We present a theory for understanding the exchange interaction between electron spins in neighboring quantum dots, either by changing the detuning of the two quantum dots or independently tuning the tunneling barrier between quantum dots. The Hubbard model and a more realistic confining-potential model are used to investigate how the tilting and barrier control affect the effective exchange coupling and thus the gate fidelity in both the detuning and symmetric regimes. We show that the exchange coupling is less sensitive to the charge noise through tunnel barrier control (while allowing for exchange coupling operations on a sweet spot where the exchange interaction has zero derivative with respect to the detuning). Both GaAs and Si quantum dots are considered, and we compare our results with experimental data showing qualitative agreements. Our results answer the open question of why barrier gates are preferable to tilt gates for exchange-based gate operations.

We have investigated the electronic, phononic, and superconducting properties of the transition-metal dichalcogenide superconductor ${\mathrm{PdTe}}_2$, and explored the origin of different superconducting behaviors between ${\mathrm{PdTe}}_2$ and its isostructural ${\mathrm{PtTe}}_2$ that is nonsuperconducting. We have found that the saddle-point van Hove singularity (vHs) near the Fermi level, which interacts strongly with Te phonon modes, plays an important role in the BCS-type superconductivity of ${\mathrm{PdTe}}_2$. We show that, with electron doping, the vHs in ${\mathrm{PdTe}}_2$ shifts down toward the Fermi level to enhance $T_c$, as is consistent with the observed enhancement of $T_c$ in Cu-doped ${\mathrm{PdTe}}_2$. We ascribe the absence of superconductivity in ${\mathrm{PtTe}}_2$ to the different dispersion behavior of the saddle-point vHs band from that of ${\mathrm{PdTe}}_2$. We also suggest that this difference in the vHs band behaviors is responsible for the different structural responses of ${\mathrm{PdTe}}_2$ and ${\mathrm{PtTe}}_2$ to external pressure.

Topological semimetals are a new class of condensed matter systems with nontrivial electronic structure topology. Their unusual observable properties may often be understood in terms of quantum anomalies. In particular, Weyl and Dirac semimetals, which have point band-touching nodes, are characterized by the chiral anomaly, which leads to the Fermi arc surface states, anomalous Hall effect, negative longitudinal magnetoresistance, and planar Hall effect. In this paper, we explore analogous phenomena in nodal line semimetals. We demonstrate that such semimetals realize a three-dimensional analog of the parity anomaly, which is a known property of two-dimensional Dirac semimetals arising, for example, on the surface of a three-dimensional topological insulator. We relate one of the characteristic properties of nodal line semimetals, namely, the drumhead surface states, to this anomaly, and derive the field theory, which encodes the corresponding anomalous response.

Van der Waals (vdW) heterostructures are promising candidates for building blocks in novel electronic and optoelectronic devices with tailored properties, since their electronic action is dominated by the band alignments upon their contact. In this work, we analyze 10 vdW heterobilayers based on tin dichalcogenides by first-principles calculations. Structural studies show that all systems are stable, and that commensurability leads to smaller interlayer distances. Using hybrid functional calculations, we derive electronic properties and band alignments for all the heterosystems and isolated two-dimensional (2D) crystals. Natural band offsets are derived from calculated electron affinities and ionization energies of 11 freestanding 2D crystals. They are compared with band alignments in true heterojunctions, using a quantum mechanical criterion, and available experimental data. For the ${\mathrm{hBN}/\mathrm{SnSe}}_2$ system, we show that hBN suffers an increase in band gap, while leaving almost unchanged the electronic properties of ${\mathrm{SnSe}}_2$. Similarly, ${\mathrm{MX}}_2$ (M = Mo, W; X = S, Se) over ${\mathrm{SnX}}_2$ preserve the natural discontinuities from each side of the heterobilayer. Significant charge transfer occurs in junctions with graphene, which becomes p-doped and forms an Ohmic contact with ${\mathrm{SnX}}_2$. Zirconium and hafnium dichalcogenides display stronger interlayer interactions, leading to larger shifts in band alignments with tin dichalcogenides. Significant orbital overlap is found, which creates zero conduction band offset systems. The validity of the Anderson electron affinity rule is discussed. Failures of this model are traced back to interlayer interaction, band hybridization, and quantum dipoles. The systematic work sheds light on interfacial engineering for future vdW electronic and optoelectronic devices.

We have measured the equilibrium melting pressure of helium-4 as a function of the crystal size. Negative compressibility of a liquid with an inclusion of solid seed is predicted theoretically and verified experimentally with helium-4 crystal-superfluid system at 0.15 K. This two-phase system is shown to be stable if the crystal size is large enough, which is proven by the experiment. Crystal seeds that are too small spontaneously either melt completely or grow to a large enough size.

We study thermalization in open quantum systems using the Lindblad formalism. A method that both thermalizes and couples to Lindblad operators only at edges of the system is introduced. Our method leads to a Gibbs state of the system, satisfies fluctuation-dissipation relations, and applies both to integrable and nonintegrable systems. Possible applications of the method include the study of systems coupled locally to multiple reservoirs. Our analysis highlights the limits of applicability of the Lindblad approach to study strongly driven systems.

We study the topological Hall (TH) effect in a diffusive ferromagnetic metal thin film by solving a Boltzmann transport equation in the presence of spin-flip scattering. A generalized spin-diffusion equation is derived which contains an additional source term associated with the gradient of the emergent magnetic field that arises from skyrmions. Because of the source term, spin accumulation may build up in the vicinity of the skyrmions. This gives rise to a spin-polarized diffusion current that in general suppresses the bulk TH current. Only when the spin-diffusion length is much smaller than the skyrmion size does the TH resistivity approach the value derived by Bruno et al. [Phys. Rev. Lett. 93, 096806 (2004)]. We derive a general expression of the TH resistivity that applies to thin-film geometries with spin-flip scattering, and show that the corrections to the TH resistivity become large when the size of room temperature skyrmions is further reduced to tens of nanometers.

Current-driven spin-orbit torques are investigated in a heterostructure composed of a ferromagnet deposited on top of a three-dimensional topological insulator using the linear response formalism. We develop a tight-binding model of the heterostructure adopting a minimal interfacial hybridization scheme that promotes induced magnetic exchange on the topological surface states, as well as induced Rashba-like spin-orbit coupling in the ferromagnet. Therefore our model accounts for the spin Hall effect from bulk states together with inverse spin galvanic and magnetoelectric effects at the interface on equal footing. By varying the transport energy across the band structure, we uncover a crossover from surface-dominated to bulk-dominated transport regimes. We show that the spin density profile and the nature of the spin-orbit torques differ substantially in both regimes. Our results, which compare favorably with experimental observations, demonstrate that the large dampinglike torque reported recently is more likely attributed to the Berry curvature of interfacial states, while spin Hall torque remains small even in the bulk-dominated regime.

We use a scanning tunneling microscope to study the superconducting density of states and vortex lattice of single crystals of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$. This material has a critical temperature of $T_c=35$ K, one of the highest among stoichiometric iron based superconductors (FeBSCs), and is comparable to $T_c$ found near optimal doping in other FeBSCs. We observe quasiparticle scattering from defects with a pattern related to interband scattering between zone centered hole sheets. We measure the tunneling conductance in vortex cores and find a peak due to Caroli–de Gennes–Matricon bound states. The peak is located above the Fermi level, showing that ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ is a clean superconductor with vortex core bound states close to the so-called extreme quantum limit. We identify locations where the superconducting order parameter is strongly suppressed due to pair breaking. Vortices are pinned at these locations, and the length scale of the suppression of the order parameter is of order of the vortex core size. As a consequence, the vortex lattice is disordered up to 8 T.

We explore the quantum phases emerging from the interplay between spin and motional degrees of freedom of a one-dimensional quantum fluid of spinful fermionic atoms, effectively interacting via a photon-mediating mechanism with tunable sign and strength $g$, as it can be realized in present-day experiments with optical cavities. We find the emergence, in the very same system, of spin- and atomic-density wave ordering, accompanied by the occurrence of superfluidity for $g>0$, while cavity photons are seen to drive strong correlations at all $g$ values, with fermionic character for $g>0$, and bosonic character for $g<0$. Due to the long-range nature of interactions, to infer these results we combine mean-field and exact-diagonalization methods supported by bosonization analysis.

The superconducting (SC) and charge-density-wave (CDW) susceptibilities of the two-dimensional Holstein model are computed using determinant quantum Monte Carlo, and compared with results computed using the Migdal-Eliashberg (ME) approach. We access temperatures as low as 25 times less than the Fermi energy, $E_F$, which are still above the SC transition. We find that the SC susceptibility at low $T$ agrees quantitatively with the ME theory up to a dimensionless electron-phonon coupling ${\lambda }_0\approx 0.4$ but deviates dramatically for larger ${\lambda }_0$. We find that for large ${\lambda }_0$ and small phonon frequency ${\omega }_0\ll E_F$ CDW ordering is favored and the preferred CDW ordering vector is uncorrelated with any obvious feature of the Fermi surface.

The magnetic properties of CuO encompass several contemporary themes in condensed-matter physics, including quantum magnetism, magnetic frustration, magnetically-induced ferroelectricity, and orbital currents. Here we report polarized and unpolarized neutron inelastic scattering measurements which provide a comprehensive map of the cooperative spin dynamics in the low-temperature antiferromagnetic (AFM) phase of CuO throughout much of the Brillouin zone. At high energies ($E\gtrsim 100\mathrm{meV}$), the spectrum displays continuum features consistent with the des Cloizeax–Pearson dispersion for an ideal $S=\frac{1}{2}$ Heisenberg AFM chain. At lower energies, the spectrum becomes more three dimensional, and we find that a linear spin-wave model for a Heisenberg AFM provides a very good description of the data, allowing for an accurate determination of the relevant exchange constants in an effective spin Hamiltonian for CuO. In the high-temperature helicoidal phase, there are features in the measured low-energy spectrum that we could not reproduce with a spin-only model. We discuss how these might be associated with the magnetically-induced multiferroic behavior observed in this phase.

We theoretically study the electronic transport through a ferromagnet-Ising superconductor junction. A tight-binding Hamiltonian describing the Ising superconductor is presented. Then by combining the nonequilibrium Green's function method, the expressions of Andreev reflection coefficient and conductance are obtained. A strong magnetoanisotropic spin-triplet Andreev reflection is shown, and the magnetoanisotropic period is $\pi$ instead of $2\pi$ as in the conventional magnetoanisotropic system. We demonstrate a significant increase of the spin-triplet Andreev reflection for the single-band Ising superconductor. Furthermore, the dependence of the Andreev reflection on the incident energy and incident angle are also investigated. A complete Andreev reflection can occur when the incident energy is equal to the superconducting gap, regardless of the Fermi energy (spin polarization) of the ferromagnet. For the suitable oblique incidence, the spin-triplet Andreev reflection can be strongly enhanced. In addition, the conductance spectroscopies of both zero bias and finite bias are studied, and the influence of gate voltage, exchange energy, and spin-orbit coupling on the conductance spectroscopy are discussed in detail. The conductance exhibits a strong magnetoanisotropy with period $\pi$ as the Andreev reflection coefficient. When the magnetization direction is parallel to the junction plane, a large conductance peak always emerges at the superconducting gap. This work offers a comprehensive and systematic study of the spin-triplet Andreev reflection and has an underlying application of $\pi$-periodic spin valve in spintronics.

The layered WHM-type ($W=\mathrm{Zr}/\mathrm{Hf}/\mathrm{La}, H=\mathrm{Si}/\mathrm{Ge}/\mathrm{Sn}/\mathrm{Sb}, M=\mathrm{S}/\mathrm{Se}/\mathrm{Te}$) materials represent a large family of topological semimetals, which provides an excellent platform to study the evolution of topological semimetal state with the fine tuning of spin-orbit coupling and structural dimensionality for various combinations of $W, H$, and $M$ elements. In this work, through high field de Haas–van Alphen (dHvA) quantum oscillation studies, we have found evidence for the predicted topological nontrivial bands in ZrSnTe. Furthermore, from the angular dependence of quantum oscillation frequency, we have revealed the three-dimensional Fermi surface topologies of this layered material owing to strong interlayer coupling.

A detailed analysis of the photoluminescence (PL) intensity distribution in singly Cr-doped CdTe/ZnTe quantum dots (QDs) is performed. First of all, we demonstrate that hole-Cr flip-flops induced by an interplay of the hole-Cr exchange interaction and the coupling with acoustic phonons are the main source of spin relaxation within the exciton-Cr complex. This spin flip mechanism appears in the excitation power dependence of the PL of the exciton as well as in the intensity distribution of the resonant PL. The resonant optical pumping of the Cr spin which was recently demonstrated can also be explained by these hole-Cr flip-flops. Despite the fast exciton-Cr spin dynamics, an analysis of the PL intensity under magnetic field shows that the hole-Cr exchange interaction in CdTe/ZnTe QDs is antiferromagnetic. In addition to the Cr spin dynamics induced by the interaction with carriers' spin, we finally demonstrate using time resolved optical pumping measurements that a Cr spin interacts with nonequilibrium acoustic phonons generated during the optical excitation inside or near the QD.

We present a theoretical study of the Kondo effect for a magnetic atom placed inside nanocorrals using Green's function calculations. Based on the standard mapping of the Anderson impurity model to a one-dimensional chain model, we formulate a weak-coupling theory to study the Anderson impurities in a hosting bath with a surface state. With further taking into account the multiple scattering effect of the surrounding atoms, our calculations show that the Kondo resonance width of the atom placed at the center of the nanocorral can be significantly tuned by the corral size, in good agreement with recent experiments [Q. L. Li et al., Phys. Rev. B 97, 035417 (2018)]. The method can also be applied to the atom placed at an arbitrary position inside the corral where our calculation shows that the Kondo resonance width also oscillates as the function of its separation from the corral center. The prediction is further confirmed by the low-temperature scanning tunneling microscopy studies where a one-to-one correspondence is found. The good agreement with the experiments validates the generality of the method to the system where multiadatoms are involved.

We use $\mathbf{k}·\mathbf{p}$ theory to estimate the Rashba spin-orbit coupling (SOC) in large semiconductor nanowires. We specifically investigate GaAs- and InSb-based devices with different gate configurations to control symmetry and localization of the electron charge density. We explore gate-controlled SOC for wires of different size and doping, and we show that in high carrier density SOC has a nonlinear electric field susceptibility, due to large reshaping of the quantum states. We analyze recent experiments with InSb nanowires in light of our calculations. Good agreement is found with the SOC coefficients reported in Phys. Rev. B 91, 201413(R) (2015), but not with the much larger values reported in Nat. Commun. 8, 478 (2017). We discuss possible origins of this discrepancy.

Solid krypton (Kr) undergoes a pressure-induced martensitic phase transition from a face-centered cubic (fcc) to a hexagonal close-packed (hcp) structure. These two phases coexist in a very wide pressure domain inducing important modifications of the bulk properties of the resulting mixed phase system. Here, we report a detailed in situ x-ray diffraction and absorption study of the influence of the fcc-hcp phase transition on the compression behavior of solid krypton in an extended pressure domain up to 140 GPa. The onset of the hcp-fcc transformation was observed in this study at around 2.7 GPa and the coexistence of these two phases up to 140 GPa, the maximum investigated pressure. The appearance of the hcp phase is also evidenced by the pressure-induced broadening and splitting of the first peak in the XANES spectra. We demonstrate that the transition is driven by a continuous nucleation and intergrowth of nanometric hcp stacking faults that evolve in the fcc phase. These hcp stacking faults are unaffected by high-temperature annealing, suggesting that plastic deformation is not at their origin. The apparent small Gibbs free-energy differences between the two structures that decrease upon compression may explain the nucleation of hcp stacking faults and the large coexistence domain of fcc and hcp krypton. We observe a clear anomaly in the equation of state of the fcc solid at ∼20 GPa when the proportion of the hcp form reaches $\sim 20\%$. We demonstrate that this anomaly is related to the difference in stiffness between the fcc and hcp phases and propose two distinct equation of states for the low and high-pressure regimes.

To date, the high-temperature phase transition R3m$\text{−}Fm\overline{3}m$ in GeTe is commonly believed to be second-order displacive with an anomalous volume contraction at the phase transition temperature, $T_{PT}$, from diffraction measurements. Three main results are here reported: (i) the phase transition is accompanied by latent heat absorption, (ii) in the high-resolution x-ray-diffraction powder pattern the cubic phase appears before the merging of the rhombohedral peaks, and (iii) the cubic phase possesses a larger amount of germanium vacancies than the rhombohedral phase. From results (i)–(iii) we conclude that the phase transition is not purely second-order displacive and that the volume contraction observed at $T_{PT}$ can be explained with the formation of vacancies in the cubic phase.