Recently, BCS superconductivity at 203 K has been observed in a highly compressed hydrogen sulfide. We use first-principles calculations to systematically examine the effects of partially substituting chalcogen atoms on the superconductivity of hydrogen chalcogenides under high pressures. We find detailed trends of how the critical temperature changes upon increasing the V-, VI- or VII-substitution rate. These trends highlight the key roles played by low atomic mass and metallized covalent bonds. In particular, a possible record high critical temperature of 280 K is predicted in stable ${\mathrm{H}}_3{\mathrm{S}}_{0.925}{\mathrm{P}}_{0.075}$ with the $Im\overline{3}m$ structure under 250 GPa.

The remarkable robustness of high-temperature superconductors against disorder remains a controversial obstacle towards the elucidation of their pairing state. Indeed, experiments report a weak suppression rate of the transition temperature $T_c$ with disorder, significantly smaller than the universal value predicted by extensions of the conventional theory of dirty superconductors. However, in many high-$T_c$ compounds, superconductivity appears near a putative magnetic quantum critical point, suggesting that quantum fluctuations, which suppress coherent electronic spectral weight, may also promote unconventional pairing. Here we investigate theoretically the impact of disorder on such a quantum critical pairing state, considering the coupling of impurities both to the low-energy electronic states and to the pairing interaction itself. We find a significant reduction in the suppression rate of $T_c$ with disorder near the magnetic quantum critical point, shedding new light on the nature of unconventional superconductivity in correlated materials.

In magnetic materials with chiral crystal structure, it has been predicted that quasiparticle flows propagating parallel and antiparallel to the external magnetic field can show different propagating character, with its sign of nonreciprocity dependent on the chirality of the underlying bulk crystal lattice. This unique phenomenon, termed magnetochiral nonreciprocity, has previously been demonstrated for the propagating light and conduction electrons but seldom for other quasiparticles. In this study, we report the experimental observation of magnetochiral nonreciprocity of propagating magnons for a chiral-lattice ferromagnet ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ by employing the spin wave spectroscopy. We found that the sign of nonreciprocity is reversed for the opposite chirality of crystal, and also directly identified the wave-number-linear term in the spin wave dispersion associated with the Dzyaloshinskii-Moriya (DM) interaction as the origin of observed nonreciprocity. Our present results pave a route for the design of efficient spin wave diode based on the bulk crystallographic symmetry breaking and also offer a unique method to evaluate the magnitude of DM interaction in chiral-lattice bulk compounds.

Using density functional theory (DFT) and its time-dependent (TD) extension, the electronic and optical properties of the hexathiapentacene (HTP) molecule, a derivative of pentacene (PNT) obtained by symmetric substitution of the six central H atoms with S atoms, are investigated for its gas and solid phases. For the molecular structure, all-electron calculations are performed using a Gaussian localized orbital basis set in conjunction with the Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid exchange-correlation functional. Electron affinities, ionization energies, quasiparticle energy gaps, optical absorption spectra, and exciton binding energies are calculated and compared with the corresponding results for PNT, as well as with the available experimental data. The DFT and TDDFT results are also validated by performing many-body perturbation theory calculations within the $GW$ and Bethe-Salpeter equation formalisms. The functionalization with S atoms induces an increase of both ionization energies and electron affinities, a sizable reduction of the fundamental electronic gap, and a redshift of the optical absorption onset. Notably, the intensity of the first absorption peak of HTP falling in the visible region is found to be nearly tripled with respect to the pure PNT molecule. For the crystal structures, pseudopotential calculations are adopted using a plane-wave basis set together with the Perdew-Burke-Ernzerhof exchange-correlation functional empirically corrected in order to take dispersive interactions into account. The electronic excitations are also obtained within a perturbative B3LYP scheme. A comparative analysis is carried out between the ground-state and excited-state properties of crystalline HTP and PNT linking to the findings obtained for the isolated molecules.

We calculate the mobility of a two-dimensional electron gas residing at the interface of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ following a three band Boltzmann approach at low temperature, where a carrier-charged impurity scattering process is assumed to be dominant. We explain the anisotropic characteristic of the dielectric function, which is a consequence of elliptical bands close to Fermi surface. The screening effect, which weakens the long-range Coulomb interaction of the electron-impurity, is considered within the random phase approximation. Working at carrier densities high enough to neglect the spin-orbit induced splitting of the bands, we find that the mobility varies inversely with the cubic power of the carrier density ($n_{\text{2D}}^{-3}$) in good agreement with the experimental results. We also investigate the role of variable dielectric constant of ${\mathrm{SrTiO}}_3$, the multiband nature of the system, and interband interactions in exploring this result.

Shaping silicon as a spherical object is not an obvious task, especially when the object size is in the micrometer range. This has the important consequence of transforming bare silicon material in a microcavity, so it is able to confine light efficiently. Here, we have explored the inside volume of such microcavities, both in their amorphous and in their polycrystalline versions. The synthesis method, which is based on chemical vapor deposition, causes amorphous microspheres to have a high content of hydrogen that produces an onionlike distributed porous core when the microspheres are crystallized by a fast annealing regime. This substantially influences the resonant modes. However, a slow crystallization regime does not yield pores, and produces higher-quality-factor resonances that could be fitted to the Mie theory. This allows the establishment of a procedure for obtaining size calibration standards with relative errors of the order of 0.1%.

We report microwave-driven photon-assisted tunneling in a suspended carbon nanotube double quantum dot. From the resonant linewidth at a temperature of 13 mK, the charge-dephasing time is determined to be $280\pm 30$ ps. The linewidth is independent of driving frequency, but increases with increasing temperature. The moderate temperature dependence is inconsistent with expectations from electron-phonon coupling alone, but consistent with charge noise arising in the device. The extracted level of charge noise is comparable with that expected from previous measurements of a valley-spin qubit, where it was hypothesized to be the main cause of qubit decoherence. Our results suggest a possible route towards improved valley-spin qubits.

We have investigated the ${\mathrm{Cu}}_3\mathrm{Au}\left(111\right)$ surface, prepared under ultrahigh vacuum conditions by sputtering and annealing, by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and normal incidence x-ray standing waves (NIXSW). We find the surface to be depleted with Cu and enriched with Au at the same time, yielding a nominal Cu:Au ratio of 0.61:0.39 in the topmost layer. The STM images reveal that the first layer is nearly closely filled with atoms and contains a small amount of vacancies with an area concentration of about $5\%$. Together with the Au enrichment, these cause local short-range disorder of the Au $p(2\times 2)$ reconstruction. From this data, the average stoichiometry of the $p(2\times 2)$ surface unit cell is estimated at $\mathrm{C}{\mathrm{u}}_{2.22}\mathrm{A}{\mathrm{u}}_{1.44}{\square }_{0.20}$ (instead of $\mathrm{C}{\mathrm{u}}_{3.00}\mathrm{A}{\mathrm{u}}_{1.00}{\square }_{0.00}$ of the ideal surface; □ denotes an atomic vacancy site). From NIXSW we find a significant outward relaxation of both the Cu and Au atoms of the topmost layer by 0.28 Å and 0.33 Å, which corresponds to $13\%$ and $15\%$ of the (111) bulk layer spacing of $\mathrm{C}{\mathrm{u}}_3\mathrm{Au}$. We suggest that this originates from a widening of the first/second layer spacing, by $6.8\%$ and $8.8\%$ for the Cu and Au atoms, respectively, plus an additional rigid increase in the second/third layer spacing by $6.2\%$. We explain this by steric repulsions between Au atoms of the topmost layer, replacing smaller Cu atoms, and Au atoms in the second layer in combination with disorder. Finally, a lateral reconstruction, similar to that on the Au(111) surface, but with a much larger periodicity of 290 Å, is identified from LEED.

We show that the critical fields of the magnetization plateaus of the Shastry-Sutherland model decrease significantly upon increasing the ratio of inter- to intradimer coupling and accordingly that the magnetization plateaus of ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ shift to lower field under pressure, making the first two plateaus at $1/8$ and $2/15$ potentially accessible to neutron scattering experiments. These conclusions are based on the derivation of an effective classical model of interacting pinwheel-shaped spin-2 bound states using a combination of perturbative and graph-based continuous unitary transformations, showing that pinwheel crystals are indeed the lowest-energy plateau structures at low magnetization and that a simple model of intermediate-range two-body repulsion between pinwheels is able to account quantitatively for the plateau sequence.

In this paper we focus on the interplay between geometry defects and topological properties in bosonic symmetry-protected topological (SPT) phases. We start from eight copies of 3D time-reversal $\left(\mathcal{T}\right)$ invariant topological superconductors (TSC) on a crystal lattice. We melt the lattice by condensation of disclinations and therefore restore the rotation symmetry. Such a disclination condensation procedure confines the fermion and afterwards turns the system into a 3D boson topological liquid crystal (TCL). The low energy effective theory of this crystalline-liquid transition contains a topological term inherited from the geometry axion response in TSC. In addition, we investigate the interplay between dislocation and superfluid vortex on the surface of TCL. We demonstrate that the $\mathcal{T}$ and translation invariant surface state is a double $\left[e\mathcal{T}m\mathcal{T}\right]$ state with intrinsic surface topological order. We also look into the exotic behavior of dislocation in the 2D boson SPT state described by an $O\left(4\right)$ nonlinear $\sigma$ model $\left(\mathrm{NL}\sigma \mathrm{M}\right)$ with topological $\mathrm{\Theta }$ term. By dressing the $O\left(4\right)$ vector with spiral order and gauging the symmetry, the dislocation has mutual semion statistics with the gauge flux. Further reducing the $O\left(4\right)\mathrm{NL}\sigma M$ to the Ising limit, we arrive at the Levin-Gu model with stripy modulation whose dislocation has nontrivial braiding statistics.

Carbon related defects are readily incorporated in $\mathrm{GaN}$ due to its abundance during growth both with MBE and MOCVD techniques. Employing first-principles calculations, we compute the migration barriers of carbon interstitials and we discuss possible relevant mechanisms of diffusion in the wurtzite $\mathrm{GaN}$ crystal. In addition, we calculate the migration barriers for the diffusion of the native defects of the crystal, i.e., gallium and nitrogen interstitials and vacancies. The minimum energy path and the migration barriers of these defects are obtained using the nudged elastic band method with the climbing image modification. In addition, the dimer method is used to independently determine the results. The results yield a quantitative description of carbon diffusion in $\mathrm{GaN}$ allowing for the determination of the most preferable migration paths.

We present calculations of the inelastic vibrational signals in the electrical current through a graphene nanoconstriction. We find that the inelastic signals are only present when the Fermi-level position is tuned to electron transmission resonances, thus, providing a fingerprint which can link an electron transmission resonance to originate from the nanoconstriction. The calculations are based on a novel first-principles method which includes the phonon broadening due to coupling with phonons in the electrodes. We find that the signals are modified due to the strong coupling to the electrodes, however, still remain as robust fingerprints of the vibrations in the nanoconstriction. We investigate the effect of including the full self-consistent potential drop due to finite bias and gate doping on the calculations and find this to be of minor importance.

Results are presented for an open Floquet topological system represented by Dirac fermions coupled to a circularly polarized laser and an external reservoir. It is shown that when the separation between quasienergy bands becomes small, and comparable to the coupling strength to the reservoir, the reduced density matrix in the Floquet basis, even at steady state, has nonzero off-diagonal elements, with the magnitude of the off-diagonal elements increasing with the strength of the coupling to the reservoir. In contrast, the coupling to the reservoir only weakly affects the diagonal elements, hence inducing an effective coherence. The steady-state reduced density matrix synchronizes with the periodic drive, and a Fourier analysis allows the extraction of the occupation probabilities of the Floquet quasienergy levels. The lack of detailed balance at steady state is quantified in terms of an entropy-production rate, and it is shown that this equals the heat current flowing out of the system and into the reservoir. It is also shown that the entropy-production rate mainly depends on the off-diagonal components of the Floquet density matrix. Thus, a stronger coupling to the reservoir leads to an enhanced entropy-production rate, implying a more efficient removal of heat from the system, which in turn helps the system maintain coherence. Analytic expressions in the vicinity of the Dirac point are derived which highlights these results, and also indicates how the reservoir may be engineered to enhance the coherence of the system.

We present here a neutron diffraction study, both in zero field and as a function of magnetic field, of the magnetic structure of the tetragonal intermetallic ${\mathrm{EuNiGe}}_3$ on a single crystalline sample. This material is known to undergo a cascade of transitions, first at 13.2 K towards an incommensurate modulated magnetic structure, then at 10.5 K to an antiferromagnetic structure. We show here that the low-temperature phase presents a spiral moment arrangement with wave vector $\mathit{k}=(\frac{1}{4},\delta ,0)$. For a magnetic field applied along the tetragonal $c$ axis, the square root of the scattering intensity of the (1 0 1) reflection matches very well the complex metamagnetic behavior of the magnetization along $c$ measured previously. For the magnetic field applied along the $b$ axis, two magnetic transitions are observed below the transition to a fully polarized state.

Ultrathin Pt films grown on insulating ferrimagnetic $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ (111) epitaxial films display a magnetoresistance upon rotating the magnetization of the magnetic layer. We report here x-ray magnetic circular dichroism (XMCD) recorded at Pt-$L_{2,3}$ and Pt-$M_3$ edges. The results indicate that the Pt magnetic moment, if any, is below the detection limit ($<0.001{\mu }_{\mathrm{B}}/\mathrm{Pt}$), thus strongly favoring the view that the presence of $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ does not induce the formation of magnetic moments in Pt. Therefore, the observed magnetoresistance cannot be attributed to some sort of proximity-induced magnetic moments at Pt ions and subsequent magnetic-field dependent scattering. It thus follows that either bulk (spin Hall and inverse spin Hall effects) or interface (Rashba) spin-orbit related effects dominate the observed magnetoresistance. Furthermore, comparison of bulk magnetization and XMCD data at (Fe,Co)-$L_{2,3}$ edges suggests the presence of some spin disorder in the $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ layer which may be relevant for the observed anomalous nonsaturating field dependence of spin Hall magnetoresistance.

Despite decades-long fascination, the difficulty of maintaining high lattice symmetry in frustrated nonbipartite $S=\frac{1}{2}$ materials that can also be made into high-quality single crystals has been a persistent challenge. Here we report magnetization studies of a single-crystal sample of barlowite, ${\mathrm{Cu}}_4{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$, which has a geometrically perfect kagome motif. At $T\le 4.2$ K and $35\le {\mu }_{0}H\le 65$ T, the interlayer spins are fully polarized, and the kagome-intrinsic magnetization is consistent with a Heisenberg model having $J/k_{\mathrm{B}}=-180$ K. Several field-driven anomalies are observed, having varied scalings with temperature. At an applied field, kagome disorder caused by the interlayer spins is smaller than that in herbertsmithite. At $T\le$ 15 K, the bulk magnetic moment comes from the interlayer spins. An almost coplanar spin order suggests that the magnitude of in-plane Dzyaloshinskii-Moriya interaction is smaller than 0.006(6) $J$. On the other hand, the possibility of a spin-liquid state in the kagome lattice coexisting with ordered interlayer spins is left open.

We consider a class of one-dimensional compass models with an XYZ-YZX type of three-site exchange interaction in an external magnetic field. We present the exact solution derived by means of Jordan-Wigner transformation, and study the excitation gap, spin correlations, and establish the phase diagram. Besides the canted antiferromagnetic and polarized phases, the three-site interactions induce two distinct chiral phases, corresponding to gapless spinless-fermion systems having two or four Fermi points. We find that the $z$ component of the scalar chirality operator can act as an order parameter for these chiral phases. We also find that the thermodynamic quantities including the Wilson ratio can characterize the liquid phases. Finally, a nontrivial magnetoelectric effect is explored, and we show that the polarization can be manipulated by the magnetic field in the absence of electric field.

The anomalous Hall effect in the ultrathin film regime is investigated in Fe(001)(1–3 nm) films epitaxial on MgO(001). The logarithmic localization correction to longitudinal resistivity and anomalous Hall resistivity are observed at low temperature. We identify that the coefficient of skew scattering has a reduction from metallic to localized regime, while the contribution of side jump has inconspicuous change except for a small drop below 10 K. Furthermore, we discover that the intrinsic anomalous Hall conductivity decreases with the reduction of thickness below 2 nm. Our results provide unambiguous experimental evidence to clarify the problem of localization correction to the anomalous Hall effect.

Exchange interactions between isolated pairs of spin centers in diamond have been calculated, based on an accurate atomistic electronic structure for diamond and any impurity atoms, for spin-center separations of up to 2 nm. The exchange interactions exceed dipolar interactions for spin-center separations of less than 3 nm. ${\mathrm{NV}}^{-}$ spin centers, which involve two lattice sites which differ from the host, interact very differently depending on the relative orientations of the symmetry axis of the spin center and the radius vector connecting the pair. Exchange interactions between transition-metal dopants behave similarly to those of ${\mathrm{NV}}^{-}$ centers. The Mn-Mn exchange interaction decays with a much longer length scale than the Cr-Cr and Ni-Ni exchange interactions, exceeding dipolar interactions for Mn-Mn separations of less than 5 nm. Calculations of these highly anisotropic and spin-center-dependent interactions provide the potential for the design of spin-spin interactions for novel nanomagnetic structures.

The phase diagram of the Be-H system is measured up to 100 GPa. ${\mathrm{BeH}}_2$ is the only stoichiometry observed. Three crystalline phases are identified using synchrotron x-ray diffraction. The $Ibam$ structure, already known at ambient pressure, is stable up to 27 GPa. The $P{4}_1{2}_{1}2$ structure, isotypical to the crystoballite-low ${\mathrm{SiO}}_2$ structure, is stable above 600 K over the same pressure range. The well-known $P\overline{3}m1$ layered structure of transition metal dichalcogenides is stable above 80 GPa, making ${\mathrm{BeH}}_2$ a quasi-two-dimensional hydride. Surprisingly, between 27 and 72 GPa, a miscibility gap opens up in the Be-H system and an equilibrium between pure Be and pure H is observed. That is at odds with the current prevailing view of an increase in solubility of H in metals under pressure and contradicts calculations of stable compounds in the Be-H system.