The electronic and galvanomagnetic properties of the equiatomic quaternary Heusler alloy CoMnFeSi, which was recently synthesized and which crystallizes in the cubic LiMgPdSb structure, are studied from first principles. We concentrate on two problems: (i) the origin of the alloy disorder, which was found to exist in this alloy, and (ii) the evaluation of basic galvanomagnetic properties such as the dc conductivity ${\sigma }_{\mathrm{tot}}$ and the anomalous Hall conductivity(AHC) for swap defects, accompanied by a comparison of the calculated results with a recent experiment. The origin of the alloy disorder is investigated by estimating corresponding formation energies of possible swap defects which preserve the sample stoichiometry. We discuss various defects with respect to the sample half metallicity. The most favorable swap types are those for which the calculated ${\sigma }_{\mathrm{tot}}$ and AHC agree reasonably with the experiment. On the basis of calculated transport quantities ${\sigma }_{\mathrm{tot}}$ (AHC), the two most favorable swap types can be identified, namely, (i) the Co-Fe swaps, which preserve the half metallicity and have the lowest formation energy, and (ii) the Co-Mn swaps, which violate the half metallicity but have larger formation energy.

The experimentally measured phase diagram of cuprate superconductors in the temperature-applied magnetic field plane illuminates key issues in understanding the physics of these materials. At low temperature, the superconducting state gives way to a long-range charge order with increasing magnetic field; both the orders coexist in a small intermediate region. The charge order transition is strikingly insensitive to temperature and quickly reaches a transition temperature close to the zero-field superconducting $T_c$. We argue that such a transition along with the presence of the coexisting phase is difficult to obtain in a weak coupling competing orders formalism. We demonstrate that for some range of parameters there is an enlarged symmetry of the strongly coupled charge and superconducting orders in the system depending on their relative masses and the coupling strength of the two orders. We establish that this sharp switch from the superconducting phase to the charge order phase can be understood in the framework of a composite SU(2) order parameter comprising the charge and superconducting orders. Finally, we illustrate that there is a possibility of the coexisting phase of the competing charge and superconducting orders only when the SU(2) symmetry between them is weakly broken due to biquadratic terms in the free energy. The relation of this sharp transition to the proximity to the pseudogap quantum critical doping is also discussed.

We construct a metamaterial from radio-frequency harmonic oscillators, and find two topologically distinct phases resulting from dissipation engineered into the system. These phases are distinguished by a quantized value of bulk energy transport. The impulse response of our circuit is measured and used to reconstruct the band structure and winding number of circuit eigenfunctions around a dark mode. Our results demonstrate that dissipative topological transport can occur in a wider class of physical systems than considered before.

Floquet symmetry protected topological (FSPT) phases are nonequilibrium topological phases enabled by time-periodic driving. FSPT phases of one-dimensional (1D) chains of bosons, spins, or qubits host dynamically protected edge states that can store quantum information without decoherence, making them promising for use as quantum memories. While FSPT order cannot be detected by any local measurement, here we construct nonlocal string order parameters that directly measure general 1D FSPT order. We propose a superconducting-qubit array based realization of the simplest Ising FSPT phase, which can be implemented with existing quantum computing hardware. We devise an interferometric scheme to directly measure the nonlocal string order using only simple one- and two-qubit operations and single-qubit measurements.

We have carried out small angle neutron scattering measurements on single crystals of two members of the family of cubic rare-earth titanate pyrochlores that display ferromagnetic Curie-Weiss susceptibilities, ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is established as displaying a prototypical classical dipolar spin ice ground state, while ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ has been purported as a candidate for a quantum spin ice ground state. While both materials have been well studied with neutron scattering techniques, neither has been previously explored in single crystal form with small angle neutron scattering (SANS). Our results for ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ show distinct SANS features below its ${\mathrm{\Theta }}_{CW}\sim 0.50\mathrm{K}$, with rods of diffuse scattering extending along $\langle 111\rangle$ directions in reciprocal space, off-rod scattering which peaks in temperature near ${\mathrm{\Theta }}_{CW}$, and quasi-Bragg scattering at very small angles which correlates well with $T_C\sim 0.26\mathrm{K}$. The quasi-Bragg scattering corresponds to finite extent ferromagnetic domains $\sim 140\AA$ across, at the lowest temperatures. We interpret the $\langle 111\rangle$ rods of diffuse scattering as arising from domain boundaries between the finite-extent ferromagnetic domains. In contrast the SANS signal in ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is isotropic within the (HHL) plane around $\mathbf{Q}=0$, consistent with the expectations of the dipolar spin ice model. However the strength of this overall SANS signal has a temperature dependence resembling that of the magnetic heat capacity, with a peak near 3 K. Below the break between the field-cooled and the zero-field cooled susceptibility in ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ at $\sim 0.60\mathrm{K}$, the SANS signal is very low, approaching zero.

By means of relativistic, first principles calculations, we investigate the microscopic origin of the vanishingly low magnetic anisotropy of Permalloy, here proposed to be intrinsically related to the local symmetries of the alloy. It is shown that the local magnetic anisotropy of individual atoms in Permalloy can be several orders of magnitude larger than that of the bulk sample and 5–10 times larger than that of elemental Fe or Ni. We furthermore show that locally there are several easy axis directions that are favored, depending on local composition. The results are discussed in the context of perturbation theory, applying the relation between magnetic anisotropy and orbital moment. Permalloy keeps its pronounced soft ferromagnetic nature due to the exchange energy to be larger than the magnetocrystalline anisotropy. Our results shine light on the magnetic anisotropy of permalloy and of magnetic materials in general, and in addition enhance the understanding of pump-probe measurements and ultrafast magnetization dynamics.

We investigated the electronic structure of a layered perovskite oxyfluoride $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ thin film by hard x-ray photoemission spectroscopy (HAXPES) and soft x-ray absorption spectroscopy (XAS) as well as density functional theory (DFT)-based calculations. The core-level HAXPES spectra suggested that $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ is a Mott insulator. The DFT calculations described the total and site-projected density of states and the band dispersion for the optimized crystal structure of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$, predicting that $\mathrm{R}{\mathrm{u}}^{4+}$ takes a high-spin configuration of ${\left(xy\right)}^{\uparrow }{\left(yz,zx\right)}^{\uparrow \uparrow }{\left(3z^2-r^2\right)}^{\uparrow }$ and that $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ has an indirect band gap of 0.7 eV with minima at the $M,A$ and $X,R$ points. HAXPES spectra near the Fermi level and the angular-dependent O $1s$ XAS spectra of the $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ thin film, corresponding to the valence band and conduction band density of states, respectively, were drastically different compared to those of the $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ film, suggesting that the changes in the electronic states were mainly driven by the substitution of an oxygen atom coordinated to Ru by fluorine and subsequent modification of the crystal field.

We demonstrate the existence of a fundamentally new type of excitation, fractonic lines, which are linelike excitations with the restricted mobility properties of fractons. These excitations, described using an amalgamation of higher-form gauge theories with symmetric tensor gauge theories, see direct physical realization as the topological lattice defects of ordinary three-dimensional quantum crystals. Starting with the more familiar elasticity theory, we show how theory maps onto a rank-4 tensor gauge theory, with phonons corresponding to gapless gauge modes and disclination defects corresponding to linelike charges. We derive flux conservation laws which lock these linelike excitations in place, analogous to the higher moment charge conservation laws of fracton theories. This way of encoding mobility restrictions of lattice defects could shed light on melting transitions in three dimensions. This new type of extended object may also be a useful tool in the search for improved quantum error-correcting codes in three dimensions.

We theoretically investigate electron spin operations driven by applied electric fields in a semiconductor double quantum dot (DQD) formed in a nanowire with longitudinal potential modulated by local gating. We develop a model that describes the process of loading and unloading the DQD taking into account the overlap between the electron wave function and the leads. Such a model considers the spatial occupation and the spin Pauli blockade in a time-dependent fashion due to the highly mixed states driven by the external electric field. Moreover, we present a road map based on the quantum optimal control theory (QOCT) to find a specific electric field that performs two-qubit quantum gates on a faster timescale and with higher possible fidelity. By employing the QOCT, we demonstrate the possibility of performing within high efficiency a universal set of quantum gates $\{$cnot, H, and $\mathrm{T}\}$, where cnot is the controlled-not gate, H is the Hadamard gate, and T is the $\pi /8$ gate, even in the presence of the loading/unloading process and charge noise effects. Furthermore, by varying the intensity of the applied magnetic field $B$, the optimized fidelity of the gates oscillates with a period inversely proportional to the gate operation time $t_f$. This behavior can be useful to attain higher fidelity for fast gate operations ($>1$ GHz) by appropriately choosing $B$ and $t_f$ to produce a maximum of the oscillation.

Using the Matsubara Green's function formalism, we calculate the temperature dependence of the nonequilibrium spin polarization induced by an external electric field in the presence of spin-orbit coupling. The model Hamiltonian includes an isotropic $k$-cubed form of the Rashba spin-orbit interaction. Such a Hamiltonian captures the electronic and the spin properties of a two-dimensional electron (hole) gas at the surfaces or interfaces of transition metal oxides or in p-doped semiconductor heterostructures. The induced spin polarization is calculated for the nonmagnetic as well as for a magnetic electron/hole gas. Relations of the spin polarization to the Berry curvature is also discussed.

We investigate nonperturbatively the effect of a magnetic dopant impurity on the edge transport of a quantum spin Hall (QSH) insulator. We show that for a strongly coupled magnetic dopant located near the edge of a system, a pair of transmission antiresonances appear. When the chemical potential is on resonance, interaction effects broaden the antiresonance width with decreasing temperature, thus suppressing transport for both repulsive and moderately attractive interactions. Consequences for the recently observed QSH insulating phase of the $1-{\mathrm{T}}^{\prime }$ of ${\mathrm{WTe}}_2$ are briefly discussed.

Acoustic vibrations at the nanoscale (GHz-THz frequencies) and their interactions with electrons, photons, and other excitations are the heart of an emerging field in physics: nanophononics. The design of ultrahigh frequency acoustic-phonon transducers, with tunable frequency, and easy to integrate in complex systems is still an open and challenging problem for the development of acoustic nanoscopies and phonon lasers. Here we show how an optimized plasmonic metasurface can act as a high-frequency phonon transducer. We report pump-probe experiments in metasurfaces composed of an array of gold nanostructures, revealing that such arrays can act as efficient and tunable photon-phonon transducers, with a strong spectral dependence on the excitation rate and laser polarization. We anticipate our work to be the starting point for the engineering of phononic metasurfaces based on plasmonic nanostructures.

Transition-metal dichalcogenides (TMDs) monolayers have been considered as important two-dimensional semiconductor materials for the study of fundamental physics in the field of spintronics. However, the out-of-plane mirror symmetry in TMDs may constrain electrons' degrees of freedom and it may limit spin-related applications. Recently, a newly synthesized Janus TMDs MoSSe was found to intrinsically possess both the in-plane inversion and the out-of-plane mirror-symmetry breaking. Here we performed first-principles calculations in order to systematically investigate the electronic band structures of a series of Janus monolayer TMDs with chemical formula $MXY\left(M=\mathrm{Mo},\mathrm{W}\mathrm{and}X,Y=\mathrm{S},\mathrm{Se},\mathrm{Te}\right)$. It is found that they possess robust electronic properties like their parent phases. We explored also the effect of perpendicular external electric field and in-plane biaxial strain on the Rashba spin splittings. The Zeeman-type spin splitting and valley polarization at $K\left(K^{\prime }\right)$ point are well preserved and we observed a Rashba-type spin splitting around the $\mathrm{\Gamma }$ point for all the $MXY$ systems. We have also found that these spin splittings can be enhanced by an external electric field collinear with the local electric field derived by the polar bonds and by the compressive strain. The Rashba parameters change linearly with the external electric field, but nonlinearly with the biaxial strain. The compressive strain is found to enhance significantly the anisotropic Rashba spin splitting.

Topological states may be protected by a lattice symmetry in a class of topological semimetals. In three spatial dimensions, the Berry flux around gapless excitations in momentum space concretely defines a chirality, so a protecting symmetry may be referred to as a chiral symmetry. Prime examples include a Dirac semimetal (DSM) in a distorted spinel, ${\mathrm{BiZnSiO}}_4$, protected by a mirror symmetry, and a DSM in ${\mathrm{Na}}_3\mathrm{Bi}$, protected by a rotational symmetry. In these states, topology and chiral symmetry are intrinsically tied. In this Rapid Communication, the characteristic interplay between a chiral symmetry order parameter and an instantaneous long-range Coulomb interaction is investigated with the standard renormalization group method. We show that a topological transition associated with chiral symmetry is stable under the presence of a Coulomb interaction and the electron velocity always becomes faster than the one of a chiral symmetry order parameter. Thus, the transition must not be relativistic, which implies that supersymmetry is intrinsically forbidden by the long-range Coulomb interaction. Asymptotically exact universal ratios of physical quantities such as the energy gap ratio are obtained, and connections with experiments and recent theoretical proposals are also discussed.

We monitor the dynamics of hot carriers in InSe by means of two-photon photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to electronic states degenerate with the $\overline{M}$ valley. Subsequently, the electronic cooling is dictated by Fröhlich coupling with phonons of small momentum transfer. Ab initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the $\overline{M}$ valley could travel through a multilayer flake of InSe with a lateral size of 1 $\mu \mathrm{m}$. The hot carriers pave a viable route to the realization of below-band-gap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides.

We identify the presence of monatomic steps at the Si/SiGe or $\mathrm{Si}/{\mathrm{SiO}}_2$ interface as a dominant source of variations in the dephasing time of silicon (Si) quantum dot (QD) spin qubits. First, using atomistic tight-binding calculations we show that the $g$-factors and their Stark shifts undergo variations due to these steps. We compare our theoretical predictions with experiments on QDs at a $\mathrm{Si}/{\mathrm{SiO}}_2$ interface, in which we observe significant differences in Stark shifts between QDs in two different samples. We also experimentally observe variations in the $g$-factors of one-electron and three-electron spin qubits realized in three neighboring QDs on the same sample, at a level consistent with our calculations. The dephasing times of these qubits also vary, most likely due to their varying sensitivity to charge noise, resulting from different interface conditions. More importantly, from our calculations we show that by employing the anisotropic nature of the spin-orbit interaction (SOI) in a Si QD, we can minimize and control these variations. Ultimately, we predict that the dephasing times of the Si QD spin qubits will be anisotropic and can be improved by at least an order of magnitude, by aligning the external dc magnetic field towards specific crystal directions, given other decoherence mechanisms do not dominate over charge noise.

We propose second-order topological insulators (SOTIs) whose lattice structure has a hexagonal symmetry $C_6$. We start with a three-dimensional weak topological insulator constructed on a stacked triangular lattice, which has only side topological surface states. We then introduce an additional mass term which gaps out the side surface states but preserves the hinge states. The resultant system is a three-dimensional SOTI. The bulk topological quantum number is shown to be the ${\mathbb{Z}}_3$ index protected by inversion time-reversal symmetry $IT$ and rotoinversion symmetry $I{C}_6$. We obtain three phases: trivial, strong, and weak SOTI phases. We argue the origin of these two types of SOTIs. A hexagonal prism is a typical structure respecting these symmetries, where six topological hinge states emerge at the side. The building block is a hexagon in two dimensions, where topological corner states emerge at the six corners in the SOTI phase. Strong and weak SOTIs are obtained when the interlayer hopping interaction is strong and weak, respectively.

Extremely large magnetoresistance (XMR) was recently discovered in many nonmagnetic materials, while its underlying mechanism remains poorly understood due to the complex electronic structure of these materials. Here we report an investigation of the $\alpha$-phase ${\mathrm{WP}}_2$, a topologically trivial semimetal with monoclinic crystal structure ($C2/m$), which contrasts with the recently discovered robust type-II Weyl semimetal phase in $\beta -{\mathrm{WP}}_2$. We found that $\alpha -{\mathrm{WP}}_2$ exhibits almost all the characteristics of XMR materials: the near-quadratic field dependence of MR, a field-induced up-turn in resistivity followed by a plateau at low temperature, which can be understood by the compensation effect, and high mobility of carriers confirmed by our Hall effect measurements. It was also found that the normalized MRs under different magnetic fields have the same temperature dependence in $\alpha -{\mathrm{WP}}_2$, the Kohler scaling law can describe the MR data in a wide temperature range, and there is no obvious change in the anisotropic parameter $\gamma$ value with temperature. The resistance polar diagram has a peanut shape when the field is rotated in the $ac$ plane, which can be understood by the anisotropy of the Fermi surface. These results indicate that both field-induced-gap and temperature-induced Lifshitz transition are not the origin of up-turn in resistivity in the $\alpha -{\mathrm{WP}}_2$ semimetal. Our findings establish $\alpha -{\mathrm{WP}}_2$ as a new reference material for exploring the XMR phenomena.

We present a comprehensive study of the earth-abundant semiconductor ${\mathrm{Cu}}_3\mathrm{N}$ as a potential solar energy conversion material, using density functional theory and experimental methods. Density functional theory indicates that among the dominant intrinsic point defects, copper vacancies ${\mathrm{V}}_{\mathrm{Cu}}$ have shallow defect levels while copper interstitials ${\mathrm{Cu}}_{\mathrm{i}}$ behave as deep potential wells in the conduction band, which mediate Shockley-Read-Hall recombination. The existence of ${\mathrm{Cu}}_{\mathrm{i}}$ defects has been experimentally verified using photothermal deflection spectroscopy. A ${\mathrm{Cu}}_3\mathrm{N}$/ZnS heterojunction diode with good current-voltage rectification behavior has been demonstrated experimentally, but no photocurrent is generated under illumination. The absence of photocurrent can be explained by a large concentration of ${\mathrm{Cu}}_{\mathrm{i}}$ recombination centers capturing electrons in $p$-type ${\mathrm{Cu}}_3\mathrm{N}$.

The out-of-equilibrium excess conductance of electron-glasses $\mathrm{\Delta }G\left(t\right)$ typically relaxes with a logarithmic time dependence. Here it is shown that the $log\left(t\right)$ relaxation of a weakly disordered ${\mathrm{In}}_{\mathrm{x}}\mathrm{O}$ film crosses over asymptotically to an exponential dependence $\mathrm{\Delta }G\left(t\right)\propto \exp \{-[t/\tau \left(\infty \right)\left]\right\}$. This allows for assigning a well-defined relaxation-time $\tau \left(\infty \right)$ for a given system disorder (characterized by the Ioffe-Regel parameter $k_{\mathrm{F}}\ell$). Near the metal-insulator transition, $\tau \left(\infty \right)$ obeys the scaling relation $\tau \left(\infty \right)\propto [{\left(k_{\mathrm{F}}\ell \right)}_{\mathrm{C}}-k_{\mathrm{F}}\ell ]$ with the same critical disorder ${\left(k_{\mathrm{F}}\ell \right)}_{\mathrm{C}}$ where the zero-temperature conductivity of this system vanishes. The latter defines the position of the disorder-driven metal-to-insulator transition which is a quantum-phase transition. In this regard the electron glass differs from classical glasses, such as the structural glass and spin glass. The ability to experimentally assign an unambiguous relaxation time allows us to demonstrate the steep dependence of the electron-glass dynamics on carrier concentration.