We present first-principles calculations of electronic structure and topological properties of thin films of tetradymite-like binary, ternary, and quaternary compounds. Two-dimensional topological insulators were found, including two septuple layer films of ${\mathrm{PbBi}}_2{\mathrm{Te}}_4$ with large band gap. We demonstrate that the Pb-based thin films inherit ${\mathbb{Z}}_2$ dependence on the film thickness from the “parent” quintuple-layer compounds. An effective way to induce the topological phase transition in the trivial thin films is proposed that does not require substantial changes in the in–plane lattice parameter and surface crystal structure.

The graphene-Ni(111) (GrNi) growth via chemical vapor deposition has been explored by resonant, angle-resolved, and dichroic photoemission spectroscopy (soft x-ray Res-ARPES) in order to identify the possible contributions to the electronic structure deriving from different phases that can coexist in this complex system. We provide evidences of electronic states so far unexplored at the $\overline{\mathrm{\Gamma }}$ point of GrNi, appearing at the C $K$-edge resonance. These states show both circular dichroism (CD) and $k$ dependence, suggesting a long-range orbital ordering, as well as a coherent matching with the underlying lattice. Through a comparison of core-level photoemission, valence band resonances, and constant initial-state spectroscopy, we demonstrate that these states are actually induced by a low residual component of nickel carbide (${\mathrm{Ni}}_2\mathrm{C}$). These results also show that caution must be exercised while interpreting x-ray magnetic circular dichroism collected on C $K$-edge with Auger partial yield method, due to the presence of CD in photoelectron spectra unrelated to magnetic effects.

Periodic driving fields can induce topological phase transitions, resulting in Floquet topological phases with intriguing properties such as very large Chern numbers and unusual edge states. Whether such Floquet topological phases could generate robust edge state conductance much larger than their static counterparts is an interesting question. In this paper, working under the Keldysh formalism, we study two-lead transport via the edge states of irradiated quantum Hall insulators using the method of recursive Floquet-Green's functions. Focusing on a harmonically-driven Hofstadter model, we show that quantized Hall conductance as large as $8e^2/h$ can be realized but only after applying the so-called Floquet sum rule. To assess the robustness of edge state transport, we analyze the DC conductance, time-averaged current profile, and local density of states. It is found that copropagating chiral edge modes are more robust against disorder and defects as compared with the remarkable counterpropagating edge modes, as well as certain symmetry-restricted Floquet edge modes. Furthermore, we go beyond the wide-band limit, which is often assumed for the leads, to study how the conductance quantization (after applying the Floquet sum rule) of Floquet edge states can be affected if the leads have finite bandwidths. These results may be useful for the design of transport devices based on Floquet topological matter.

Group/subgroup structural phase transitions are exploited in a wide variety of technologies, including those that rely on shape-memory behavior and on transformation toughening. Here, we introduce an approach to identify symmetry-adapted strain and shuffle order parameters for any group/subgroup structural transition between a high-symmetry parent phase and its symmetrically equivalent low-symmetry product phases. We show that symmetry-adapted atomic shuffle order parameters can be determined by the diagonalization of an orbital covariance matrix, formed by taking the covariance among the atomic displacement vectors of all symmetrically equivalent product phase variants. We use this approach to analyze the technologically important tetragonal to monoclinic structural phase transformation of ${\mathrm{ZrO}}_2$. We explore the energy landscapes, as calculated with density functional theory, along distinct paths that connect $m{\mathrm{ZrO}}_2$ to $t{\mathrm{ZrO}}_2$ and to other $m{\mathrm{ZrO}}_2$ variants. The calculations indicate favorable pairs of variants and reveal intermediate structures likely to exist at coherent twin boundaries and having relatively low deformation energy. We identify crystallographic features of the monoclinic ${\mathrm{ZrO}}_2$ variant that make it very sensitive to shape changing strains.

In this work, polarized pump-probe spectroscopy was carried out to investigate the effects of a structural ordering of molecules on photoinduced phase separation (PIPS) in the organic conductors $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_{2}X$ [$X=\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br} (\kappa -Br)$ and $\mathrm{Cu}{\left(\mathrm{NCS}\right)}_2$ ($\kappa$-NCS)]. We found that the anisotropic response for the probe polarization appeared at around $T_{\mathrm{g}}$, where the glasslike structural transition occurs. The anisotropy can be a result of a transient destruction of the local ordering of molecules, indicating a connection between the glasslike transition and PIPS. Moreover, we found that the PIPS response gradually develops with decreasing temperature in $\kappa$-Br, whereas it steeply increases in $\kappa$-NCS. This qualitative difference suggests that the structural ordering caused by a PIPS is more crucial in $\kappa$-NCS than in $\kappa$-Br.

The magnetic structure of the mixed antiferromagnet ${\mathrm{NdMn}}_{0.8}{\mathrm{Fe}}_{0.2}{\mathrm{O}}_3$ was resolved. Neutron powder diffraction data definitively resolve the Mn sublattice with a magnetic propagation vector $\mathbf{k}=\left(000\right)$ and with the magnetic structure ($A_x,F_y,G_z$) for 1.6 K $<T<T_{\mathrm{N}}(\approx 59$ K). The Nd sublattice has a ($0,f_y,0$) contribution in the same temperature interval. The Mn sublattice undergoes a spin-reorientation transition at $T_1\approx 13$ K while the Nd magnetic moment abruptly increases at this temperature. Powder x-ray diffraction shows a strong magnetoelastic effect at $T_{\mathrm{N}}$ but no additional structural phase transitions from 3 to 300 K. Density functional theory calculations confirm the magnetic structure of the undoped ${\mathrm{NdMnO}}_3$ as part of our analysis. Taken together, these results show that the magnetic structure of the Mn sublattice in ${\mathrm{NdMn}}_{0.8}{\mathrm{Fe}}_{0.2}{\mathrm{O}}_3$ is a combination of the Mn and Fe parent compounds, but the magnetic ordering of the Nd sublattice spans a broader temperature interval than in the case of ${\mathrm{NdMnO}}_3$ and ${\mathrm{NdFeO}}_3$. This result is a consequence of the fact that the Nd ions do not order independently, but via polarization from the Mn/Fe sublattice.

High fidelity models, which are able to both support accurate device characterization and correctly account for environmental effects, are crucial to the engineering of scalable quantum technologies. As it ensures positivity of the density matrix, one preferred model of open systems describes the dynamics with a master equation in Lindblad form. In practice, Linblad operators are rarely derived from first principles, and often a particular form of annihilator is assumed. This results in dynamical models that miss those additional terms which must generally be added for the master equation to assume the Lindblad form, together with the other concomitant terms that must be assimilated into an effective Hamiltonian to produce the correct free evolution. In first principles derivations, such additional terms are often canceled (or countered), frequently in a somewhat ad hoc manner, leading to a number of competing models. Whilst the implications of this paper are quite general, to illustrate the point we focus here on an example anharmonic system; specifically that of a superconducting quantum interference device (SQUID) coupled to an Ohmic bath. The resulting master equation implies that the environment has a significant impact on the system's energy; we discuss the prospect of keeping or canceling this impact and note that, for the SQUID, monitoring the magnetic susceptibility under control of the capacitive coupling strength and the externally applied flux results in experimentally measurable differences between a number of these models. In particular, one should be able to determine whether a squeezing term of the form $\widehat{X}\widehat{P}+\widehat{P}\widehat{X}$ should be present in the effective Hamiltonian or not. If model generation is not performed correctly, device characterization will be prone to systemic errors.

Multiple condensates in a superconducting material can interfere constructively or destructively and this leads to unconventional effects not inherent in single-band superconductors. Such effects can be pronounced when the spatial scales (healing lengths) of different band condensates deviate from each other. Here we show that, contrary to usual expectations, this deviation can be considerable even far beyond the regime of nearly decoupled bands, being affected by difference between the band Fermi velocities. Our study is performed within the extended Ginzburg-Landau (GL) formalism that goes to one order beyond the GL theory in the perturbation expansion of the microscopic equations over the proximity to $T_c$. The formalism makes it possible to obtain closed analytical results for the profiles of the band condensates and for their healing lengths and, at the same time, captures the difference between the healing lengths which does not appear in the standard GL domain.

The knowledge of the free-energy barriers separating different states is critically important for the assessment of the long-term stability of information stored in magnetic devices. This information, however, is not directly accessible by standard simulations of microscopic models because of the ubiquitous time-scale problem, related to the fact that the transitions among different free-energy minima are characteristic of rare events. Here, we show that by employing the metadynamics algorithm based on suitably chosen collective variables, namely, helicity and circulation, it is possible to reliably recover the free-energy landscape. We demonstrate the effectiveness of this approach on an example of a vortex nucleation process in a magnetic nanodot with lowered spatial symmetry. With the help of reconstructed free-energy surfaces, we show the origin of symmetry broken vortex nucleation, where one polarity of the nucleated vortex core is preferred, even though only an in-plane magnetic field is present.

We study the effect of a tetragonal, symmetry-conserving epitaxial strain on the phase transition sequence and the onset of three symmetry-breaking modes in ${\mathrm{SrBi}}_2{\mathrm{Nb}}_2{\mathrm{O}}_9$ by first-principles calculations. The interplay of the strain variation of energy expansion coefficients results in an interesting phase diagram which includes proper, hybrid improper, and a different kind of improper ferroelectric phase transitions. The symmetries of the epitaxially stabilized phases and the possible phase-transition sequences are compared with the unstrained case.

We have investigated the anomalous Hall effect and magnetoresistance of the noncentrosymmetric itinerant ferromagnet ${\mathrm{Cr}}_{11}{\mathrm{Ge}}_{19}$. We have found a characteristic temperature ($\sim 30$ K) below the ferromagnetic transition temperature $T_{\mathrm{C}}=85$ K. While a conventional anomalous Hall conductivity that is proportional to the magnetization smoothly grows below $T_{\mathrm{C}}$, the anomalous Hall conductivity shows an additional enhancement below the characteristic temperature. Concomitantly, the AMR emerges and the magnitude of negative magnetoresistance begins to increase toward low temperature. Because there is no anomaly in the temperature dependence of magnetization around the characteristic temperature, the origin of these observations in transport properties is ascribed to some electronic structure with the energy scale of 30 K. We speculate this is caused by the spin splitting due to breaking of spatial inversion symmetry.

Within a Ginzburg-Landau formalism we establish analytically the necessary and sufficient conditions to realize a doubly degenerate superconducting ground state with broken time-reversal symmetry (BTRS) in a multiband superconductor. Using these results we analyze the ground state of a three-band superconductor in the cylindrical geometry in an external magnetic field. We show that depending on the interband coupling constants, a magnetic flux can induce current density jumps in such superconducting geometries that are related to adiabatic or nonadiabatic transitions from BTRS to time-reversal-symmetric states and vice versa. This unusual current-induced magnetic flux response can in principle be used experimentally to detect superconducting BTRS ground states as well as corresponding metastable excited states.

We theoretically study the effect of impurity scattering in a magnetically doped Dirac semimetal. It is found that the magnetic impurity potential can split a Dirac point into a pair of Weyl nodes, due to the broken time-reversal symmetry, and the DSM undergoes a topological phase transition to a Weyl semimetal (WSM). The Chern numbers on a cross section between the Weyl nodes in the Brillouin zone are integer quantized. As a consequence, open Fermi arcs emerge, which connect the projections of the Weyl nodes on the surface Brillouin zone. However, a relatively strong magnetic potential, in addition to separating the Weyl nodes, will introduce some localized states to the system. These localized states smear the Weyl nodes, and the anomalous Hall conductivity deviates from the linear dependence on the momentum distance of the Weyl nodes. Interestingly, though the nonmagnetic charge potential of the impurities is shown to be irrelevant to the topological phase transition, it can modify the Berry curvature and anomalous Hall conductivity of the impurity-induced WSM phase, by redistribution of the energies of the localized states.

To explore the origin of the phase transitions in ${\mathrm{Na}}_2{\mathrm{Ti}}_2{P}_2\mathrm{O}$ ($P$ = Sb, As), we have investigated their band structures and phonon dispersions based on the $abinitio$ density functional theory. We have found that the phonon softening instabilities occur for both compounds at q${}_X$ and q${}_M$, which lead to charge density wave (CDW) instabilities through the electron-phonon coupling. When the Coulomb correlation effect of Ti $d$ electrons is taken into account, the CDW transition to a $2\times 1\times 1$ supercell driven by the normal mode at q${}_X$ produces the most stable state for both compounds. In the CDW ground states, ${\mathrm{Na}}_2{\mathrm{Ti}}_2{\mathrm{Sb}}_2\mathrm{O}$ and ${\mathrm{Na}}_2{\mathrm{Ti}}_2{\mathrm{As}}_2\mathrm{O}$ have the partial and full gap openings in the band structures, respectively, which are in good agreement with the observed transport and angle-resolved photoemission spectroscopy results. Our paper reveals that the Coulomb correlation effects of Ti $d$ electrons are essential to properly describe the CDW transitions in ${\mathrm{Na}}_2{\mathrm{Ti}}_2{P}_2\mathrm{O}$.

The recent progress in formation of two-dimensional (2D) GaN by a migration-enhanced encapsulated technique opens up new possibilities for group III-V 2D semiconductors with a band gap within the visible energy spectrum. Using first-principles calculations we explored alloying of 2D-GaN to achieve an optically active material with a tunable band gap. The effect of isoelectronic III-V substitutional elements on the band gaps, band offsets, and spatial electron localization is studied. In addition to optoelectronic properties, the formability of alloys is evaluated using impurity formation energies. A dilute highly mismatched solid solution 2D-${\mathrm{GaN}}_{1-x}{\mathrm{P}}_x$ features an efficient band gap reduction in combination with a moderate energy penalty associated with incorporation of phosphorous in 2D-GaN, which is substantially lower than in the case of the bulk GaN. The group-V alloying elements also introduce significant disorder and localization at the valence band edge that facilitates direct band gap optical transitions, thus implying the feasibility of using III-V alloys of 2D-GaN in light-emitting devices.

Small numbers of nitrogen dopants dramatically modify the electronic properties of GaAs, generating spatially localized resonant states within the conduction band, pair and cluster states in the band gap, and very large shifts in the conduction-band energies with nonlinear concentration dependence. Cross-sectional scanning tunneling microscopy provides the local electronic structure of single nitrogen dopants at the (110) GaAs surface, yielding highly anisotropic spatial shapes when the empty states are imaged. Measurements of the resonant states relative to the GaAs surface states and their spatial extent allow an unambiguous assignment of specific features to nitrogen atoms at different depths below the cleaved (110) surface. Multiband tight-binding calculations around the resonance energy of nitrogen in the conduction band match the imaged features, verifying that the Green's function method can accurately describe the isolated isovalent nitrogen impurity. The spatial anisotropy is attributed to the tetrahedral symmetry of the bulk lattice and will lead to a directional dependence for the interaction of nitrogen atoms. Additionally, the voltage dependence of the electronic contrast for two features in the filled state imaging suggests these features could be related to a locally modified surface state.

We have investigated the quantum transport properties of high-mobility electrons and holes in atomically thin black phosphorus ambipolar devices. The two-dimensional hole system exhibits unambiguously the quantum Hall effect in a magnetic field up to 30 T, while the electron system shows clearly developing Hall plateaus at integer Landau level filling factors accompanied by $R_{xx}$ oscillations, signaling the onset of the quantum Hall effect. By bringing the spin-resolved Landau levels of the electron system to a coincidence, we determine an electron spin susceptibility to be ${\chi }_{se}=m^{*}{g}^{*}=1.1\pm 0.03$, which, combined with the electron mass $m^{*}=0.39m_0$, yields a Landé $g$ factor $g^{*}=2.8\pm 0.2$. The enhancement of spin susceptibility in the black phosphorus two-dimensional electron system is around 50% compared with band susceptibility, which agrees well with various two-dimensional charge-carrier systems with weak spin-orbit coupling, suggesting the important role played by the exchange interaction.

We report $ab$-plane Hall-effect and magnetoresistivity measurements on $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{C}{\mathrm{e}}_x\mathrm{Cu}{\mathrm{O}}_4$ thin films as a function of doping for magnetic fields up to 14 T and temperatures down to 1.8 K. A dramatic change in the low-temperature (1.8 K) normal-state Hall coefficient is found near a doping $\mathrm{Ce}=0.14$. This, along with a nonlinear Hall resistance as a function of magnetic field, suggests that the Fermi surface reconstructs at a critical doping of $\mathrm{Ce}=0.14$. A competing antiferromagnetic phase is the likely cause of this Fermi surface reconstruction. Low-temperature linear-in-$T$ resistivity is found at $\mathrm{Ce}=0.14$, but anomalously, also at higher doping. We compare our data with similar behavior found in hole-doped cuprates at a doping where the pseudogap ends.