Layered transition metal dichalcogenides ${\mathrm{MoTe}}_2$ and ${\mathrm{WTe}}_2$ share almost similar lattice constants as well as topological electronic properties except their structural phase transitions. While the former shows a first-order phase transition between monoclinic and orthorhombic structures, the latter does not. Using a recently proposed van der Waals density functional method, we investigate structural stability of the two materials and uncover that the disparate phase transitions originate from delicate differences between their interlayer bonding states near the Fermi energy. By exploiting the relation between the structural phase transitions and the low energy electronic properties, we show that a charge doping can control the transition substantially, thereby suggesting a way to stabilize or to eliminate their topological electronic energy bands.

Density-functional theory forces, stresses, and energies comprise a database from which the optimal parameters of a spline-based empirical potential combining Stillinger-Weber and modified embedded-atom forms are determined. The accuracy of the potential is demonstrated by calculations of ideal shear, stacking fault, vacancy migration, elastic constants, and phonons all between 0 and 100 GPa. Consistency with existing models and experiments is demonstrated by predictions of screw dislocation core structure and deformation twinning in a tungsten nanorod. Last, the potential is used to study the stabilization of fcc tungsten at high pressure.

First-principles calculations predict the emergence of magnetoelectric coupling mediated by two-dimensional electron gas (2DEG) at the ferroelectric ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructure. Free electrons endowed by naturally existing oxygen vacancies in ${\mathrm{SrTiO}}_3$ are driven to the heterostructure interface under the polarizing field of ferroelectric ${\mathrm{PbTiO}}_3$ to form a 2DEG. The electrons are captured by interfacial Ti atoms, which surprisingly exhibits ferromagnetism even at room temperature with a small critical density of $\sim 15.5\mu \mathrm{C}/{\mathrm{cm}}^2$. The ferroelectricity-controlled ferromagnetism mediated by interfacial 2DEG shows strong magnetoelectric coupling strength, enabling convenient control of magnetism by electric field and vice versa. The ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructure is cheap, easily grown, and controllable, promising future applications in low-cost spintronics and information storage at ambient condition.

We report an investigation of the superconducting states of ${\mathrm{Lu}}_3{\mathrm{Os}}_4{\mathrm{Ge}}_{13}$ and ${\mathrm{Y}}_3{\mathrm{Ru}}_4{\mathrm{Ge}}_{13}$ single crystals by measurements of the electrical resistivity, ac susceptibility, and London penetration depth. The analysis of the penetration depth and the derived superfluid density indicates the presence of nodeless superconductivity and suggests that there are multiple superconducting gaps in both materials. Furthermore, ac susceptibility measurements of both compounds display the peak effect in the low-temperature region of the $H\text{−}T$ phase diagram. This anomalous increase of the critical current with field gives an indication of a change of the arrangement of flux lines in the mixed state, as found in some of the isostructural stannide materials.

We performed optical spectroscopy on a thin film of FeSe, in which orbital ordering shows up in the orthorhombic phase below $T_{\mathrm{s}}$. The optical conductivity spectrum for FeSe exhibits no gap feature in the orbitally ordered state, in contrast to those for iron pnictides showing the gap opening in the magnetostructurally ordered phase. Instead, we observed a gradual suppression of the coherent carrier density with temperature. This highlights a peculiar metallic state in FeSe that the Fermi surfaces are gradually modified below $T_{\mathrm{s}}$. We also found an anomalous behavior of the optical phonon mode, indicating the intimate connection between the orbital and lattice degrees of freedom. The result implies the orbital origin of the structural transition.

We present a resistivity study of single- and polycrystalline ${\mathrm{Nb}}_3\mathrm{Sn}$ under pressure up to 9.3 GPa. Despite quite different pressure responses, $T_{\mathrm{c}}$ of both samples is diminished with increasing pressure. Above $T_{\mathrm{c}}$, their resistivity follows a $T^2$ dependence whose coefficient $A$ decreases with pressure. We show that the bulk $T_{\mathrm{c}}$ depends linearly on $\sqrt{A}$, and the slope of this linear contribution becomes larger with the increase of disorder. Our results suggest that the density of states at the Fermi level plays an important role in governing the pressure dependence of $T_{\mathrm{c}}$ in ${\mathrm{Nb}}_3\mathrm{Sn}$.

In this paper, we study phase diagrams of the extended Bose-Hubbard model (EBHM) in one dimension by means of the quantum Monte-Carlo (QMC) simulation using the stochastic-series expansion (SSE). In the EBHM, there exists a nearest-neighbor repulsion as well as the on-site repulsion. In the SSE-QMC simulation, the highest particle number at each site, $n_c$, is also a controllable parameter, and we found that the phase diagrams depend on the value of $n_c$. It is shown that in addition to the Mott insulator, superfluid, density wave, the phase so-called Haldane insulator, and supersolid appear in the phase diagrams, and their locations in the phase diagrams are clarified.

Recent investigations of excitonic absorption spectra in cuprous oxide (${\mathrm{Cu}}_2\mathrm{O}$) have shown that it is indispensable to account for the complex valence-band structure in the theory of excitons. In ${\mathrm{Cu}}_2\mathrm{O}$, parity is a good quantum number and thus the exciton spectrum falls into two parts: the dipole-active exciton states of negative parity and odd angular momentum, which can be observed in one-photon absorption (${\mathrm{\Gamma }}_4^{-}$ symmetry), and the exciton states of positive parity and even angular momentum, which can be observed in two-photon absorption (${\mathrm{\Gamma }}_5^{+}$ symmetry). The unexpected observation of $D$ excitons in two-photon absorption has given first evidence that the dispersion properties of the ${\mathrm{\Gamma }}_5^{+}$ orbital valence band are giving rise to a coupling of the yellow and green exciton series. However, a first theoretical treatment by Uihlein et al. [Phys. Rev. B 23, 2731 (1981)] was based on a simplified spherical model. The observation of $F$ excitons in one-photon absorption is a further proof of a coupling between yellow and green exciton states. Detailed investigations on the fine structure splitting of the $F$ exciton by F. Schweiner et al. [Phys. Rev. B 93, 195203 (2016)] have proved the importance of a more realistic theoretical treatment including terms with cubic symmetry. In this paper we show that the even and odd parity exciton system can be consistently described within the same theoretical approach. However, the Hamiltonian of the even parity system needs, in comparison to the odd exciton case, modifications to account for the very small radius of the yellow and green $1S$ exciton. In the presented treatment, we take special care of the central-cell corrections, which comprise a reduced screening of the Coulomb potential at distances comparable to the polaron radius, the exchange interaction being responsible for the exciton splitting into ortho and para states, and the inclusion of terms in the fourth power of $p$ in the kinetic energy being consistent with $O_{\mathrm{h}}$ symmetry. Since the yellow $1S$ exciton state is coupled to all other states of positive parity, we show how the central-cell corrections affect the whole even exciton series. The close resonance of the $1S$ green exciton with states of the yellow exciton series has a strong impact on the energies and oscillator strengths of all implied states. The consistency between theory and experiment with respect to energies and oscillator strengths for the even and odd exciton system in ${\mathrm{Cu}}_2\mathrm{O}$ is a convincing proof for the validity of the applied theory.

We present a theoretical investigation of the single-electron electronic structure of polyatomic phosphorus donor molecular planar structures embedded in silicon. Using an effective mass theory and multivalley envelope function representation, the effect of the valley-orbit coupling is systematically analyzed in such systems. The valley composition of the single-electron states strongly depends on the geometry of the dopant molecule and on its orientation relative to the crystallographic axes. The electron binding energy of a triatomic linear molecule is larger than that of a diatomic one oriented along the same crystallographic axis, but the energy gap between the ground state and the first excited state is not significantly different for internuclear distances from 1.5 to 6.6 nm. Three donor atoms arranged in a triangle geometry have larger binding energies than a triatomic linear chain of dopants with the same internuclear distance. Planar donor molecules are characterized by a strong polarization in favor of the valleys oriented perpendicular to the molecular plane, an effect that increases with the number of atoms in the molecule and is not present in diatomics. As a result, the amplitude of the in-plane wave function oscillations caused by the valley interference decreases which reduces the sensitivity of the electronic states to random displacements of dopants. The effect of weak spatial disorder and of the molecular orientation relative to the crystallographic axes are studied in detail for a hexagonal structure. The valley properties that we characterize are fundamental for the implementation of robust multiqubits systems on many-body states of interacting dopants.

We investigate the relaxation of hot carriers by emission of phonons in bulk Si and Si nanowires (NWs) with an identical atomistic methodology. The phonon scattering is strongly enhanced in NWs. At high excitation energy, the carrier cooling is faster in NWs than in bulk, mainly due to the coupling to surface phonons. Slow relaxation is only noticed for electrons at low energy in the conduction band of thin NWs due to the quantum confinement that lifts the degeneracy of the valleys. This work gives insight into the complexity of the carrier cooling in semiconductor nanostructures.

We consider the effect of electron-electron interactions on a voltage biased quantum point contact in the tunneling regime used as a detector of a nearby qubit. We model the leads of the quantum point contact as Luttinger liquids, incorporate the effects of finite temperature and analyze the detection-induced decoherence rate and the detector efficiency, $Q$. We find that interactions generically reduce the induced decoherence along with the detector's efficiency, and strongly affect the relative strength of the decoherence induced by tunneling and that induced by interactions with the local density. With increasing interaction strength, the regime of quantum-limited detection ($Q\to 1$) is shifted to increasingly lower temperatures or higher bias voltages respectively. For small to moderate interaction strengths, $Q$ is a monotonously decreasing function of temperature as in the noninteracting case. Surprisingly, for sufficiently strong interactions we identify an intermediate temperature regime where the efficiency of the detector increases with rising temperature.

Extraordinary electronic phases can form in artificial oxide heterostructures, which will provide a fertile ground for new physics and also give rise to novel device functions. Based on a systematic first-principles density functional theory study of the magnetic and electronic properties of the (111) superlattices ${\left({AB\text{O}}_3\right)}_2/{\left({AB}^{\prime }{\mathrm{O}}_3\right)}_{10}$ of $4d$ and $5d$ transition metal perovskite ($B$ = Ru, Rh, Ag, Re, Os, Ir, Au; ${AB}^{\prime }{\mathrm{O}}_3={\mathrm{LaAlO}}_3, {\mathrm{SrTiO}}_3)$, we demonstrate that due to quantum confinement, bilayers ${\left({\text{La}B\text{O}}_3\right)}_2$ ($B$ = Ru, Re, Os) and ${\left({\text{Sr}B\text{O}}_3\right)}_2$ ($B$ = Rh, Os, Ir) are ferromagnetic with ordering temperatures up to room temperature. In particular, bilayer ${\left({\mathrm{LaOsO}}_3\right)}_2$ is an exotic spin-polarized quantum anomalous Hall insulator, while the other ferromagnetic bilayers are metallic with large Hall conductances comparable to the conductance quantum. Furthermore, bilayers ${\left({\mathrm{LaRuO}}_3\right)}_2$ and ${\left({\mathrm{SrRhO}}_3\right)}_2$ are half metallic, while the bilayer ${\left({\mathrm{SrIrO}}_3\right)}_2$ exhibits a peculiar colossal magnetic anisotropy. Our findings thus show that $4d$ and $5d$ metal perovskite (111) bilayers are a class of quasi-two-dimensional materials for exploring exotic quantum phases and also for advanced applications such as low-power nanoelectronics and oxide spintronics.

We investigate from first principles the fieldlike spin-orbit torques (SOTs) in a ${\mathrm{Ag}}_2\mathrm{Bi}$-terminated Ag(111) film grown on ferromagnetic Fe(110). We find that a large part of the SOT arises from the spin-orbit interaction (SOI) in the ${\mathrm{Ag}}_2\mathrm{Bi}$ layer far away from the Fe layers. These results clearly hint at a long-range spin transfer in the direction perpendicular to the film that does not originate in the spin Hall effect. In order to bring evidence of the nonlocal character of the computed SOT, we show that the torque acting on the Fe layers can be engineered by the introduction of Bi vacancies in the ${\mathrm{Ag}}_2\mathrm{Bi}$ layer. Overall, we find a drastic dependence of the SOT on the disorder type, which we explain by a complex interplay of different contributions to the SOT in the Brillouin zone.

A symmetry broken phase of a system with internal degrees of freedom often features a complex order parameter, which generates a rich variety of topological excitations and imposes topological constraints on their interaction (topological influence); yet the very complexity of the order parameter makes it difficult to treat topological excitations and topological influence systematically. To overcome this problem, we develop a general method to calculate homotopy groups and derive decomposition formulas which express homotopy groups of the order parameter manifold $G/H$ in terms of those of the symmetry $G$ of a system and those of the remaining symmetry $H$ of the state. By applying these formulas to general monopoles and three-dimensional skyrmions, we show that their textures are obtained through substitution of the corresponding $\mathfrak{su}\left(2\right)$ subalgebra for the $\mathfrak{su}\left(2\right)$ spin. We also show that a discrete symmetry of $H$ is necessary for the presence of topological influence and find topological influence on a skyrmion characterized by a non-Abelian permutation group of three elements in the ground state of an SU(3)-Heisenberg model.

We investigate the connection between local structure and dynamics in the liquid and undercooled $\mathrm{A}{\mathrm{l}}_{93}\mathrm{C}{\mathrm{r}}_7$ alloy using ab initio molecular dynamics simulations. In the liquid phase, we show the striking effects of Cr alloying on the atomic-scale structure, characterized by a heterogeneous local ordering around each component, due to a strong interplay between chemical short-range order and icosahedral short-range order. In the undercooled phase, this interplay leads to the formation of an icosahedral-based medium-range order (IMRO) referring to Cr atoms. In examining the dynamic properties, we observe that this chemically induced structural heterogeneity gives rise to a substantial decoupling of component diffusion in the liquid phase, far above the liquidus temperature. In the undercooled regime, we find the breakdown of the Stokes-Einstein relation and a pronounced increase of the non-Gaussian parameter, indicating the onset of dynamic heterogeneities. Using the isoconfigurational ensemble method, we evidence that the structural origin of dynamics heterogeneities is clearly related to the formation of IMRO. Finally, we discuss the role played by IMRO in nucleation on the preexisting icosahedral cluster in the liquid phase.

For metallic glasses, “flexibility volume” has recently been introduced as a property-revealing indicator of the structural state the glass is in. This parameter incorporates the atomic volume and the vibrational mean-square displacement, to combine both static structure and dynamics information. Flexibility volume was shown to quantitatively correlate with the properties of metallic glasses [J. Ding et al., Nat. Commun. 7, 13733 (2016)]. However, it remains to be examined if this parameter is useful for other types of glasses with bonding characteristics, atomic packing structures, as well as properties that are distinctly different from metallic glasses. In this paper, we tackle this issue through systematic molecular-dynamics simulations of amorphous silicon $(a$-Si) models produced with different cooling rates, as $a$-Si is a prototypical covalently bonded network glass whose structure and properties cannot be characterized using structural parameters such as free volume used for metallic and polymeric glasses. Specifically, we demonstrate a quantitative prediction of the shear modulus of $a$-Si from the flexibility for atomic motion. This flexibility volume descriptor, when evaluated on the atomic scale, is shown to also correlate well with local packing, as well as with the propensity for thermal relaxations and shear transformations, providing a metric to map out and explain the structural and mechanical heterogeneity in the amorphous material. This case study of a model of covalently bonded network $a$-Si, together with our earlier demonstration for metallic glasses, points to the universality of flexibility volume as an indicator of the structure state to link with properties, applicable across amorphous materials with different chemical bonding and atomic packing structures.

We show that the application of circularly polarized electromagnetic radiation on the surface of a Weyl semimetal can generate states at that surface. These states can be characterized by their surface momentum. The Floquet eigenvalues $e^{i\theta }$ of these states come in complex conjugate pairs rather than being equal to $\pm 1$. If the amplitude of the radiation is small, we find some unusual bulk-boundary relations: the values of $\theta$ of the surface states lie at the extrema of the $\theta$'s of the bulk system, and the peaks of the Fourier transforms of the surface state wave functions lie at the momenta where the bulk $\theta$'s have extrema. For the case of zero surface momentum, we can analytically derive scaling relations between the decay length of the surface states and the amplitude and penetration length of the radiation. For topological insulators, we again find that circularly polarized radiation can generate states on the top surface; these states have much larger decay lengths than the surface states which are present even in the absence of radiation. Finally, we show that radiation can generate surface states for trivial insulators also.

We examine the possibility of using graphene nanoribbons (GNRs) with directly substituted chromium atoms as a spintronic device. Using density functional theory, we simulate a voltage bias across a constructed GNR in a device setup where a magnetic dimer has been substituted into the lattice. Through this first-principles approach, we calculate the electronic and magnetic properties as a function of Hubbard $U$, voltage, and magnetic configurations. By calculating the total energy of each magnetic configuration, we determine that the initial antiferromagnetic ground state flips to a ferromagnetic state with applied bias. Mapping this transition point to the calculated conductance for the system reveals that there is a distinct change in conductance through the GNR, which indicates the possibility of a spin valve. We also show that this corresponds to a distinct shift in the induced magnetization within graphene.

Given recent progress in the realization of Majorana zero modes in semiconducting nanowires with proximity-induced superconductivity, a crucial next step is to attempt an experimental demonstration of the predicted braiding statistics associated with the Majorana mode. Such a demonstration should, in principle, confirm that the experimentally observed zero-bias anomalies are indeed due to the presence of anyonic Majorana zero modes. Moreover, such a demonstration would be a breakthrough at the level of fundamental physics: the first clear demonstration of a non-Abelian excitation. It is therefore important to clarify the expected signals of Majorana physics in the braiding context and to differentiate these signals from those that might also arise in nontopological variants of the same system. A definitive and critical distinction between signals expected in topological (i.e., anyonic) and nontopological (i.e., trivial) situations is therefore essential for future progress in the field. In this paper, we carefully examine the expected signals of proposed braiding and fusion experiments in topological and nontopological variants of the experimental nanowire systems in which Majoranas are predicted to occur. We point out situations where ‘trivial’ and ‘anyonic’ signatures may be qualitatively similar experimentally, necessitating a certain level of caution in the interpretation of various proposed fusion and braiding experiments. We find in particular that braiding experiments consisting of full braids (two Majorana exchanges) are better at distinguishing between topological and nontopological systems than fusion experiments or experiments with an odd number of Majorana exchanges. Successful fusion experiments, particularly in nanowires where zero bias conductance peaks are also observed, can also provide strong evidence for the existence of Majorana modes, but such fusion evidence without a corresponding braiding success is not definitive.