Superconductivity results from a Bose condensate of Cooper-paired electrons with a macroscopic quantum wave function. Dramatic effects can occur when the region of the condensate is shaped and confined to the nanometer scale. Recent progress in nanostructured superconductors has revealed a route to topological superconductivity, with possible applications in quantum computing. However, challenges remain in controlling the shape and size of specific superconducting materials. Here, we report a method to create nanostructured superconductors by partial crystallization of the half-Heusler material, YPtBi. Superconducting islands, with diameters in the range of 100 nm, were reproducibly created by local current annealing of disordered YPtBi in the tunneling junction of a scanning tunneling microscope. We characterize the superconducting island properties by scanning tunneling spectroscopic measurements to determine the gap energy, critical temperature and field, coherence length, and vortex formations. These results show unique properties of a confined superconductor and demonstrate that this method holds promise to create tailored superconductors for a wide variety of nanometer scale applications.

Magnetoelectric effects in the chiral magnet ${\mathrm{CsCuCl}}_3$ have been investigated through magnetization and electric polarization measurements in pulsed high magnetic fields. If the magnetic field is applied normal to the spin chiral $c$ axis, a plateau and a jump in magnetization are observed. Between the plateau and the jump in magnetization, a small but significant electric polarization of about $0.25\mu \mathrm{C}/{\mathrm{m}}^2$ along the $a$ axis is observed with the field applied almost perpendicular to the $ac$ plane. In addition, the paramagnetoelectric effect expected from the point group symmetry is confirmed. The emergence of electric polarization is explained by the cooperation of the local electric polarization on the chiral solitonic spin arrangement and the paramagnetoelectric effect. Hence, we interpret this multiferroic phase in ${\mathrm{CsCuCl}}_3$ as a polar solitonic phase.

Using large supercells models and the KKRnano multiple scattering approach, statistically meaningful information is obtained on the distribution of local densities of states, magnetic moments, and distance-dependent exchange interactions for interstitial N or O or Ga vacancies in Gd-doped GaN. The exchange interactions between N interstitials (${\mathrm{N}}_i$) and ${\mathrm{N}}_i$ with Gd are found to be short-ranged and mainly antiferromagnetic, while exchange interactions between Gd are negligible. For O interstitials, the ferro- and antiferromagnetic interactions between Gd and ${\mathrm{O}}_i$ are roughly canceling each other, and the ${\mathrm{O}}_i-{\mathrm{O}}_i$ interactions are ferromagnetic but very short-ranged. The Fermi level dependence of these interactions is studied. The difference between ${\mathrm{N}}_i$ and ${\mathrm{O}}_i$ behavior is related to the filling of up and down spin partial densities of states, which promotes antiferromagnetic superexchange and ferromagnetic double exchange for ${\mathrm{N}}_i$ and ${\mathrm{O}}_i$, respectively. On the other hand, Ga vacancies provide significantly stronger and more robust ferromagnetic interactions between moments localized on N near the vacancies and may reach the percolation threshold for concentrations of order 5%. The role of strain in films grown under different conditions on the vacancy concentration is discussed.

Simultaneous detection of terahertz (THz) emission and transient magneto-optical response is employed to study ultrafast laser-induced magnetization dynamics in three different types of amorphous metallic alloys: Co, GdFeCo, and NdFeCo. A satisfactory agreement between the dynamics revealed with the help of these two techniques is obtained for Co and GdFeCo. For NdFeCo the THz emission indicates faster dynamics than the magneto-optical response. This observation indicates that in addition to spin dynamics of Fe, ultrafast laser excitation of NdFeCo triggers faster magnetization dynamics of Nd originating from its orbital momentum.

We provide a general method to calculate and maximize the purity of a qubit interacting with an anisotropic non-Markovian environment. Counter to intuition, we find that the purity is often maximized by preparing and storing the qubit in a superposition of noninteracting eigenstates. For a model relevant to decoherence of a heavy-hole spin qubit in a quantum dot or for a singlet-triplet qubit for two electrons in a double quantum dot, we show that preparation of the qubit in its noninteracting ground state can actually be the worst choice to maximize purity. We further give analytical results for spin-echo envelope modulations of arbitrary spin components of a hole spin in a quantum dot, going beyond a standard secular approximation. We account for general dynamics in the presence of a pure-dephasing process and identify a crossover time scale at which it is again advantageous to initialize the qubit in the noninteracting ground state. Finally, we consider a general two-axis dynamical decoupling sequence and determine initial conditions that maximize purity, minimizing leakage to the environment.

The usual paradigm in the theory of electron transport is related to the fact that the dielectric permittivity of the insulator is assumed to be constant, with no time dispersion. We take into account the “slow” polarization dynamics of the dielectric layers in the tunnel barriers in the fluctuating electric fields induced by single-electron tunneling events and study transport in the single-electron transistor (SET). Here “slow” dielectric implies a time scale that is slow compared to the characteristic time scales of the SET charging-discharging effects. We show that for strong enough polarizability, such that the induced charge on the island is comparable to the elementary charge, the transport properties of the SET substantially deviate from the known results of transport theory of the SET. In particular, the Coulomb blockade is more pronounced at finite temperature, the conductance peaks change their shape, and the current-voltage characteristics show the memory effect (hysteresis). However, in contrast to SETs with ferroelectric tunnel junctions, here the periodicity of the conductance in the gate voltage is not broken; instead, the period strongly depends on the polarizability of the gate dielectric. We uncover the fine structure of the hysteresis effect where the “large” hysteresis loop may include a number of “smaller” loops. Also we predict the memory effect in the current-voltage characteristics $I\left(V\right)$, with $I\left(V\right)\ne -I(-V)$.

We present an effective hydrodynamic theory of electronic transport in graphene in the interaction-dominated regime. We derive the emergent hydrodynamic description from the microscopic Boltzmann kinetic equation taking into account dissipation due to Coulomb interaction and find the viscosity of Dirac fermions in graphene for arbitrary densities. The viscous terms have a dramatic effect on transport coefficients in clean samples at high temperatures. Within linear response, we show that viscosity manifests itself in the nonlocal conductivity as well as dispersion of hydrodynamic plasmons. Beyond linear response, we apply the derived nonlinear hydrodynamics to the problem of hot-spot relaxation in graphene.

Peeling, shearing, and sliding are important mechanical phenomena in van der Waals solids. However, theoretically they have been studied mostly using minimal periodic cells and in the context of accurate quantum simulations. Here we investigate the peeling of large-scale multilayer graphene stacks with varying thicknesses, stackings, and peeling directions by using classical molecular dynamics simulations with a registry-dependent interlayer potential. Simulations show that, while at large scale the peeling proceeds smoothly, at small scale the registry shifts and sliding patterns of the layers are unexpectedly intricate and depend both on the initial stacking and on the peeling direction. These observations indicate that peeling and concomitant kink formations may well transform stacking order and thereby profoundly influence the electronic structures of such multilayer solids.

Magnetic impurities in three-dimensional Dirac and Weyl systems are shown to exhibit a fascinatingly diverse range of Kondo physics, with distinctive experimental spectroscopic signatures. When the Fermi level is precisely at the Dirac point, Dirac semimetals are in fact unlikely candidates for a Kondo effect due to the pseudogapped density of states. However, the influence of a nearby quantum critical point leads to the unconventional evolution of Kondo physics for even tiny deviations in the chemical potential. Separating the degenerate Dirac nodes produces a Weyl phase: Time-reversal symmetry breaking precludes Kondo physics due to an effective impurity magnetic field, but different Kondo variants are accessible in time-reversal invariant Weyl systems.

We consider an interaction quench in the critical $\text{spin}-\frac{1}{2}$ Heisenberg XXZ chain. We numerically compute the time evolution of the two-point correlation functions of spin operators in the thermodynamic limit and compare the results to predictions obtained in the framework of the Luttinger liquid approximation. We find that the transverse correlation function $\langle S_j^x{S}_{j+\ell }^x\rangle$ agrees with the Luttinger model prediction to a surprising level of accuracy. The agreement for the longitudinal two-point function $\langle S_j^z{S}_{j+\ell }^z\rangle$ is found to be much poorer. We speculate that this difference between transverse and longitudinal correlations has its origin in the locality properties of the respective spin operator with respect to the underlying fermionic modes.

The distribution of Yang-Lee zeros in the ferromagnetic Ising model in both two and three dimensions is studied on the complex field plane directly in the thermodynamic limit via the tensor network methods. The partition function is represented as a contraction of a tensor network and is efficiently evaluated with an iterative tensor renormalization scheme. The free-energy density and the magnetization are computed on the complex field plane. Via the discontinuity of the magnetization, the density of the Yang-Lee zeros is obtained to lie on the unit circle, consistent with the Lee-Yang circle theorem. Distinct features are observed at different temperatures—below, above, and at the critical temperature. Application of the tensor network approach is also made to the $q$-state Potts models in both two and three dimensions and a previous debate on whether, in the thermodynamic limit, the Yang-Lee zeros lie on a unit circle for $q>2$ is resolved: they clearly do not lie on a unit circle except at the zero temperature. For the Potts models $\left(q=3,4,5,6\right)$ investigated in two dimensions, as the temperature is lowered the radius of the zeros at a fixed angle from the real axis shrinks exponentially towards unity with the inverse temperature.

Because of its narrow split-off conduction band, doping of ${\mathrm{V}}_2{\mathrm{O}}_5$ leads to interesting strongly correlated electrons. We study the effects of doping on ${\mathrm{V}}_2{\mathrm{O}}_5$'s electronic and magnetic properties, either by adding electrons compensated by an artificial homogeneous background, or a virtual crystal approximation (VCA), by changing the atomic number $Z_{\mathrm{V}}$, so as to keep charge neutrality, or by explicitly introducing Na as a dopant. The former two are considered as a way to simulate injected charge by gating, the latter occurs in the vanadium bronze ${\mathrm{NaV}}_2{\mathrm{O}}_5$. We also simulate ${\mathrm{Na}}_{1-x}{\mathrm{V}}_2{\mathrm{O}}_5$ using a virtual crystal approximation by changing the atomic number $10\le Z_{\mathrm{Na}}\le 11$. The differences in the band structure, which result from how the electrons added to the band are compensated by positive charge in the three models, are compared. The electronic band structures are calculated using the quasiparticle self-consistent $QSGW$ method including a lattice polarization correction and the local spin density functional method with Hubbard-$U$ corrections (LSDA+$U$). For ${\mathrm{NaV}}_2{\mathrm{O}}_5$, the half-filling leads to a splitting of the up- and down-spin lowest $d_{xy}$ band. The spins are found to prefer an antiferromagnetic ordering along the chain direction. Other spin configurations are shown to have higher energy and the exchange interactions are extracted and compared with literature. The optical conductivities are calculated and compared with experiment. Similar results are found for simply doping the band compensated by a background or virtual crystal approximation. However, the position of the occupied bands depends on the method chosen for compensating the charge. The most realistic way to simulate gating in which the compensating charge is kept away from the ${\mathrm{V}}_2{\mathrm{O}}_5$ layer is the VCA with varying $Z_{\mathrm{Na}}$. The splitting between the up- and down-spin bands depends on the filling. We find that below a certain concentration of about 0.88 electrons per V, the FM arrangement becomes preferable over the antiferromagnetic one. The magnetic moments then gradually decrease as we lower the filling of the split-off band.

Antimony sulphide ${\mathrm{Sb}}_2{\mathrm{S}}_3$ has emerged as a promising material for a variety of energy applications ranging from solar cells to thermoelectrics and solid-state batteries. The most distinctive feature of ${\mathrm{Sb}}_2{\mathrm{S}}_3$ is its crystal structure, which consists of parallel 1-nm-wide ribbons held together by weak van der Waals forces. This structure clearly suggests that it should be possible to isolate individual ${\mathrm{Sb}}_2{\mathrm{S}}_3$ ribbons using micromechanical or liquid-phase exfoliation techniques. However, it is not clear yet how to identify the ribbons postexfoliation using standard optical probes. Using state-of-the-art first-principles calculations based on many-body perturbation theory, here we show that individual ribbons of ${\mathrm{Sb}}_2{\mathrm{S}}_3$ carry optical signatures clearly distinct from those of bulk ${\mathrm{Sb}}_2{\mathrm{S}}_3$. In particular, we find a large blueshift of the optical absorption edge (from 1.38 to 2.30 eV) resulting from the interplay between a reduced screening and the formation of bound excitons. In addition, we observe a transition from an indirect band gap to a direct gap, suggesting an enhanced photoluminescence in the green. These unique fingerprints will enable extending the research on van der Waals materials to the case of one-dimensional chalchogenides.

Valley currents and nonlocal resistances of graphene nanostructures with broken inversion symmetry are considered theoretically in the linear response regime. Scattering state wave functions of electrons entering the nanostructure from the contacts represented by groups of ideal leads are calculated by solving the Lippmann-Schwinger equation and are projected onto the valley state subspaces to obtain the valley velocity fields and total valley currents in the nanostructures. In the tunneling regime when the Fermi energy is in the spectral gap around the Dirac point energy, inversion symmetry breaking is found to result in strong enhancement of the nonlocal four-terminal Büttiker-Landauer resistance and in valley currents several times stronger than the conventional electric current. These strong valley currents are the direct result of the injection of electrons from a contact into the graphene in the tunneling regime. They are chiral and occur near contacts from which electrons are injected into the nanostructure whether or not a net electric current flows through the contact. It is also pointed out that enhanced nonlocal resistances in the linear response regime are not a signature of valley currents arising from the combined effect of the electric field and Berry curvature on the velocities of electrons.

Phase-field simulations demonstrate that the polarization order-parameter field in the Ginzburg-Landau-Devonshire model of rhombohedral ferroelectric ${\mathrm{BaTiO}}_3$ allows for an interesting linear defect, stable under simple periodic boundary conditions. This linear defect, here called the Ising line, can be described as an about 2-nm-thick intrinsic paraelectric nanorod acting as a highly mobile borderline between finite portions of Bloch-like domain walls of opposite helicity. These Ising lines play the role of domain boundaries associated with the Ising-to-Bloch domain-wall phase transition.

We investigated theoretically electronic and magnetic properties of the perovskite material ${\mathrm{SrCoO}}_{3-\delta }$ with $\delta \le 0.15$ using a projector-augmented plane-wave method and a Green's function method. This material is known from various experiments to be ferromagnetic with a Curie temperature of $260\mathrm{K}$ to $305\mathrm{K}$ and a magnetic moment of $1.5{\mu }_{\mathrm{B}}$ to $3.0{\mu }_{\mathrm{B}}$. Applying the magnetic force theorem as it is formulated within the Green's function method, we calculated for ${\mathrm{SrCoO}}_{3-\delta }$ the magnetic exchange interactions and estimated the Curie temperature. Including correlation effects by an effective $U$ parameter within the $\mathrm{GGA}+U$ approach and verifying this by hybrid functional calculations, we obtained the Curie temperatures in dependence of the oxygen deficiency close to the experimental values.

We report on magnetotransport properties on $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3/\mathrm{MgO}/\mathrm{Fe}$ tunnel junctions grown epitaxially on top of (001)-oriented $\mathrm{SrTi}{\mathrm{O}}_3$ substrates by sputtering. It is shown that the magnetoresistive response depends critically on the $\mathrm{MgO}/\mathrm{Fe}$ interfacial properties. The appearance of an $\mathrm{Fe}{\mathrm{O}}_X$ layer by the interface destroys the ${\mathrm{\Delta }}_1$ symmetry filtering effect of the $\mathrm{MgO}/\mathrm{Fe}$ system and only a small negative tunneling magnetoresistance (TMR) $(\sim -3\%)$ is measured. However, in annealed samples a switchover from positive TMR ($\sim +25\%$ at 70 K) to negative TMR $(\sim -1\%)$ is observed around 120 K. This change is associated with the transition from semiconducting at high $T$ to insulating at low $T$ taking place at the Verwey transition $(T_{\mathrm{V}}\sim 120\mathrm{K})$ in $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4,$ thus suggesting the formation of a very thin slab of magnetite at the $\mathrm{MgO}/\mathrm{Fe}$ interface during annealing treatments. These results highlight the relevance of interfacial properties on the tunneling conduction process and how it can be substantially modified through appropriate interface engineering.

We show the existence of massive Dirac fermions in the electronic band structures of a few Fe atomic layers with perpendicular magnetization. Based on a tight-binding model fitted to ab initio band structure, we observe four distinct massive Dirac fermions near the Fermi level, which result from atomic spin-orbit coupling of Fe and a band inversion between an Fe $4s\text{−}3d_{x^2-y^2}$ hybrid orbital band and a $3d_{xy}$ orbital band. These led to a valence band with a finite Chern integer $\left(+2\right)$ and chiral edge modes near the Fermi level. When the chemical potential is set inside the Dirac gap by carrier doping, the Hall conductivity exhibits a plateaulike structure with a quantized value $2(e^2/h)$, and orbital magnetization shows a prominent increase, the latter of which is mostly due to the chiral orbital motion of electrons along the edge modes. We discuss the stability of the Dirac fermions in the Fe(001) monolayer on an MgO(001) substrate and an Fe(001) bilayer case.

We carry out large-scale micromagnetic simulations that demonstrate that due to topological constraints, internal domain walls (Bloch lines) within extended domain walls are more robust than domain walls in nanowires. Thus, the possibility of spintronics applications based on their motion channeled along domain walls emerges. Internal domain walls are nucleated within domain walls in perpendicularly magnetized media concurrent with a Walker breakdown-like abrupt reduction of the domain wall velocity above a threshold driving force, and may also be generated within pinned, localized domain walls. We observe fast field and current driven internal domain wall dynamics without a Walker breakdown along pinned domain walls, originating from topological protection of the internal domain wall structure due to the surrounding out-of-plane domains.

The mechanical properties and electronic structure of recently synthesized $\mathrm{Pt}{\mathrm{N}}_2$, proposed as a potential candidate for superhard materials, have been investigated by means of density functional theory. Although it shows a clear band gap indicating a covalent bonding nature, the calculated shear moduli and ideal strengths of both proposed $\mathrm{Pt}{\mathrm{N}}_2$ polymorphs are much lower than those of $\mathrm{Re}{\mathrm{B}}_2$, suggesting that it should be weaker than $\mathrm{Re}{\mathrm{B}}_2$, whose load-invariant hardness is less than 30 GPa. The anisotropic strength of the pyrite $\mathrm{Pt}{\mathrm{N}}_2$ polymorph is significantly higher than that of the fluorite polymorph due to a larger covalent contribution. The shear instability for both polymorphs occurs in a cleavagelike mode between the weakly bonded crystal planes. This behavior is different from transition-metal (TM) diborides where the TM-TM or TM-B bonds are the carriers of the shear instability.