states in a monolayer candidate of $\beta -{\mathrm{Bi}}_2\mathrm{Pd}$ with intrinsic full-gap superconductivity.

Quantum Monte Carlo (QMC) methods are some of the most accurate methods for simulating correlated electronic systems. We investigate the compatibility, strengths, and weaknesses of two such methods, namely, diffusion Monte Carlo (DMC) and auxiliary-field quantum Monte Carlo (AFQMC). The multideterminant trial wave functions employed in both approaches are generated using the configuration interaction using a perturbative selection made iteratively (CIPSI) technique. Complete basis-set full configuration interaction energies estimated with CIPSI are used as a reference in this comparative study between DMC and AFQMC. By focusing on a set of canonical finite–size solid-state systems, we show that both QMC methods can be made to systematically converge towards the same energy once basis-set effects and systematic biases have been removed. AFQMC shows a much smaller dependence on the trial wave function than DMC while simultaneously exhibiting a much larger basis-set dependence. We outline some of the remaining challenges and opportunities for improving these approaches.

Using tensor network methods, we perform finite-size scaling analysis to study the parameter-induced phase transitions of two-dimensional deformed Affleck-Kennedy-Lieb-Tasaki (AKLT) states. We use the higher-order tensor renormalization group method to evaluate the moments and the correlations. Then, the critical point and critical exponents are determined simultaneously by collapsing the data. Alternatively, the crossing points of the dimensionless ratios are used to determine the critical point, and the scaling at the critical point is used to determine the critical exponents. For the transition between the disordered AKLT phase and the ferromagnetic (FM) ordered phase, we demonstrate that both the critical point and the exponents can be determined accurately. Furthermore, the values of the exponents confirm that the AKLT-FM transition belongs to the two-dimensional Ising universality class. We also investigate the Berezinskii-Kosterlitz-Thouless transition from the AKLT phase to the critical $\mathit{XY}$ phase. In this case we show that the critical point can be located by the crossing point of the correlation ratio.

We study the ultrafast electron and lattice dynamics of thin gold films with mid-infrared transmission and reflection pump-probe spectroscopy. We determine the time dependence of the dielectric function, which yields the electron-phonon coupling constant $G$ that determines the energy exchange rate between the electrons and the lattice. From the experimentally obtained dielectric function, we determine the dependence of the electron-electron and the electron-lattice scattering rates on the electron and the lattice temperature.

Structural and magnetic studies on $\mathrm{SrLa}\left({\mathrm{Fe}}_{0.25}{\mathrm{Mn}}_{0.25}\right){\mathrm{Co}}_{0.5}{\mathrm{O}}_4$ (FMC1) and ${\mathrm{SrLaFe}}_{0.5}\left({\mathrm{Mn}}_{0.25}{\mathrm{Co}}_{0.25}\right){\mathrm{O}}_4$ (FMC2) reveal unusually large exchange bias behavior in these atomically disordered quasi-two-dimensional layered perovskites. Powder x-ray as well as neutron diffraction confirm tetragonal crystal structure with $I4/mmm$ space group for both the compounds. Magnetization measurements on FMC1 as well as FMC2 reveal short-range antiferromagnetic ordering around room temperature and frozen magnetic clusters at lower temperatures ($T<23\mathrm{K}$ for FMC1 and $T<43\mathrm{K}$ for FMC2). The random occupancy of mixed-valent magnetic ions ($\mathrm{F}{\mathrm{e}}^{3+}/\mathrm{F}{\mathrm{e}}^{4+}, \mathrm{M}{\mathrm{n}}^{3+}/\mathrm{M}{\mathrm{n}}^{4+}, \mathrm{C}{\mathrm{o}}^{2+}/\mathrm{C}{\mathrm{o}}^{3+}$) at the perovskite octahedral sites give rise to locally varying competing antiferromagnetic and ferromagnetic exchange interactions, resulting in low temperature frozen spin states. Giant exchange bias values of ∼6 and ∼9.5 kOe were observed for FMC1 and FMC2, respectively, at 2 K under an applied field of 50 kOe. In comparison, the Co-rich FMC1 exhibits higher magnetization and coercivity, whereas Fe-rich FMC2 possesses a higher exchange bias effect. Our experimental results reveal compositional tuning induced exotic magnetic behavior in quasi-two-dimensional layered oxides.

In this work we report the temperature and doping-level evolution of the crystallographic and magnetic properties of the Sr-doped cobaltates ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{BaCo}}_4{\mathrm{O}}_7$. The noncollinear ferrimagnetic phase known for the magnetoelectric parent compound is found to persist only for a small amount of Sr doping ($x=0.02$) and is accompanied by a strong unit cell distortion. In turn, further Sr doping blurs this distortion of lattice parameters and favors other magnetic arrangements. In particular, this work focuses on an intermediate temperature region $62\mathrm{K}<T<82\mathrm{K}$ that shows a plateau in the magnetization. Using neutron powder diffraction at selected temperatures and compositions, we have solved the magnetic structure of this intermediate phase. By means of the superspace group theory and its implementation in the Rietveld refinement, we found that the phase belongs to the superspace group $Pna{2}_1{1}^{\prime }(1/2,1/2,g)qq0s$ with a basic orthorhombic structure and a modulation of the magnetic moments along the $c$ axis. The propagation vector is $\mathbf{k}=(\frac{1}{2},\frac{1}{2},g)$ with $g\sim 0.02$ and possibly dependent on temperature. A modulated spin structure with distinct behaviors of the triangular and kagome cobalt sites is reported in “114” cobaltates.

Motivated by the magnetic phase transition of a proximate Kitaev system $\alpha -{\mathrm{RuCl}}_3$ in the presence of a magnetic field, we study the simplest but essential quantum spin model with the ferromagnetic nearest neighboring (NN) Kitaev interaction and additional antiferromagnetic third NN Heisenberg interaction. Employing both exact diagonalization and density matrix renormalization group methods, we demonstrate that the model shows the magnetic phase transition from the zigzag order phase to the spin polarized phase through an intermediate phase in both cases when an in-plane magnetic field is applied perpendicular to the NN bond direction and when an out-of-plane field is applied, in good agreement with experimental observations. Furthermore, we verify that additional symmetric off-diagonal $\mathrm{\Gamma }$ interaction and ferromagnetic Heisenberg interaction between NN spins can both suppress the intermediate phase with the in-plane field. Our result gives important clues for determining relevant interactions in the field-induced magnetic phase transition of proximate Kiteav systems.

We report the enhancement of the magneto-optical Faraday effect in a two-dimensional magnetoplasmonic grating with a square lattice. The structure consists of dielectric magnetic Bi-substituted iron garnet film with low thickness (100 nm) coated by a perforated gold layer. Enhancement of the Faraday rotation by about 2 times is shown experimentally and theoretically despite the suppression of the waveguiding modes. This happens primarily through excitation of surface plasmon polaritons propagating at the Au-garnet interface in two orthogonal directions along lattice vectors of the plasmonic grating.