We study the equilibrium dc Josephson current in a junction between an $s$-wave and a topological superconductor. Cooper pairs from the $s$-wave superconducting lead can transfer to the topological side either via an unpaired Majorana zero mode localized near the junction or via the above-gap continuum states. We find that the Majorana contribution to the supercurrent can be switched on when time-reversal symmetry in the conventional lead is broken, e.g., by an externally applied magnetic field inducing a Zeeman splitting. Moreover, if the magnetic field has a component in the direction of the effective spin-orbit field, there will be a Majorana-induced anomalous supercurrent at zero phase difference. These behaviors may serve as a signature characteristic of Majorana zero modes and are accessible to devices with only superconducting contacts.

We discuss theoretically the magnetically ordered phase induced by magnetic and nonmagnetic impurities in three-dimensional and quasi-low-dimensional systems with singlet ground states separated by a gap from excited triplet states. Using ideas of the percolation theory, we estimate the transition temperature $T_N\left(n\right)$ to the Néel phase at a small concentration $n$ of defects, derive the density of states of low-energy elementary excitations, and examine the contribution of these excitations to the specific heat and magnetization. Our expressions for $T_N\left(n\right)$ and for the specific heat describe well available experimental findings obtained in various appropriate systems: spin-$\frac{1}{2}$ dimer materials, spin-ladder compounds, spin-Peierls, and Haldane chain materials. However, our expression for $T_N\left(n\right)$ differs considerably from many of those proposed before.

Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled $3d$ band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys.

$\mathrm{Tm}\mathrm{Fe}{\mathrm{O}}_3$ (TFO) is a canted antiferromagnet that undergoes a spin reorientation transition (SRT) with temperature between 82 and 94 K in single crystals. In this temperature region, the Néel vector continuously rotates from the crystallographic $c$ axis (below 82 K) to the $a$ axis (above 94 K). The SRT allows for a temperature control of distinct antiferromagnetic states without the need for a magnetic field, making it apt for applications working at terahertz frequencies. For device applications, thin films of TFO are required as well as an electrical technique for read-out of the magnetic state. Here, we demonstrate that orthorhombic TFO thin films can be grown by pulsed laser deposition and the detection of the SRT in TFO thin films can be accessed by making use of the all-electrical spin Hall magnetoresistance, in good agreement for the temperature range where the SRT occurs in bulk crystals. Our results demonstrate that one can electrically detect the SRT in insulators.

The metamaterial approach to dielectric response engineering for enhancing the transition temperature, $T_c$, of a superconductor has been demonstrated in several recent reports. One example of this effect is ${\mathrm{Al}}_2{\mathrm{O}}_3$ coated aluminum nanoparticles that form an epsilon near zero (ENZ) core-shell metamaterial superconductor with a $T_c$ that is nearly three times that of pure aluminum. Since the $T_c$ of a conventional low temperature superconductor is determined by the Debye temperature and the coupling strength, which is expressed as the product of the single particle density of state (DOS) at the Fermi energy and an attractive electron-electron potential, it is natural to explore these properties to determine whether the attractive potential is responsible for the enhancement. In this report, we present specific heat results obtained from ${\mathrm{Al}}_2{\mathrm{O}}_3$ coated aluminum nanoparticle composite pellets with an enhanced $T_c$ that demonstrate that the Debye temperature and DOS are similar to that of pure aluminum indicating that the source of the $T_c$ enhancement is indeed a modification of the attractive electron-electron interaction.

The ${\pi }^{*}$-band renormalization of Li-doped quasifreestanding graphene has been investigated by means of isotope (${}^{13}\mathrm{C}$) substitution and angle-resolved photoemission spectroscopy. The well documented sudden slope change (known as “kink”) located at 169 meV from the Fermi level in the graphene made of ${}^{12}\mathrm{C}$ atoms shifts to 162 meV once the carbon monolayer is composed by ${}^{13}\mathrm{C}$ isotope. Such an energy shift is in excellent agreement with the expected softening of the phonon energy distribution due to the isotope substitution and provides, therefore, an indisputable experimental proof of the electron-phonon coupling origin of this well known many-body feature in the electronic structure of graphene.

Eight decades ago, Schottky proposed that the energy barrier at the metal-semiconductor interface, which now bears his name, should be compared with the difference of two surface quantities, the work function (WF) of the metal and the ionization potential of the semiconductor. This tradition of plotting and modeling the Schottky barrier height (SBH) against the metal WF has been followed ever since. However, success in general quantitative understanding of the SBH from physical principles has been limited, and empirical models are still relied upon. Here, we show that the stumbling block that has prevented a broadly applicable physical explanation of the SBH is the presence of surface dipole terms, inherently included in the traditional, Schottky-Mott styled analyses. By removing these surface contributions with the help of the recently developed neutral polyhedra theory, we show that the SBHs calculated for a very large number of epitaxial interfaces between metals and zinc-blende semiconductors are quantitatively explained from general chemical principles. Amazingly, SBHs calculated for 17 different semiconductors fit onto the same universal plot. Previously, SBHs needed to be grouped according to the semiconductor before analyses could be conducted separately for, and with empirical parameters specific to, each semiconductor. This work shows that the mechanism for SBH formation at metal-semiconductor interface is none other than the same chemistry responsible for charge distribution in molecules. There is no need for empirical modeling once the traditional beginning-point of SBH analysis is abandoned and the proposed new one is used.

In this paper, the completed investigation of a possible superconducting phase in monolayer indium selenide is determined using first-principles calculations for both the hole and electron doping systems. The hole-doped dependence of the Fermi surface is exclusively fundamental for monolayer InSe. It leads to the extensive modification of the Fermi surface from six separated pockets to two pockets by increasing the hole densities. For low hole doping levels of the system, below the Lifshitz transition point, superconductive critical temperatures $T_c\sim 55–75$ K are obtained within anisotropic Eliashberg theory depending on varying amounts of the Coulomb potential from 0.2 to 0.1. However, for some hole doping above the Lifshitz transition point, the combination of the temperature dependence of the bare susceptibility and the strong electron-phonon interaction gives rise to a charge density wave that emerged at a temperature far above the corresponding $T_c$. Having included nonadiabatic effects, we could carefully analyze conditions for which either a superconductive or charge density wave phase occurs in the system. In addition, monolayer InSe becomes dynamically stable by including nonadiabatic effects for different carrier concentrations at room temperature.

One of the most fundamental and exotic properties of three-dimensional (3D) topological insulators (TIs) is spin-momentum locking (SML) of their topological surface states (TSSs), promising for potential applications in future spintronics. However, other possible conduction channels, such as a trivial two-dimensional electron gas (2DEG) with strong Rashba-type spin-orbit interaction (SOI) and bulk-conducting states that may possess a spin Hall effect (SHE), can coexist in 3D TIs, making determining the origin of the current-induced spin polarization (CISP) difficult. In this work, we directly compared the CISP between bulk-insulating ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ (BTS221) and bulk-metallic ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin flakes using spin potentiometry. In the bulk-insulating BTS221, the observed CISP has a sign consistent with the expected helicity of the SML of the TSS, but an opposite sign to its calculated bulk spin Hall conductivity. However, compared to BTS221, an opposite CISP is observed in the bulk-metallic ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, consistent with both the expectations of its Rashba-Edelstein effect of the band-bending induced 2DEG and bulk intrinsic spin Hall Effect (SHE). If one assumes a representative occupation of the Rashba band of $3\times {10}^{13}\mathrm{c}{\mathrm{m}}^{-2}$ in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with a relevant relaxation time of 100 fs, the contribution to the CISP could be more dominated by the bulk intrinsic SHE. Our results provide an electrical way to distinguish the TSS from other possible conducting channels in spin transport measurements on 3D TIs, and open ways for the potential applications in charge-spin conversion devices.

We theoretically describe the quantum Zeno effect in a spin-photon interface represented by a charged quantum dot in a micropillar cavity in the strong coupling regime. This simplest model allows for various generalizations for the different systems. We derive a simple expression for the spin measurement rate, which allows one to tune the electron spin precession frequency in an external magnetic field and spin relaxation time. We calculate the spin noise bispectrum, which reveals the qualitative change of the spin dynamics with an increase of the measurement strength and proves the quantum nature of the spin noise. We also calculate the quantum information gain rate and find the conditions when it equals the spin dephasing rate, i.e., reaches the quantum limit.

We consider a quantum dot weakly tunnel coupled to superconducting reservoirs. A finite superconducting pair amplitude can be induced on the dot via the proximity effect. We investigate the dynamics of the induced pair amplitude after a quench and under periodic driving of the system by means of a real-time diagrammatic approach. We find that the quench dynamics is dominated by an exponential decay towards equilibrium. In contrast, the periodically driven system can sustain coherent oscillations of both the amplitude and the phase of the induced pair amplitude in analogy to Higgs and Nambu-Goldstone modes in driven bulk superconductors.

We show that ground state phase competition significantly decreases the critical cooling rate required for the kinetic avoidance of a first-order phase transition and thus facilitates the creation of metastable electronic states. This principle is revealed for the phase transition between antiferromagnetic charge-orbital-ordered insulating and ferromagnetic metallic states in the so-called colossal magnetoresistive manganite ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$. In a phase competition situation, we achieve reversible and nonvolatile control of bulk magnetization by $1{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ and resistivity by $\sim 1000\%$ by applying electric pulses to exploit electric heating and subsequent rapid cooling.

Helium bubbles are a severe form of radiation damage that has been frequently observed. It would be possible to understand the complex processes that cause bubble formation if suitable interatomic potentials were available to enable molecular dynamics simulations. In this paper, Pd-H-He embedded-atom method potentials based on both Daw-Baskes and Finnis-Sinclair formalisms have been developed to enable modeling of He bubbles formed by the radioactive decay of tritium in Pd. Our potentials incorporate helium into an existing Pd-H potential while addressing two challenging paradoxes: (a) Interstitial He atoms can dramatically lower their energies by forming dimers and larger clusters in Pd but are only bound by weak van der Waals forces in the gas phase. (b) He atoms diffuse readily in Pd yet significantly distort the Pd lattice with large volume expansions. We demonstrate that both of our potentials reproduce density functional theory results for (b). However, the Daw-Baskes formalism fails to resolve paradox (a) because it cannot reproduce the experimental helium equation of state. We resolved this problem through a modification of the Finnis-Sinclair formalism in which a (fictitious) negative embedding charge density is produced by Pd at the He binding sites. In addition to molecular statics validation of static properties, molecular dynamics simulation tests establish that our Finnis-Sinclair potential leads to the nucleation of helium bubbles from an initial random distribution of helium interstitial atoms.

We identify topological aspects of the subextensive magnetic moment contributed by the surfaces of a three-dimensional crystallite—assumed to be insulating in the bulk as well as on all surface facets, with trivial Chern invariants in the bulk. The geometric component of this subextensive moment is given by its derivative with respect to the chemical potential, at zero temperature and zero field, per unit surface area, and hence corresponds to the surface magnetic compressibility. The sum of the surface compressibilities contributed by two opposite facets of a cube-shaped crystallite is quantized to an integer multiple of the fundamental constant $e/hc$; this integer is in one-to-one correspondence with the net chirality of hinge modes on the surface of the crystallite, manifesting a link with higher-order topology. The contribution by a single facet to the magnetic compressibility need not be quantized to integers; however, symmetry and/or Hilbert-space constraints can fix the single-facet compressibility to half-integer multiples of $e/hc$, as will be exemplified by the Hopf insulator.

The Nd permanent magnet (${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$) is an indispensable material used in modern energy conversion devices. The realization of high coercivity at finite temperatures is an important issue. One of the important ingredients for controlling the coercive force is the surface property of magnetic grains. It has been found by first-principles studies that the Nd atoms in the first (001) surface layer facing the vacuum have in-plane anisotropy perpendicular to the $c$ axis, which may decrease the coercivity. Focusing on the surface anisotropy effect on the coercivity, we examine the coercivity at zero and finite temperatures using an atomistic model reflecting the lattice structure of the Nd magnet with a stochastic Landau-Lifshitz-Gilbert equation method. We study three general cases, in which the Nd atoms in surface layers have (1) no anisotropy, (2) in-plane anisotropy, and (3) reinforced anisotropy for two types of surface, (001) and (100) surfaces. We find that in contrast to the zero-temperature case, owing to the thermal fluctuation effect, the modification of only the first surface layer has little effect on the coercivity at finite temperatures. However, the modification of a few layers results in significant effects. We discuss the details of the dependences of the coercivity on temperature, the type of surface, and modified layer depth, and also the features of domain growth in magnetization reversal.

The recent advances of high spin-orbit torque efficiency in ferromagnetic/topological insulator (TI) structures hold great promise for the development of high-performance spintronic devices. However, the roles of spin-momentum-locked Dirac surface states (SSs) and the interfacial magnetic proximity effect (MPE) on dynamic magnetic properties are still under debate. Here, we quantitatively distinguish the manipulation effects of SSs and MPE on magnetic damping and surface anisotropy in TI/nonmagnet/CoFe heterostructures. We found that, in addition to the common spin pumping contribution stemming from SSs, damping enhancement also consists of an obvious MPE contribution to the TI/CoFe material system. Moreover, the increased surface magnetic anisotropy for the CoFe films grown on top of a TI layer is believed to arise mainly from the interfacial atomic intermixing. Our paper sheds light on the effects of SSs and MPE on magnetization dynamics, which offer exciting opportunities for developing TI-based spintronics.

We apply torque equilibrium spin wave theory (TESWT) to investigate an anisotropic XXZ antiferromagnetic model with Dzyaloshinskii-Moriya interaction in a triangular lattice. Considering the quasiparticle vacuum as our reference, we provide an accurate analysis of the noncollinear ground state of a frustrated triangular lattice magnet using the TESWT formalism. We elucidate the effects of quantum fluctuations on the ordering wave vector based on model system parameters. We study the single-magnon dispersion, the two-magnon continuum using the spectral function, and the Raman spectrum of bimagnon and trimagnon excitations. We present our results for the $HH,VV$, and the $HV$ polarization Raman geometry dependence of the bimagnon and trimagnon excitation spectra where $H\left(V\right)$ represents horizontal (vertical) polarization. Our calculations show that both the $HH$ and the $HV$ polarization spectra can be used to determine the degree of anisotropy of our system. We calculate the Raman spectra of ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ and ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$.

We theoretically investigate spin-wave dispersion in rare-earth magnet compounds by using first-principles calculations and a method we call the reciprocal-space algorithm. The value of the calculated exchange stiffness for ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ is within the range of reported experimental values. We find that the exchange stiffness is considerably anisotropic when only short-range exchange couplings are considered, whereas inclusion of long-range couplings weakens the anisotropy. In contrast, $R{\mathrm{Fe}}_{11}\mathrm{Ti}$ ($R=\mathrm{Y},\mathrm{Nd},\mathrm{Sm}$) shows large anisotropy in the exchange stiffness.

We show that in superconductors of type II, where Abrikosov vortices with different polarities are present, the areas where they meet and annihilate each other ($B=0$ lines) “attract” a significant part of the magnetization current $j$. This leads to redistribution of $j$ over the sample, and as a result, the rate of magnetic relaxation is reduced. This effect is significant in the case of weak dependence of the activation energy $U$ on $j$, particularly for the flux flow ($U=0$) and at early stages of flux creep ($U\gtrsim kT$). The slowdown of the relaxation is mostly pronounced in the remanent state, where the $B=0$ lines are located at the edges of the sample. In the case of flux flow, the remanent magnetization decays as $m\propto 1/t$ instead of the usual “field-on” exponential dependence $m\propto exp(-t/\tau )$. The effect is important and observable in the magnetization measurements, for instance, in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ crystals and other novel superconducting materials.

We investigate the dynamics of superconducting fluctuations in the attractive three-dimensional Hubbard model after a quench from the disordered phase to the ordered regime. While the long-time evolution is well understood in terms of dissipative time-dependent Ginzburg-Landau models with unstable potentials, early times are more demanding due to the inseparable dynamics of the pairing fluctuations and the electronic quasiparticles. Our simulation using the time-dependent fluctuation exchange approximation treats both degrees of freedom on the same footing and reveals a nonthermal electronic regime causing a nonmonotonous growth of the fluctuations. This feature is not directly captured by the Ginzburg-Landau theory but nevertheless remains observable beyond the thermalization time of the electrons. We further explore how the growth of the order parameter fluctuations leads to an opening of a pseudogap in the electronic spectrum and identify Andreev reflections as the dominant mechanism behind the gap opening.