A comprehensive study of magnetoresistance and Hall effect has been performed for the set of the single crystals of nonmagnetic metal $\mathrm{Lu}{\mathrm{B}}_{12}$ with the Jahn-Teller instability of the boron cage and dynamic charge stripes forming along $\langle 110\rangle$ direction. An anomalous positive contribution to Hall effect for a particular direction of magnetic field $\mathbf{H}//\left[001\right]$ is found in the single crystals of $\mathrm{Lu}{\mathrm{B}}_{12}$ of the highest quality. This contribution arising at $T_{\mathrm{E}}\sim 150\mathrm{K}$ is shown to increase drastically when approaching the disordered ground state below $T^{*}\sim 60\mathrm{K}$. The Hall effect anomaly is shown to appear in combination with the peak of magnetoresistance. The various scenarios allowing for the topology of Fermi surface, anisotropy of relaxation time for charge carriers, and interaction of external magnetic field with the filamentary structure of fluctuating charge stripes are analyzed to explain the features of magnetotransport in this metal with inhomogeneous distribution of electron density. The origin of SdH oscillations, which are observed in this nonequilibrium metal with electron phase separation and strong charge carrier scattering, is discussed.

We report implementation of the electron localization function (ELF) within the exact muffin-tin orbitals (EMTO) formalism. The ELF is often used to study the nature of electronic bonding in different types of materials, and it is also an important ingredient in meta-generalized gradient approximations, which are one of the classes of exchange-correlation functionals. The correctness of the ELF implementation is verified with test calculations and comparison with previous literature results. The implementation supports not only regular ordered systems but also disordered systems that have been calculated using the coherent potential approximation method.

The double quantum well systems consisting of two HgTe layers separated by a tunnel-transparent barrier are expected to manifest a variety of phase states including two-dimensional gapless semimetal and two-dimensional topological insulator. The presence of several subbands in such systems leads to a rich filling factor diagram in the quantum Hall regime. We have performed magnetotransport measurements of the HgTe-based double quantum wells in both a gapless and gapped state and observed numerous crossings between the Landau levels belonging to different subbands. We analyze the Landau level crossing patterns and compare them to the results of theoretical calculations.

The $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}R{30}^{\circ }\text{−}\mathrm{B}\left[\mathrm{Si}\left(111\right)\text{−}\mathrm{B}\right]$ surface has evolved into a particularly interesting surface in the context of on-surface molecular self-assembly. Photoemission spectroscopy is a powerful tool to understand the interaction between the surface and the adsorbates. Previous studies of $\mathrm{Si}\left(111\right)\text{−}\mathrm{B}$ contain many inconsistencies with regard to the Si $2p$ core level and valence-band dispersion. Here we employ synchrotron-based core-level and angle-resolved photoemission spectroscopy measurements in combination with density functional theory (DFT) calculations to address these issues. DFT calculations of the Si $2p$ core-level spectra accurately identify contributions from one bulk and five surface components, which allows us to obtain a comprehensive understanding of the spectra recorded at different photon energies. As an archetypal example, this refined decomposition is employed to understand the changes in Si $2p$ spectra upon the adsorption of cobalt phthalocyanine molecules. Regarding the valence-band dispersion of the clean $\mathrm{Si}\left(111\right)\text{−}\mathrm{B}$ surface, our comprehensive DFT and photoemission investigations are able to reconcile the inconsistencies appearing in previous studies and reveal several yet unreported surface states. Furthermore, we are able to theoretically and experimentally resolve the distribution of these surface states in constant energy plots.

Motivated by recent experimental progress in transition metal oxides with the ${\mathrm{K}}_2\mathrm{Ni}{\mathrm{F}}_4$ structure, we investigate the magnetic and orbital ordering in $\alpha \text{−}{\mathrm{Sr}}_2\mathrm{Cr}{\mathrm{O}}_4$. Using first-principles calculations, first we derive a three-orbital Hubbard model, which reproduces the ab initio band structure near the Fermi level. The unique reverse splitting of $t_{2g}$ orbitals in $\alpha \text{−}{\mathrm{Sr}}_2\mathrm{Cr}{\mathrm{O}}_4$, with the $3d^2$ electronic configuration for the ${\mathrm{Cr}}^{4+}$ oxidation state, opens up the possibility of orbital ordering in this material. Using real-space Hartree-Fock for multiorbital systems, we constructed the ground-state phase diagram for the two-dimensional compound $\alpha \text{−}{\mathrm{Sr}}_2\mathrm{Cr}{\mathrm{O}}_4$. We found stable ferromagnetic, antiferromagnetic, antiferro-orbital, and staggered orbital stripe ordering in robust regions of the phase diagram. Furthermore, using the density matrix renormalization group method for two-leg ladders with the realistic hopping parameters of $\alpha \text{−}{\mathrm{Sr}}_2\mathrm{Cr}{\mathrm{O}}_4$, we explore magnetic and orbital ordering for experimentally relevant interaction parameters. Again, we find a clear signature of antiferromagnetic spin ordering along with antiferro-orbital ordering at moderate to large Hubbard interaction strength. We also explore the orbital-resolved density of states with Lanczos, predicting insulating behavior for the compound $\alpha \text{−}{\mathrm{Sr}}_2\mathrm{Cr}{\mathrm{O}}_4$, in agreement with experiments. Finally, an intuitive understanding of the results is provided based on a hierarchy between orbitals, with $d_{xy}$ driving the spin order, while electronic repulsion and the effective one dimensionality of the movement within the $d_{xz}$ and $d_{yz}$ orbitals driving the orbital order.

Correlated Dirac semimetal phases emerge in lightly doped (Tb- or La-doped) Mott insulator ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$, where a $d$-wave symmetry-breaking order underlying a pseudogap plays a crucial role in determining the nature of Dirac degeneracy, i.e., whether it is a Dirac line node or Dirac point node. Here, using a realistic five-orbital tight-binding model with a Hubbard $U$ and a semiclassical Boltzmann transport theory, we systematically study the low-energy electrodynamic properties of the Dirac semimetal phases in the paramagnetic lightly doped ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$. We investigate the effects of the $d$-wave electronic order and electron doping concentration on the electronic band structures and optical properties of various Dirac semimetal phases. We calculate the intraband optical conductivity and obtain electrodynamic parameters of dc conductivity, scattering rate, and Drude weight for three Dirac semimetal phases: two are Dirac point-node states observed in the 3% Tb-doped and 5% La-doped ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$, and the other is a Dirac line-node state. Our results show that the temperature dependence of the electrodynamic parameters is strong in the Tb-doped system while weak in the La-doped and Dirac line-node systems, which are consistent with available experimental data. Moreover, using the low-energy effective theory, we also compare the temperature-dependent screening effect in the Tb- and La-doped systems using graphene as a reference. Our paper provides valuable insight for understanding the transport and optical properties of correlated Dirac semimetal phases in the doped ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$.

In this work, we present measurements of the temperature dependence of the resonance Raman spectra of $\mathrm{Mo}{\mathrm{S}}_2$ in two-dimensional and bulk forms, performed to identify the processes related to different combinations of two acoustic phonons involved in the intervalley scattering. The resonance Raman spectra of samples of different thicknesses (single layer, bilayer, trilayer, and bulk) were measured near the resonance with the $A$ excitonic transition and at different temperatures. Measurements of the Raman spectra of bulk $\mathrm{Mo}{\mathrm{S}}_2$ were performed using several laser energies across the resonances with the $A$ and $B$ excitonic transitions. Based on the electronic and vibrational structures of the samples with different thicknesses and the evolution of the bands as a function of the laser excitation energy and temperature, we propose correct assignments to the Raman bands appearing at approximately 380, 395, and $405\mathrm{c}{\mathrm{m}}^{-1}$. According to our measurements and data analysis, the peaks at 380, 395, and $405\mathrm{c}{\mathrm{m}}^{-1}$ correspond to the combinations of 2TA around the K point, LA and TA phonons around the M point, and LA and out-of-plane acoustic (ZA) phonons around the M point, respectively. This work sheds light on the double-resonance processes of $\mathrm{Mo}{\mathrm{S}}_2$ and how it is related to the electronic structure of this material. The results presented here establish the assignment and the scattering mechanism for some two-phonon and double-resonance Raman bands whose origins were still a matter of debate in the literature. It can also be an important basis to explain the double-resonance processes in other transition-metal dichalcogenides since we present the fundamental electron scattering mechanism near the resonance with the $A$ and $B$ excitonic transitions, and how it is affected by thermal effects.

Monolayer (ML) transition metal dichalcogenides (TMDCs) emerged as ideal materials to combine spin and momentum of charge carriers for spintronics and valleytronics applications. Despite its relevance for TMDC-based technology, the impact of the various many-body-like interactions of charge carriers with defects, phonons, and other system's quasiparticles on the charge dynamics and transport properties remains experimentally elusive, being commonly overshadowed by the strong ML interactions with other materials (such as substrate and contacts) introduced in standard experimental approaches. Here, a method combining interface engineering and angle-resolved photoemission spectroscopy (ARPES) enables a direct investigation of the impact of many-body effects on the hole quasiparticle dynamics of ${\mathrm{WS}}_2$ ML on graphite and extraction of relevant transport parameters. In particular, at the valence band edge, a clear ARPES line-shape asymmetry is observed, mainly reflecting the hole interaction with intralayer electrons and defects while a negligible hole-phonon coupling is found. Using the valley hole quasiparticle lifetime and effective mass extracted from ARPES data at different temperatures, we estimated a hole mobility of $\sim 300\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, comparable to some of the highest values reported by transport measurements.

${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is an ideal three-dimensional topological insulator in which the chemical potential can be brought into the bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize the photoexcited carrier decay in ${\mathrm{Bi}}_{2-x}{\mathrm{Sb}}_x{\mathrm{Se}}_3$ nanoplatelets. We report a substantial slowing of the bulk carrier relaxation time in bulk-insulating ${\mathrm{Bi}}_{2-x}{\mathrm{Sb}}_x{\mathrm{Se}}_3$ as compared to $n$-type bulk-metallic ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ at low temperatures, which approaches $3.3\mathrm{ns}$ in the zero pump fluence limit. This long-lived decay is correlated across different fluences and antimony concentrations, revealing unique decay dynamics not present in $n$-type ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, namely the slow bimolecular recombination of bulk carriers.

Dangling edge spins of two-dimensional quantum critical antiferromagnets display strongly enhanced spin correlations with scaling dimensions that fall outside of the classical theory of surface critical phenomena. We provide large-scale quantum Monte Carlo results for both spin-correlation and bond-correlation functions for the case of the columnar dimer model in particular. Unlike the spin correlations, we find the bond correlations to differ starkly between the spin-1/2 and spin-1 case. Furthermore, we compare the corresponding scaling dimensions to recent theoretical predictions. These predictions are, in part, supported by our numerical data, but cannot explain our findings completely. Our results thus put further constraints on completing the understanding of dangling edge correlations, as well as surface phenomena in strongly correlated quantum systems in general.

Integer quantum Hall plateau transitions are usually modeled by a system of noninteracting electrons moving in a random potential. The physics of the most relevant degrees of freedom, the edge states, is captured by a recently proposed random network model, in which randomness is induced by a parameter-dependent modification of a regular network. In this paper we formulate a specific map from random potentials onto two-dimensional (2D) discrete surfaces, which indicates that 2D gravity emerges in all quantum phase transitions characterized by the presence of edge states in a disordered environment. We also establish a connection between the parameter in the network model and the Fermi level in the random potential.

First-principles electronic structure calculations have been employed to identify the nature of spin textures induced by Rashba-Dresselhaus effect in the ferroelectric systems ${\mathrm{LaWN}}_3$ (LWN), ${\mathrm{Ag}}_2{\mathrm{BiO}}_3$ (ABO), and ${\mathrm{BiInO}}_3$ (BIO). Calculations reveal Rashba spin texture in LWN, Dresselhaus spin texture in ABO and unidirectional persistent spin helix (PSH) in BIO hosted by the conduction bands around the high-symmetry $Z$ point of the Brilliouin zone (BZ) despite all these systems belonging to the same point group symmetry ${\mathrm{C}}_{2v}$. We propose that in addition to the local symmetry, the orbital character of the bands at the high-symmetry point of the BZ play a crucial role in determining the spin textures. A systematic study indeed reveals that the orbital characters of the conduction bands at the $Z$ point for these three systems are markedly different, where in the presence of SOC it is predominantly W-$d$ character in LWN, Bi-$s+\mathrm{O}\text{−}{p}_z$ character in ABO and Bi-$p_x$ character in BIO, suggesting the important role of orbital degrees of freedom in tailoring spin texture. Further we explain, while nonsymmorphic symmetry hosts out of plane spin component in the spin texture for LWN and BIO, enhancing the band splitting at the ${\mathrm{k}}_z=\pi /c$ plane, this is however not present in ABO. Our first-principles results are supplemented with an effective $\mathbf{k}·\mathbf{p}$ model confirming the important role of orbital symmetry.

The formation of lattice defects and their effect on the electronic properties of single-layer ${\mathrm{FeCl}}_2$ are investigated by means of first-principles calculations. Among the vacancy defects, namely mono-, di-, and three-Cl vacancies and mono-Fe vacancy, the formation of mono-Cl vacancy is the most preferable. Comparison of two different antisite defects reveals that the formation of the Fe-antisite defect is energetically preferable to the Cl-antisite defect. While a single Cl vacancy leads to a $1{\mu }_B$ decrease in the total magnetic moment of the host lattice, each Fe vacant site reduces the magnetic moment by $4{\mu }_B$. However, adsorption of an excess Cl atom on the surface changes the electronic structure to a ferromagnetic metal or to a ferromagnetic semiconductor depending on the adsorption site without changing the ferromagnetic state of the host lattice. Both Cl-antisite and Fe-antisite defected domains change the magnetic moment of the host lattice by $-1{\mu }_B$ and $+3{\mu }_B$, respectively. The electronic ground state of defected structures reveals that (i) single-layer ${\mathrm{FeCl}}_2$ exhibits half-metallicity under the formation of vacancy and Cl-antisite defects; (ii) ferromagnetic metallicity is obtained when a single Cl atom is adsorbed on upper-Cl and Fe sites, respectively; and (iii) ferromagnetic semiconducting behavior is found when a Cl atom is adsorbed on a lower-Cl site or a Fe-antisite defect is formed. Simulated scanning electron microscope images show that atomic-scale identification of defect types is possible from their electronic charge density. Further investigation of the periodically Fe-defected structures reveals that the formation of the single-layer ${\mathrm{FeCl}}_3$ phase, which is a dynamically stable antiferromagnetic semiconductor, is possible. Our comprehensive analysis on defects in single-layer ${\mathrm{FeCl}}_2$ will complement forthcoming experimental observations.

We use the phase-field crystal model to study nucleation of edge dislocations in two dimensions under an applied stress field. A dislocation dipole nucleates under the applied stress, consistent with Burgers vector conservation. The phase field correctly accounts for elastic energy storage prior to nucleation and for dissipative relaxation during the nucleation event. We show that a lattice incompatibility field is a sensitive diagnostic of the location of the nucleation event and of the Burgers vector and slip direction of the dislocations that will be nucleated above threshold. A direct calculation of the phase-field energy accurately correlates with the nucleation event, as signaled by the lattice incompatibility field. We show that a Schmid-like criterion concerning the resolved stress at the nucleation site correctly predicts the critical nucleation stress. Finally, we present preliminary results for a three-dimensional, bcc lattice. The phase field allows direct computation of the lattice incompatibility tensor for both dislocation lines and loops.

The periodically driven $O\left(N\right)$ model is studied near the critical line separating a disordered paramagnetic phase from a period doubled phase, the latter being an example of a Floquet time crystal. The time evolution of one-point and two-point correlation functions are obtained within the Gaussian approximation and perturbatively in the drive amplitude. The correlations are found to show not only period doubling, but also power-law decays at large spatial distances. These features are compared with the undriven $O\left(N\right)$ model, within the Gaussian approximation, in the vicinity of the paramagnetic-ferromagnetic critical point. The algebraic decays in space are found to be qualitatively different in the driven and the undriven cases. In particular, the spatiotemporal order of the Floquet time crystal leads to position-momentum and momentum-momentum correlation functions which are more long-ranged in the driven than in the undriven model. The light-cone dynamics associated with the correlation functions is also qualitatively different as the critical line of the Floquet time crystal shows a light cone with two distinct velocities, with the ratio of these two velocities scaling as the square-root of the dimensionless drive amplitude. The Floquet unitary, which describes the time evolution due to a complete cycle of the drive, is constructed for modes with small momenta compared to the drive frequency, but having a generic relationship with the square-root of the drive amplitude. At intermediate momenta, which are large compared to the square-root of the drive amplitude, the Floquet unitary is found to simply rotate the modes. On the other hand, at momenta which are small compared to the square-root of the drive amplitude, the Floquet unitary is found to primarily squeeze the modes, to an extent which increases upon increasing the wavelength of the modes, with a power-law dependence on it.

A quadruple perovskite oxide $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Co}}_2{\mathrm{Re}}_2{\mathrm{O}}_{12}$ was synthesized by high-pressure annealing. This compound crystallizes in an $A$- and $B$-site ordered quadruple perovskite structure with space group $Pn\text{−}3$. The charge combination is determined to be $\mathrm{Ca}{{\mathrm{Cu}}^{2+}}_3{{\mathrm{Co}}^{2+}}_2{{\mathrm{Re}}^{6+}}_2{\mathrm{O}}_{12}$ by bond valence sum analysis and x-ray absorption spectroscopy. In contrast to other isostructural $A{\mathrm{Cu}}_3{B}_2{B}_2^{\prime }{O}_{12}$ compounds with a single magnetic transition, a long-range antiferromagnetic phase transition originating from the $A^{\prime }$-site ${\mathrm{Cu}}^{2+}$ sublattice is found to occur at $T_N\approx 28\mathrm{K}$. Subsequently, the spin coupling between the $B$-site ${\mathrm{Co}}^{2+}$ and $B^{\prime }$-site ${\mathrm{Re}}^{6+}$ ions contributes to a ferrimagnetic transition around $T_{\mathrm{C}}\approx 20\mathrm{K}$. Strong electrical insulating behavior is identified by optical measurement with an energy gap of approximately 3.75 eV. The mechanisms of the spin interactions are discussed in detail.

Spin pumping is a technique widely used to generate pure spin current and characterize the spin-charge conversion efficiency of heavy metals. Upon microwave excitation, the sample may also be heated, and the parasitic thermoelectric signals could contaminate the spin pumping results. Owing to their identical angular dependences with respect to the magnetic field, it is difficult to isolate one from the other. In this paper, we present a quantitative method to separate thermoelectric contributions from spin pumping signals in both $\mathrm{Py}\left({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}\right)/\mathrm{Pt}$ and $\mathrm{YIG}\left({\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}\right)/\mathrm{Pt}$ bilayers through microwave photoresistance measurements. We find that the microwave absorption indeed can raise the temperature of samples, resulting a field-dependent thermoelectric hysteresis loop. However, the additional heat dissipation due to the resonant precession of the magnetization in the ferromagnet is negligibly small compared with the measured spin pumping signal. Thus, we conclude that the spin pumping signal is free of any detectable thermoelectric contributions.

Geometric frustration in the kagome lattice makes it a great host for the flat electronic band, nontrivial topological properties, and novel magnetism. Here, we use magnetotransport measurements to map out the field-temperature phase diagram of the centrosymmetric $\mathrm{Y}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ with a Mn kagome lattice and show that the system exhibits the topological Hall effect (THE) with an in-plane applied magnetic field around 240 K. In addition, our neutron diffraction results demonstrate that the observed THE cannot arise from a magnetic skyrmion lattice, but instead from an in-plane field-induced double-fan spin structure with $c$-axis components. This paper provides a platform to understand the influence of a field-induced novel magnetic structure on magnetoelectric response in topological kagome metals.

Intrinsic topological superconductors with $p$-wave pairing are rare in nature. Its underlying reason is due to the fact that it is usually difficult to change the relative strength between the singlet and triplet channels for the electron-electron interaction in material. Here we show that by considering superconductivity occurring on surfaces of topological insulators (TIs), the relative strength between the singlet and triplet channels can be changed by geometry and sizes of TIs. Specifically, we show that pairing of electrons at different locations on the surface of a topological insulator generally tends to favor the triplet pairing and can induce topological superconductivity by controlling the surface curvature and size of the topological insulator. We illustrate the effects in two configurations, thin-film geometry and the spherical geometry with a sphere or a hemisphere, and find that topological superconductivity arises with the $p\pm ip$ pairing symmetry dominated in nanoscale size of the TI. As a consequence, vortices can spontaneously form on surfaces of topological insulators with roughness of appropriate curvature. These vortices support a Majorana zero mode inside each core and can be used as a platform to host Majorana zero modes without invoking real magnetic fields. Our theoretical discovery opens a new route to realize topological superconductivity in material.

Exploration of new superconductors still relies on the experience and intuition of experts, and is largely a process of experimental trial and error. In one study, only 3% of the candidate materials showed superconductivity [Hosono et al., Sci. Technol. Adv. Mater. 16, (2015)]. Here, we report a deep learning model for finding new superconductors. We introduced the method named “reading periodic table” that represented the periodic table in a way that allows deep learning to learn to read the periodic table and to learn the law of elements for the purpose of discovering novel superconductors which are outside the training data. It is recognized that it is difficult for deep learning to predict something outside the training data. Although we used only the chemical composition of materials as information, we obtained an $R^2$ value of 0.92 for predicting $T_{\text{c}}$ for materials in a database of superconductors. We also introduced the method named “garbage-in” to create synthetic data of nonsuperconductors that do not exist. Nonsuperconductors are not reported, but the data must be required for deep learning to distinguish between superconductors and nonsuperconductors. We obtained three remarkable results. The deep learning can predict superconductivity for a material with a precision of 62%, which shows the usefulness of the model; it found the recently discovered superconductor ${\mathrm{CaBi}}_2$ and another one ${\mathrm{Hf}}_{0.5}{\mathrm{Nb}}_{0.2}{\mathrm{V}}_2{\mathrm{Zr}}_{0.3}$, neither of which is in the superconductor database; and it found Fe-based high-temperature superconductors (discovered in 2008) from the training data before 2008. These results open the way for the discovery of new high-temperature superconductor families. The candidate materials list, data, and method are openly available on the Internet.