Perpendicular magnetic tunnel junctions in the next-generation magnetic memory using current induced magnetization switching will likely rely on a material design that can enhance the perpendicular magnetic anisotropy of heterojunctions containing only light elements. Using first-principles calculations, we investigated the effect of compressive and tensile strain on the perpendicular magnetic anisotropy of light element heterostructures of Co films, Co/graphene, and Co/BN. We found that the perpendicular magnetic anisotropy of Co/graphene is greatly enhanced compared to the Co films, while that of Co/BN is reduced compared to the Co films. In addition, tensile strain can further enhance perpendicular magnetic anisotropy of Co/graphene and Co/BN heterojunctions by 48.5% and 80.8%, respectively, compared to the unstrained systems. A density of state analysis, combined with layer and orbital magnetic anisotropy contributions obtained from a second-order perturbation theory of the spin-orbit coupling, reveals that the tensile strain effect arises from the increase of the hybridization between same spin $d_{xy}$ and $d_{x^2-y^2}$ states of the surface Co film. Our results suggest that strain engineering is an effective approach to enhance the perpendicular magnetic anisotropy of light element heterostructures.

The standard interpretation of the phase diagram of type-II superconductors was developed in the 1960s and has since been considered a well-established part of classical superconductivity. However, upon closer examination a number of fundamental issues arises that leads one to question this standard picture. To address these issues we studied equilibrium properties of niobium samples near and above the upper critical field $H_{c2}$ in parallel and perpendicular magnetic fields. The samples investigated were very high quality films and single-crystal disks with the Ginzburg-Landau parameters 0.8 and 1.3, respectively. A range of complementary measurements has been performed, which include dc magnetometry, electrical transport, muon spin rotation spectroscopy, and scanning Hall-probe microscopy. Contrary to the standard scenario, we observed that a superconducting phase is present in the sample bulk above $H_{c2}$ and the field $H_{c3}$ is the same in both parallel and perpendicular fields. Our findings suggest that above $H_{c2}$ the superconducting phase forms filaments parallel to the field regardless of the field orientation. Near $H_{c2}$ the filaments preserve the hexagonal structure of the preceding vortex lattice of the mixed state, and the filament density continuously falls to zero at $H_{c3}$. Our paper has important implications for the correct interpretation of the properties of type-II superconductors and can be essential for practical applications of these materials.

We successfully fabricated and studied planar superconductor-normal-superconductor (SNS) Josephson junctions made of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) high-${\mathrm{T}}_C$ superconductor (S) and Au normal metal (N). The crucial step in fabrication was done by using a Pt/Au bilayer encapsulation of the patterned YBCO nanogap. This procedure preserves the YBCO nanogap from the degradation due to the contact with resists, water, and chemicals involved in further processing for hybrid devices. The junctions exhibit Fraunhofer-like modulation patterns and Shapiro steps when they are irradiated by a rf field. Furthermore, we analyzed the $I_C\left(T\right)$ characteristics of our hybrid junctions employing a ${\mathrm{SINI}}^{\prime }\mathrm{S}$ (superconductor–insulator–normal conductor–insulator–superconductor) model.

We investigate the quench dynamics of a one-dimensional incommensurate lattice described by the Aubry-André model by a sudden change of the strength of incommensurate potential $\mathrm{\Delta }$ and unveil that the dynamical signature of localization-delocalization transition can be characterized by the occurrence of zero points in the Loschmidt echo. For the quench process with quenching taking place between two limits of $\mathrm{\Delta }=0$ and $\mathrm{\Delta }=\infty$, we give analytical expressions of the Loschmidt echo, which indicate the existence of a series of zero points in the Loschmidt echo. For a general quench process, we calculate the Loschmidt echo numerically and analyze its statistical behavior. Our results show that if both the initial and post-quench Hamiltonian are in extended phase or localized phase, Loschmidt echo will always be greater than a positive number; however if they locate in different phases, Loschmidt echo can reach nearby zero at some time intervals.

The combined effect of magnetic field and current on domain wall motion is investigated in epitaxial [Co/Ni] microwires. Both thermally activated and flow regimes are found to be strongly affected by current. All experimental data can be understood by taking into account both adiabatic and nonadiabatic components of the spin transfer torque, the parameters of which are extracted. In the precessional flow regime, it is shown that the domain wall can move in the electron flow direction against a strong applied field, as previously observed. In addition, for a large range of applied magnetic field and injected current, a stochastic domain wall displacement after each pulse is observed. Two-dimensional micromagnetic simulations, including some disorder, show a random fluctuation of the domain wall position that qualitatively matches the experimental results.

Quantum magnets, which involve strong quantum fluctuation stemming from noncommutative properties of spin operators, have attracted much attention because of those fascinating properties. In this paper, we report detailed behaviors of the ferroelectricity, driven by the field-induced Bose-Einstein condensation of magnon quasiparticles in the quantum spin gap system ${\mathrm{TlCuCl}}_3$. Superposition of the wave functions inherent in a quantum magnet in its ground state plays a key role in the appearance of this ferroelectricity. The field dependence of the spontaneous electric polarization clearly demonstrates that the ferroelectricity is caused by the emergence of the vector spin chirality in the magnon Bose-Einstein condensate. The ferroelectricity is suggested to be significantly enhanced by quantum entanglement in the spin dimer. Furthermore, reflecting the isotropic nature of ${\mathrm{TlCuCl}}_3$, the ferroelectricity is very soft with a low electric coercive field $E_{\mathrm{r}}\simeq 0.03$ MV/m. Our analysis indicates that vector components of the electric polarization, which are not caused by the spin current mechanism, appear in ${\mathrm{TlCuCl}}_3$.

Majorana bound states (MBSs) are well established in the clean limit in chains of ferromagnetically aligned impurities deposited on conventional superconductors with finite spin-orbit coupling. Here we show that these MBSs are very robust against disorder. By performing self-consistent calculations we find that the MBSs are protected as long as the surrounding superconductor show no large signs of inhomogeneity. We also find that longer chains offer more stability against disorder for the MBSs, albeit the minigap decreases, as do increasing strengths of spin-orbit coupling and superconductivity.

We present a new model to explain the difference between the transport and spectroscopy gaps in samarium hexaboride (${\mathrm{SmB}}_6$), which has been a mystery for some time. We propose that ${\mathrm{SmB}}_6$ can be modeled as an intrinsic semiconductor with an accumulation length that diverges at cryogenic temperatures. In this model, we find a self-consistent solution to Poisson's equation in the bulk, with boundary conditions based on Fermi energy pinning due to surface charges. The solution yields band bending in the bulk; this explains the difference between the two gaps because spectroscopic methods measure the gap near the surface, while transport measures the average over the bulk. We also connect the model to transport parameters, including the Hall coefficient and thermopower, using semiclassical transport theory. The divergence of the accumulation length additionally explains the 10–12 K feature in data for these parameters, demonstrating a crossover from bulk dominated transport above this temperature to surface-dominated transport below this temperature. We find good agreement between our model and a collection of transport data from 4–40 K. This model can also be generalized to materials with similar band structure.

Tungstates $A{\mathrm{WO}}_4$ with the wolframite structure characterized by the $A{\mathrm{O}}_6$ octahedral zigzag chains along the $c$ axis can be magnetic if $A=\mathrm{Mn}$, Fe, Co, Cu, Ni. Among them, ${\mathrm{MnWO}}_4$ is a unique member with a cycloid ${\mathrm{Mn}}^{2+}$ spin order developed at low temperature, leading to an interesting type-II multiferroic behavior. However, so far no other multiferroic material in the tungstate family has been found. In this work, we present the synthesis and the systematic study of the double tungstate $\mathrm{LiFe}{\left({\mathrm{WO}}_4\right)}_2$. Experimental characterizations including structural, thermodynamic, magnetic, neutron powder diffraction, and pyroelectric measurements unambiguously confirm that $\mathrm{LiFe}{\left({\mathrm{WO}}_4\right)}_2$ is the secondly found multiferroic system in the tungstate family. The cycloidal magnetism driven ferroelectricity is also verified by density functional theory calculations. Although here the magnetic couplings between Fe ions are indirect, namely via the so-called super-super-exchanges, the temperatures of magnetic and ferroelectric transitions are surprisingly much higher than those of ${\mathrm{MnWO}}_4$.

Electric field control of magnetic structures, particularly topological defects in magnetoelectric materials, has drawn a great deal of attention in recent years, which has led to experimental success in creation and manipulation by an electric field of single magnetic defects, such as domain walls and skyrmions. In this work we explore a scenario of electric field creation of another type of topological defect, magnetic vortices and antivortices, which are characteristic of materials with easy-plane (XY) symmetry. Each magnetic (anti)vortex in magnetoelectric materials (such as type-II multiferroics) possesses a quantized magnetic and an electric charge, where the former is responsible for interaction between vortices and the latter couples the vortices to an electric field. This property of magnetic vortices opens a peculiar possibility of creation of magnetic vortex plasma by nonuniform electric fields. We show that the electric field, created by a cantilever tip, produces a “magnetic atom” with a localized spatially ordered spot of vortices (“nucleus” of the atom) surrounded by antivortices (“electronic shells”). We analytically find the vortex density distribution profile and temperature dependence of polarizability of this structure and confirm it numerically. We show that electric polarizability of the “magnetic atom” depends on temperature as $\alpha \sim 1/T^{1-\eta }$ $\left(\eta >0\right)$, which is consistent with Euclidean random matrix theory prediction.

Elucidating the interaction between magnetic moments and itinerant carriers is an important step to spintronic applications. Here, we investigate magnetic and transport properties in $d^0$ ferromagnetic SiC single crystals prepared by postimplantation pulsed laser annealing. Magnetic moments are contributed by the $p$ states of carbon atoms, but their magnetic circular dichroism is different from that in semi-insulating SiC samples. The anomalous Hall effect and negative magnetoresistance indicate the influence of $d^0$ spin order on free carriers. The ferromagnetism is relatively weak in N-implanted SiC compared with that in Al-implanted SiC after annealing. The results suggest that $d^0$ magnetic moments and itinerant carriers can interact with each other, which will facilitate the development of SiC spintronic devices with $d^0$ ferromagnetism.

Polarization selectivity of light-field-induced charge localization was investigated in an organic metal $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ with a triangular lattice. Dependences of transient reflectivity spectra on polarizations of the 7-fs pump and probe lights indicate that a short-range charge order (CO) is induced efficiently from the metallic phase for the pump polarization perpendicular to the 1010-type CO axis. A numerical solution of a time-dependent Schrödinger equation clarified that the 1010 CO is induced by field-induced re-distribution of charges cooperating with competing intersite Coulomb repulsions in the triangular lattice.

Motivated by the direct observation of electronic phase separation in first-order Mott transitions, we model the interface between thermodynamically coexisting metals and Mott insulators. We show how to model the required slab geometry and extract the electronic spectra. We construct an effective Landau free energy and compute the variation of its parameters across the phase diagram. Finally, using a linear mixture of the density and double occupancy, we identify a natural Ising order parameter which unifies the treatment of the bandwidth and filling controlled Mott transitions.

Understanding the interface formation of a conventional semiconductor with a monolayer of transition-metal dichalcogenides provides a necessary platform for the anticipated applications of dichalcogenides in electronics and optoelectronics. We report here, based on the density functional theory, that under in-plane tensile strain, a 2H semiconducting phase of the molybdenum ditelluride $\left({\mathrm{MoTe}}_2\right)$ monolayer undergoes a semiconductor-to-metal transition and in this form bonds covalently to bilayers of Ge stacked in the [111] crystal direction. This gives rise to the stable bonding configuration of the ${\mathrm{MoTe}}_2$/Ge interface with the $\pm \mathrm{K}$ valley metallic, electronic interface states exclusively of a Mo $4d$ character. The atomically sharp Mo layer represents therefore an electrically active (conductive) subsurface $\delta$-like two-dimensional profile that can exhibit a valley-Hall effect. Such system can develop into a key element of advanced semiconductor technology or a novel device concept.

We investigate the effects of strain on the topological order of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ family of topological insulators by ab initio first-principles methods. Strain can induce a topological phase transition and we present the phase diagram for the 3D topological insulators, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, and ${\mathrm{Sb}}_2{\mathrm{Se}}_3$, under combined uniaxial and biaxial strain. Their phase diagram is universal and shows metallic and insulating phases, both topologically trivial and nontrivial. In particular, uniaxial tension can drive the four compounds into a topologically trivial insulating phase. We propose a ${\mathrm{Sb}}_2{\mathrm{Te}}_3/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ heterojunction in which a strain-induced topological interface state arises in the common gap of this normal insulator–topological insulator heterojunction. Unexpectedly, the interface state is confined in the topologically trivial subsystem and is physically protected from ambient impurities. It can be switched on or off by means of uniaxial strain and therefore ${\mathrm{Sb}}_2{\mathrm{Te}}_3/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ heterojunctions provide a topological system which hosts tunable robust helical interface states with promising spintronic applications.

Understanding how surrounding environments act on the functional properties of switchable nano-objects across extended and multiple length scales is of growing interest in many areas of material science. Here, we examine in details, using a microscopic model, the interplay between the structural properties of an inert shell and a spin-active spin-crossover (SCO) core, composed of atoms which can switch thermally between the low-spin (LS) and high-spin (HS) states, a transition which is accompanied with a volume expansion. To come closer to realistic experimental data, we considered a shell having the lattice parameter of the HS state. Intensive Monte Carlo simulations, running on the spin states and atomic positions, are performed on the core-shell spin-crossover nanoparticle using an electroelastic model based on a compressible 2D lattice. A detailed analysis of the effect of the shell's size and rigidity on the magnetostructural properties of the core allows us to address the following issues: (i) the heteroelastic properties of the lattice induce a spatially inhomogeneous pressure (negative in the shell and positive in the core) which strongly distorts the lattice when the core is in the LS state, creating a visible spatial deflection of the shell/core interface; (ii) the thermally-induced first-order SCO transition of the core is significantly affected by the increase of the shell size, which lowers the transition temperature and reduces the thermal hysteresis width; (iii) the shell's rigidity dependence of the thermal hysteresis of the nanoparticle exhibited a resonance behavior when the shell's rigidity equals that of the core, a feature that is analyzed on the basis of acoustic impedance mismatch between the core and the shell. All these outcomes reflect the crucial influence of the surrounding environment on the structural properties of the nanoparticle and provide potentialities in the control of the bistability and cooperativity of the SCO nanoparticles by acting on their shell's rigidity.

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements were carried out on a multilayered cuprate superconductor ${\mathrm{Ba}}_2{\mathrm{Ca}}_5{\mathrm{Cu}}_6{\mathrm{O}}_{12}\left({\mathrm{O}}_{1-x},{\mathrm{F}}_x\right){}_2$. STM topography revealed random spot structures with the characteristic length $\le 0.5$ nm. The conductance spectra $dI/dV\left(V\right)$ show the coexistence of smaller gaps ${\mathrm{\Delta }}_S$ and large gaps (pseudogaps) ${\mathrm{\Delta }}_L$. The pseudogap-related features in the superconducting state were traced with the spatial resolution of $\sim 0.07$ nm. Here, $I$ and $V$ are the tunnel current and bias voltage, respectively. The temperature, $T$, dependence of ${\mathrm{\Delta }}_S$ follows the reduced Bardeen-Cooper-Schrieffer (BCS) dependence. The hallmark ratio $2{\mathrm{\Delta }}_S(T=0)/k_B{T}_c$ equals to 4.9, which is smaller than those of other cuprate superconductors. Here, $T_c$ is the superconducting critical temperature and $k_B$ is the Boltzmann constant. The larger gap ${\mathrm{\Delta }}_L$ survives in the normal state and even increases with $T$ above $T_c$. The $T$ dependencies of the spatial distributions for both relevant gaps $(\mathrm{\Delta }$ map), as well as for each gap separately $({\mathrm{\Delta }}_S$ and ${\mathrm{\Delta }}_L)$, were obtained. From the histogram of $\mathrm{\Delta }$ map, the averaged gap values were found to be ${\overline{\mathrm{\Delta }}}_S=\sim 24$ meV and ${\overline{\mathrm{\Delta }}}_L=\sim 79$ meV. The smaller gap ${\mathrm{\Delta }}_S$ shows a spatially homogeneous distribution while the larger gap ${\mathrm{\Delta }}_L$ is quite inhomogeneous, indicating that rather homogeneous superconductivity coexists with the patchy distributed pseudogap. The spatial variation length ${\xi }_{{\mathrm{\Delta }}_L}$ of ${\mathrm{\Delta }}_L$ correlates with the scale of the topography spot structures, being approximately 0.4 nm. This value is considerably smaller than the coherence length of this class of superconductors, suggesting that ${\mathrm{\Delta }}_L$ is strongly affected by the disorder of the apical O/F.

Theoretical predictions of pressure-induced phase transformations often become long-standing enigmas because of limitations of contemporary available experimental possibilities. Hitherto the existence of a nonicosahedral boron allotrope has been one of them. Here we report on a nonicosahedral boron allotrope, which we denoted as $\zeta$-B, with the orthorhombic $\alpha$-Ga-type structure (space group Cmce) synthesized in a diamond-anvil cell at extreme high-pressure high-temperature conditions (115 GPa and 2100 K). The structure of $\zeta$-B was solved using single-crystal synchrotron x-ray diffraction and its compressional behavior was studied in the range of very high pressures (115–135 GPa). Experimental validation of theoretical predictions reveals the degree of our up-to-date comprehension of condensed matter and promotes further development of solid-state physics and chemistry.