The electronic structure, magnetic exchange interactions, and spin-driven ferroelectricity in skyrmion host ${\mathrm{GaV}}_4{\mathrm{S}}_8$ have been studied on the basis of first-principles calculations and symmetry analysis. We find the reconstruction of band structure with the increasing value of Hubbard parameter U. The ferromagnetic ground state is determined by the symmetric exchange interaction along interplane bonds. The antisymmetric Dzyaloshinskii-Moriya interaction along intraplane bonds plays a major role in the origins of cycloidal and skyrmion phases. The exchange-striction mechanism is identified as the main origin of spin-driven polarization. Moreover, two spin-current mechanisms expressed by ${\mathbf{P}}_1\propto {\mathbf{r}}_{ab}\times ({\mathbf{S}}_a\times {\mathbf{S}}_b)$ and ${\mathbf{P}}_2\propto {\mathbf{S}}_a\times {\mathbf{S}}_b$ appear in the cycloid, while only the former mechanism appears in the skyrmion.

Twisted bilayer graphene (tBLG) represents a family of unique materials with optoelectronic properties tuned by the rotation angle between the two layers. The presented work shows an additional way of tweaking the electronic structure of tBLG by modifying the interlayer distance, for example by a small uniaxial out-of-plane compression. We have focused on the optical transition energy, which shows a clear dependence on the interlayer distance, both experimentally and theoretically.

We investigate the temperature-dependent conductivity of epitaxial multilayer graphene using THz time-domain spectroscopy and find evidence that the Fermi level in quasineutral graphene layers is pinned at the Dirac point by midgap states. We demonstrate that the scattering mechanisms result from the interplay between midgap states that dominate in the vicinity of the Dirac point and short-range potentials that govern at higher energies (>8 meV). Our results highlight the potential of multilayer epitaxial graphene for probing low-energy Dirac particles and also for THz optics.

The spin-orbit interaction of holes in zinc-blende semiconductors is much stronger than that of electrons. This makes the hole systems very attractive for possible spintronics applications. In three dimensions (3D), the dynamics of holes is described by well-known Luttinger Hamiltonian. However, most recent spintronics applications are related to two-dimensional (2D) heterostructures where dynamics in one direction is frozen due to quantum confinement. The confinement results in dimensional reduction of the Luttinger Hamiltonian, $3\mathrm{D}\to 2\text{D}$. Due to the interplay of the spin-orbit interaction, the external magnetic field, and the lateral gate potential imposed on the heterostructure, the reduction is highly nontrivial and as yet unknown. In the present work we perform the reduction and hence derive the general effective Hamiltonian which describes spintronics effects in symmetric 2D heterostructures. In particular, we do the following: (i) derive the spin-orbit interaction and the Darwin interaction related to the lateral gate potential, (ii) determine the momentum-dependent out-of-plane $g$-factor, (iii) point out that there are two independent in-plane $g$-factors, (iv) determine momentum dependencies of the in-plane $g$-factors.

An electromagnetic response of a single graphene layer to a uniform, arbitrarily strong electric field $\mathit{E}\left(t\right)$ is calculated by solving the kinetic Boltzmann equation within the relaxation-time approximation. The theory is valid at low (microwave, terahertz, infrared) frequencies satisfying the condition $\hslash \omega \lesssim 2E_F$, where $E_F$ is the Fermi energy. We investigate the saturable absorption and higher harmonics generation effects, as well as the transmission, reflection, and absorption of radiation incident on the graphene layer, as a function of the frequency and power of the incident radiation and of the ratio of the radiative to scattering damping rates. We show that the optical bistability effect, predicted in Phys. Rev. B 90, 125425 (2014) on the basis of a perturbative approach, disappears when the problem is solved exactly. We show that under the action of a high-power radiation $\left(\gtrsim 100\mathrm{kW}/{\mathrm{cm}}^2\right)$ both the reflection and absorption coefficients strongly decrease and the layer becomes transparent.

High-precision measurements of the specific volume and electrical resistivity of $\mathrm{A}{\mathrm{s}}_2\mathrm{T}{\mathrm{e}}_3$ glasses are performed under hydrostatic pressures up to 8.5 GPa. A smooth transformation and logarithmic relaxation of the density is observed at pressures higher than 1 GPa. The softening of the effective bulk modulus and the relaxation rate have a sharp maximum at 2.5 GPa, which is indicative of the existence of polyamorphism. At pressures above 4.5 GPa, a new relaxation process begins. During decompression, the pressure dependence of the compressibility exhibits a kink near 4 GPa. The electrical resistivity decreases by almost 8 orders of magnitude, most sharply in the range of 2–3.5 GPa (by 3 orders of magnitude). Smooth metallization occurs at a pressure of 5 GPa; the resistivity decreases to a value of $1.7\times {10}^{-4}\mathrm{\Omega }\mathrm{cm}$ at 8.1 GPa. Under decompression, the electrical resistivity exhibits a hysteresis and returns to values 3 orders of magnitude smaller than the initial one. The volume and electrical resistivity under normal conditions relax to quasiequilibrium values in several months. The relaxed glasses with a smaller chemical disorder have a lower electrical resistivity. The results together with the data on the structure and dynamics in $\mathrm{A}{\mathrm{s}}_2\mathrm{T}{\mathrm{e}}_3$ glasses allow conclusions on the mechanism of pressure-induced transformations.

The Hohenberg-Kohn theorem establishes a basis for mapping the exact energy functional to a model one provided that their charge densities coincide. We suggest to use a mapping in a similar spirit, but here the parameters of the formulated multiorbital model should minimize the difference between the self-consistent charge and spin densities. The analysis of the model allows for detailed understanding of the role played by different parameters of the model in the physics of interest. After finding the areas of interest in the phase diagram of the model, we return to the ab initio calculations and check if the effects discovered are confirmed or not. Because of the last controlling step, we call this approach hybrid self-consistent mapping approach (HSCMA). As an example of the approach we present a study of the effect of silicon atoms substitution by the iron atoms and vice versa on the magnetic properties in the iron silicide $\alpha -{\mathrm{FeSi}}_2$. We find that while the stoichiometric $\alpha -{\mathrm{FeSi}}_2$ is nonmagnetic, the substitutions generate different magnetic structures depending on the type of local environment of the substitutional Fe atoms. Besides, contrary to the commonly accepted statement that the destruction of the magnetic moment is controlled only by the number of Fe-Si nearest neighbors, we find that actually it is controlled by the Fe-Fe next-nearest-neighbor hopping parameter. This finding led us to the counterintuitive conclusion: an increase of Si concentration in ${\mathrm{Fe}}_{1-x}{\text{Si}}_{2+x}$ ordered alloys may lead to ferromagnetism. The calculation within GGA-to-DFT confirms this conclusion.

In superconductors with strong coupling between superconductivity and elasticity manifested in a strong dependence of transition temperature on pressure, there is an additional contribution to intervortex interactions due to the strain field generated by vortices. When vortex lines are along the $c$ axis of a tetragonal crystal, a square vortex lattice (VL) is favored at low vortex densities, because the vortex-induced strains contribution to the intervortex interactions is long range. At intermediate magnetic fields, the triangular lattice is stabilized. The triangular lattice evolves to the square lattice upon increasing magnetic field, and eventually the system locks to the square structure. We argue, however, that as magnetic field approaches the upper critical field $H_{c2}$ the elastic intervortex interactions disappear faster than the standard London interactions, so that VL should return to the triangular structure. Our results are compared to VLs observed in the heavy fermion superconductor ${\mathrm{CeCoIn}}_5$.

Using muon spin rotation and infrared spectroscopy, we study the relation between magnetism and superconductivity in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals from the underdoped to the slightly overdoped regime. We find that the Fe magnetic moment is only moderately suppressed in most of the underdoped region where it decreases more slowly than the Néel temperature $T^{\mathrm{N}}$. This applies for both the total Fe moment obtained from muon spin rotation and for the itinerant component that is deduced from the spectral weight of the spin-density-wave pair-breaking peak in the infrared response. In the moderately underdoped region, superconducting and static magnetic orders coexist on the nanoscale and compete for the same electronic states. The static magnetic moment disappears rather sharply near optimal doping, however, in the slightly overdoped region there is still an enhancement or slowing down of spin fluctuations in the superconducting state. Similar to the gap magnitude reported from specific-heat measurements, the superconducting condensate density is nearly constant in the optimally and slightly overdoped region, but exhibits a rather pronounced decrease on the underdoped side. Several of these observations are similar to the phenomenology in the electron-doped counterpart $\mathrm{Ba}({\mathrm{Fe}}_{1-y}{\mathrm{Co}}_y{)}_2{\mathrm{As}}_2$.

We show that under an ac magnetic field excitation the vortex lattice in a superconductor with a periodic array of holes can undergo a transition from a Mott-like state where each vortex is localized in a hole, to a metal-like state where the vortices get delocalized. The vortex dynamics is studied through the magnetic shielding response which is measured using a low-frequency two-coil mutual inductance technique on a disordered superconducting NbN film having a periodic array of holes. We observe that the shielding response of the vortex state is strongly dependent on the amplitude of the ac magnetic excitation. At low amplitude the shielding response varies smoothly with the excitation amplitude, corresponding to the elastic deformation of the vortex lattice. However, above a threshold value of excitation the response shows a series of sharp jumps, signaling the onset of the Mott-to-metal transition. A quantitative analysis reveals that this is a collective phenomenon which depends on the filling fraction of vortices in the antidot lattice.

We study Hanbury Brown and Twiss current cross-correlations in a three-terminal junction where a central topological superconductor (TS) nanowire, bearing Majorana bound states at its ends, is connected to two normal leads. Relying on a nonperturbative Green function formalism, our calculations allow us to provide analytical expressions for the currents and their correlations at subgap voltages, while also giving exact numerical results valid for arbitrary external bias. We show that when the normal leads are biased at voltages $V_1$ and $V_2$ smaller than the gap, the sign of the current cross-correlations is given by $-\text{sgn}\left(V_1{V}_2\right)$. In particular, this leads to positive cross-correlations for opposite voltages, a behavior in stark contrast with the one of a standard superconductor, which provides direct evidence of the presence of the Majorana zero mode at the edge of the TS. We further extend our results, varying the length of the TS (leading to an overlap of the Majorana bound states) as well as its chemical potential (driving it away from half-filling), generalizing the boundary TS Green function to those cases. In the case of opposite bias voltages, $\text{sgn}\left(V_1{V}_2\right)=-1$, driving the TS wire through the topological transition leads to a sign change of the current cross-correlations, providing yet another signature of the physics of the Majorana bound state.

We report the measurement of the statistics of photons in the nonclassical radiation emitted by a tunnel junction. This is obtained by measuring up to the fourth cumulant of the voltage fluctuations generated by the sample. When the electron shot noise generates a squeezed electromagnetic field, the measurement provides a strong signature of the presence of photon pairs, characterized by a Fano factor of the photon flux above unity.

We study the quantum spin dynamics of nearly isotropic ${\mathrm{Gd}}^{3+}$ ions entrapped in polyoxometalate molecules and diluted in crystals of a diamagnetic ${\mathrm{Y}}^{3+}$ derivative. The full energy-level spectrum and the orientations of the magnetic anisotropy axes have been determined by means of continuous-wave electron paramagnetic resonance experiments, using X-band (9–10 GHz) cavities and on-chip superconducting waveguides and 1.5-GHz resonators. The results show that seven allowed transitions between the $2S+1$ spin states can be separately addressed. Spin coherence ${\mathrm{T}}_2$ and spin-lattice relaxation ${\mathrm{T}}_1$ rates have been measured for each of these transitions in properly oriented single crystals. The results suggest that quantum spin coherence is limited by residual dipolar interactions with neighbor electronic spins. Coherent Rabi oscillations have been observed for all transitions. The Rabi frequencies increase with microwave power and agree quantitatively with predictions based on the spin Hamiltonian of the molecular spin. We argue that the spin states of each ${\mathrm{Gd}}^{3+}$ ion can be mapped onto the states of three addressable qubits (or, alternatively, of a $d=8$-level “qudit”), for which the seven allowed transitions form a universal set of operations. Within this scheme, one of the coherent oscillations observed experimentally provides an implementation of a controlled-controlled-NOT (or Toffoli) three-qubit gate.

The two-dimensional Weyl superconductor is the most elusive member of a group of materials with Weyl fermions as low-energy excitations. Here, we propose to realize this state in a heterostructure consisting of thin films of half-metal and spin-singlet superconductors. In particular, for the $d$-wave case, a very robust two-dimensional Weyl superconductor ($d$WSC) is realized independently of the orientation of the spontaneous magnetization of the half metal. The quasiparticle spectra of the $d$WSC show interesting evolution with the direction of the magnetization, featured by a series of Lifshitz transitions in the zero-energy contour of the quasiparticle spectrum. In addition, we find a transition between type-I and type-II Weyl nodes. This is an example of a two-dimensional type-II Weyl node in the presence of a superconducting correlation. For a general magnetization orientation of the half metal, the state is a combination of a superconducting component and a normal fluid component and is different from all known forms of pairings. The symmetries and topological properties of the system are analyzed. We also study the phases in the heterostructure with the half metal replaced by a ferromagnetic metal with a partially spin-polarized Fermi surface.

We investigate the ground state and finite-temperature properties of the spin-1/2 Heisenberg antiferromagnet on an infinite octa-kagome lattice by utilizing state-of-the-art tensor network-based numerical methods. It is shown that the ground state has a vanishing local magnetization and possesses a $1/2$-magnetization plateau with an up-down-up-up spin configuration. A quantum phase transition at the critical coupling ratio $J_d/J_t=0.6$ is found. When $0<J_d/J_t<0.6$, the system is in a valence bond state, where an obvious zero-magnetization plateau is observed, implying a gapful spin excitation; when $J_d/J_t>0.6$, the system exhibits a gapless excitation, in which the dimer-dimer correlation is found decaying in a power law, while the spin-spin and chiral-chiral correlation functions decay exponentially. At the isotropic point ($J_d/J_t=1$), we unveil that at low temperature $T$, the specific heat depends linearly on $T$, and the susceptibility tends to a constant for $T\to 0$, giving rise to a Wilson ratio around unity, implying that the system under interest is a fermionic algebraic quantum spin liquid.

In this paper, we present evidence for the resonances between higher subbands in an asymmetric quadruple-quantum-well (AQQW) superlattice (SL) by photoluminescence (PL) spectra. Since each QW in an AQQW is separated by thin barriers, subband interferences can easily arise. As a result, various PL branches caused by the subband resonances are observed, including PL emissions from the higher-energy Γ states, the X state in a barrier with longitudinal optical phonon replica, and long-range Γ-X transfer with Γ-X mixing. However, in conventional QW systems, the PL emissions from higher-energy subbands and their interference have not been clearly observed yet; in our system, the interferences between higher-energy subbands can be observed through PL emission, since we achieved effective electron injection into the higher subbands under very thin barriers. From these observations, we exposed the existence of such interferences and transport processes.

We consider a problem of persistent magnetization precession in a single-domain ferromagnetic nanoparticle under the driving by the spin-transfer torque. We find that the adjustment of the electronic distribution function in the particle renders this state unstable. Instead, abrupt switching of the spin orientation is predicted upon increase of the spin-transfer torque current. On the technical level, we derive an effective action of the type of Ambegaokar-Eckern-Schön action for the coupled dynamics of magnetization [gauge group $SU\left(2\right)]$ and voltage [gauge group $U\left(1\right)]$.

We study the unique physical properties of topological nodal-loop semimetals protected by the coexistence of time-reversal and inversion symmetries with negligible spin-orbit coupling. We argue that strong correlation effects occur at the surface of such systems for relatively small Hubbard interaction $U$, due to the narrow bandwidth of the “drumhead” surface states. In the Hartree-Fock approximation, at small $U$ we obtain a surface ferromagnetic phase through a continuous quantum phase transition characterized by the surface-mode divergence of the spin susceptibility, while the bulk states remain very robust against local interactions and remain nonordered. At slightly increased interaction strength, the system quickly changes from a surface ferromagnetic phase to a surface charge-ordered phase through a first-order transition. When Rashba-type spin-orbit coupling is applied to the surface states, a canted ferromagnetic phase occurs at the surface for intermediate values of $U$. The quantum critical behavior of the surface ferromagnetic transition is nontrivial in the sense that the surface spin order parameter couples to Fermi-surface excitations from both surface and bulk states. This leads to unconventional Landau damping and consequently a naïve dynamical critical exponent $z\approx 1$ when the Fermi level is close to the bulk nodal energy. We also show that, already without interactions, quantum oscillations arise due to bulk states, despite the absence of a Fermi surface when the chemical potential is tuned to the energy of the nodal loop. The bulk magnetic susceptibility diverges logarithmically whenever the nodal loop exactly overlaps with a quantized magnetic orbit in the bulk Brillouin zone. These correlation and transport phenomena are unique signatures of nodal-loop states.

We use angle-resolved photoemission spectroscopy (ARPES) to explore the electronic structure of single crystals of FeSe over a wide range of binding energies and study the effects of strong electron-electron correlations. We provide evidence for the existence of “Hubbard-like bands” at high binding energies consisting of incoherent many-body excitations originating from Fe $3d$ states in addition to the renormalized quasiparticle bands near the Fermi level. Many high-energy features of the observed ARPES data can be accounted for when incorporating the effects of strong local Coulomb interactions in calculations of the spectral function via dynamical mean-field theory, including the formation of a Hubbard-like band. This shows that over the energy scale of several eV, local correlations arising from the on-site Coulomb repulsion and Hund's coupling are essential for a proper understanding of the electronic structure of FeSe and other related iron-based superconductors.