Most recent observations of topological Fermi arcs on the surface of manyfold degenerate B20 systems, CoSi and RhSi, have attracted enormous research interests. Although another isostructural system, FeSi, has been predicted to show bulk chiral fermions, it is yet to be clear theoretically and as well experimentally that whether FeSi possesses the topological surface Fermi arcs associated with the exotic chiral fermions in vicinity of the Fermi level. In this contribution, using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), we present the low-energy electronic structure of FeSi. We further report the surface state calculations to provide insights into the surface band structure of FeSi near the Fermi level. Unlike in CoSi or RhSi, FeSi has no topological Fermi arcs near the Fermi level as confirmed both from ARPES and surface state calculations. Further, the ARPES data show spin-orbit coupling (SOC) band splitting of 40 meV, which is in good agreement with bulk band-structure calculations. We noticed an anomalous temperature-dependent resistivity in FeSi which can be understood through the electron-phonon interactions as we find a Debye energy of 80 meV from the ARPES data.

We investigated the thermoelectric transport properties of ${\mathrm{EuNi}}_2{\mathrm{P}}_2$ and ${\mathrm{EuIr}}_2{\mathrm{Si}}_2$ to evaluate the relevance of Kondo interaction and valence fluctuations in these materials. While the thermal conductivities behave conventionally, the thermopower curves exhibit large values with pronounced maxima as typically observed in Ce- and Yb-based heavy-fermion materials. However, neither the positions of these maxima nor the absolute thermopower values at low temperature are in line with the heavy-fermion scenario and the moderately enhanced effective charge carrier masses. Instead, we may relate the thermopower in our materials to the temperature-dependent Eu valence by taking into account changes in the chemical potential. Our analysis confirms that valence fluctuations play an important role in ${\mathrm{EuNi}}_2{\mathrm{P}}_2$ and ${\mathrm{EuIr}}_2{\mathrm{Si}}_2$.

The $\mathrm{spin}\text{−}\frac{1}{2}$ Heisenberg antiferromagnetic chain is ideal for realizing one of the simplest gapless quantum spin liquids (QSLs), supporting a many-body ground state whose elementary excitations are fractional fermionic excitations called spinons. Here we report the discovery of such a one-dimensional (1D) QSL in $\mathrm{C}{\mathrm{s}}_4\mathrm{CuS}{\mathrm{b}}_2\mathrm{C}{\mathrm{l}}_{12}$. Compared to previously reported $S=\frac{1}{2}$ 1D chains, this material possesses a wider temperature range over which the QSL state is stabilized. We identify spinon excitations extending at $T>0.8\mathrm{K}$, with a large $T$-linear contribution to the specific heat, $\gamma =31.5\left(2\right)\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}$, which contribute itinerantly to thermal transport up to temperatures as high as $T=35\mathrm{K}$. At $T=0.7\mathrm{K}$, we find a second-order phase transition that is unchanged by a ${\mu }_{0}H=5\mathrm{T}$ magnetic field. $\mathrm{C}{\mathrm{s}}_4\mathrm{CuS}{\mathrm{b}}_2\mathrm{C}{\mathrm{l}}_{12}$ reveals new phenomenology deep in the 1D QSL regime, supporting a gapped QSL phase over a wide temperature range compared to many other experimental realizations.

We analyze in depth an $S_3$-invariant nearest-neighbor quantum chain in the region of a $U\left(1\right)$-invariant self-dual multicritical point. We find four distinct proximate gapped phases. One has three-state Potts order, corresponding to topological order in a parafermionic formulation. Another has “representation” symmetry-protected topological (RSPT) order, while its dual exhibits an unusual “not-$A$” order, where the spins prefer to align in two of the three directions. Within each of the four phases, we find a frustration-free point with exact ground state(s). The exact ground states in the not-$A$ phase are product states, each an equal-amplitude sum over all states where one of the three spin states on each site is absent. Their dual, the RSPT ground state, is a matrix product state similar to that of Affleck-Kennedy-Lieb-Tasaki. A field-theory analysis shows that all transition lines are in the universality class of the critical three-state Potts model. They provide a lattice realization of a flow from a free-boson field theory to the Potts conformal field theory.

We theoretically investigate the spin Hall conductivity (SHC) in topological Dirac semimetals (TDSMs) the Dirac points of which are protected by rotational symmetry. On the basis of a general phase diagram of the system with time-reversal, inversion, and fourfold rotational symmetries, we reveal that the SHC is sensitive to the phase to which the system belongs. The phase and the SHC are characterized by the mirror Chern numbers and the presence or absence of gapless bulk Dirac points. It is also found that the representative TDSM ${\mathrm{Cd}}_3{\mathrm{As}}_2$ supports a large and negative SHC ${\sigma }_{xy}^z\sim -{10}^4(\hslash /e){\left(\mathrm{\Omega }\text{m}\right)}^{-1}$. The principle behind the dependency of SHC on the phase diagram is also explained.

We propose an experiment to test the uniform-Berry-curvature picture of composite fermions. We show that the asymmetry of geometrical resonances observed in a periodically modulated composite fermion system can be explained with the uniform-Berry-curvature picture. Moreover, we show that an alternative way of modulating the system, i.e., modulating the external magnetic field, will induce an asymmetry opposite to that of the usual periodic grating modulation which effectively modulates the Chern-Simons field. The experiment can serve as a critical test of the uniform-Berry-curvature picture and probe the dipole structure of composite fermions proposed by Read.

We determine the band alignment at the ${\mathrm{CaF}}_2/\mathrm{Si}\left(111\right)$ interface through various advanced electronic structure methods. This interface is experimentally well studied and serves as an ideal test case to examine the accuracy of theoretical schemes. We use both global and range-separated hybrid functionals as well as $GW$ calculations including self-consistency and vertex corrections. Our calculation procedure accounts for residual strain resulting from the small mismatch in the lateral lattice constants at the interface to minimize the systematic error in the comparison with experiment. Both the hybrid functional and the $GW$ schemes give band alignments in overall good agreement with the experimental characterization. However, the considered methods yield sizable variations in the calculated band offsets, which do not originate from incorrect evaluations of the band gaps but rather from different inherent relative positions of the band edges. The comparison with experiment reveals that the global hybrid functional and the quasiparticle self-consistent $GW$ with vertex corrections give the most accurate description of the band alignment. We then determine the variation of the band offsets as a function of the amount of excess fluorine at the interface and attribute the experimental spread in the measured offsets to uncontrolled fluorine contamination.

We study the optical reflectivity of real three-dimensional (3D) photonic band-gap crystals with increasing thickness. The crystals consist of GaAs plates with nanorod arrays that are assembled by an advanced stacking method into high-quality 3D woodpile structures. We observe intense and broad reflectivity peaks with stop bands that correspond to a broad gap in the photonic band structures. The maximum reflectivity quickly reaches high values, even for a few crystal layers. Remarkably, the bandwidth of the stop bands hardly decreases with increasing crystal thickness, in good agreement with finite-difference time domain (FDTD) simulations. This behavior differs remarkably from the large changes observed earlier in weakly interacting 3D photonic crystals. The nearly constant bandwidth and high reflectivity are rationalized by multiple Bragg interference that occurs in strongly interacting photonic band-gap crystals, whereby the incident light scatters from multiple reciprocal lattice vectors simultaneously, in particular, from oblique ones that are parallel to a longer crystal dimension and thus experience hardly any finite-size effects. Our insights have favorable consequences for the application of 3D photonic band-gap crystals, notably since even thin structures reveal the full band-gap functionality, including devices that shield quantum bits from vacuum fluctuations.

We examine the bound-state and free-state contributions to the density of states in a three-dimensional electron gas with a two-dimensional interface or quantum well with Rashba spin-orbit coupling. The motivation for our research comes from the interest in materials that exhibit Rashba spin splitting of energy bands and the Edelstein and inverse Edelstein effects in quantum wells or interfaces. By modifying the Hamiltonian of three-dimensional electron gas models to include a two-dimensional component with Rashba spin-orbit coupling, we are able to calculate the bound-state and free-state wave functions and corresponding densities of states analytically. In the case of weak asymmetry across the interface, we find that one of the spin-split energy bands has an upper bound where it merges into a three-dimensional energy band. On the other hand, with strong asymmetry across the interface, only one spin-momentum-locked band exists as an interface energy band, which emerges from a three-dimensional energy band.

Statistical properties of the electron transport flowing through nanostructures are strongly influenced by the interactions, geometry of the system, and/or by type of the external electrodes. These factors affect not only the average current induced in the system but also contribute to fluctuations in the flux of charges and their correlations. Due to possible applications of the hybrid nanosystems, containing one or more superconducting electrodes, a detailed understanding of the flow of charge and its fluctuations seems to be of primary importance. Coulomb repulsion between electrons usually strongly affect the current-current correlation function. In this work we study the correlations in the charge flow through such an interacting quantum dot contacted to one superconducting and two normal electrodes. This setup allows for analysis of the Andreev scattering events in the correlations of currents flowing between external electrodes and, in particular, gives access to cross-correlations between currents to or from different normal electrodes. Our approach relies on the master equation technique, which properly captures the Coulomb interactions. We study the finite-frequency correlations and find the relaxation processes, related to the high-frequency charge and low-frequency polarization fluctuations. The multiterminal structure of single-electron device studied here allows us to analyze a competition between the intra- and interchannel correlations. In the appropriate limit of the interacting quantum dot embedded between two normal electrodes our calculations quantitatively describe the recent experimental data on the frequency-dependent correlations. This shows a promising potential of the method for description of the hybrid systems with superconducting electrode(s).

We theoretically investigate a periodically driven semimetal based on a square lattice. The possibility of engineering both a Floquet topological insulator featuring Floquet edge states and a Floquet higher order topological insulating phase accommodating topological corner modes has been demonstrated starting from the semimetal phase, based on a Floquet Hamiltonian picture. A topological phase transition takes place in the bulk quasienergy spectrum with the variation of the drive amplitude where the Chern number changes signs from $+1$ to $-1$. This can be attributed to broken time-reversal invariance ($\mathcal{T}$) due to circularly polarized light. When the discrete fourfold rotational symmetry (${\mathcal{C}}_4$) is also broken by adding a Wilson mass term along with broken $\mathcal{T}$, a higher order topological insulator (HOTI), hosting in-gap modes at all the corners, can be realized. The Floquet quadrupolar moment, calculated with the Floquet states, exhibits a quantized value of $0.5$ (modulo 1), identifying the HOTI phase. We also show the emergence of the dressed corner modes at quasienergy $\omega /2$ (remnants of zero modes in the quasistatic high frequency limit), where $\omega$ is the driving frequency, in the intermediate frequency regime.

The transport behavior of a double quantum dot side-attached to a topological superconducting wire hosting Majorana zero-energy modes is studied theoretically in the strong correlation regime. It is shown that Majorana modes can leak to the whole nanostructure, giving rise to a subtle interplay between the two-stage Kondo screening and the half-fermionic nature of Majorana quasiparticles. In particular, coupling to the topological wire is found to reduce the effective exchange interaction between the two quantum dots in the absence of normal leads. Interestingly, it also results in an enhancement of the second-stage Kondo temperature when the normal leads are attached. Moreover, it is shown that the second stage of the Kondo effect can become significantly modified in one of the spin channels due to the interference with the Majorana zero-energy mode, yielding low-temperature conductance equal to $G=G_0/4$, where $G_0=2e^2/h$, instead of $G=0$ in the absence of the topological superconducting wire. We also identify a nontrivial spin-charge $\mathrm{SU}\left(2\right)$ symmetry present in the system at a particular point in parameter space, despite lack of the spin and charge conservation. Finally, we discuss the consequences of a finite overlap between two Majorana modes, as relevant for short Majorana wires.

Transition-metal dichalcogenide $1T\text{−}{\mathrm{VSe}}_2$ experimentally exhibits multiple charge density wave (CDW) orders, but its origin is still under debate. Using first-principles calculations, we investigate the origin of CDW orders in $1T\text{−}{\mathrm{VSe}}_2$ and clarify the ground state of CDW in the freestanding monolayer. Our results show that both Fermi-surface nesting and electron-phonon coupling account for the $4\times 4\times 3$ CDW superstructure in bulk $1T\text{−}{\mathrm{VSe}}_2$, while the momentum-dependent electron-phonon coupling-induced $\sqrt{7}\times \sqrt{3}$ CDW superstructure is most stable in the freestanding monolayer $1T\text{−}{\mathrm{VSe}}_2$. For monolayer $1T\text{−}{\mathrm{VSe}}_2$, the substrate-induced compressive strain can turn the ground state into the $4\times 4$ CDW superstructure, while tensile strain preserves the $\sqrt{7}\times \sqrt{3}$ superstructure. Our results demonstrate the origin of the CDW orders in $1T\text{−}{\mathrm{VSe}}_2$ and shed light on the experimental observation of multiple CDW orders in monolayer $1T\text{−}{\mathrm{VSe}}_2$.

We study the thermoelectric transport properties of fractional quantum Hall systems based on an exact diagonalization calculation. Based on the relation between the thermoelectric response and thermal entropy, we demonstrate that thermoelectric Hall conductivity ${\alpha }_{xy}$ has power-law scaling ${\alpha }_{xy}\propto T^{\eta }$ for gapless composite Fermi-liquid states at filling numbers $\nu =1/2$ and $1/4$ at low temperatures ($T$), with an exponent $\eta \sim 0.5$ distinctly different from Fermi liquids. The power-law scaling remains unchanged for different forms of interaction including Coulomb and short-range ones, demonstrating the robustness of non-Fermi-liquid behavior of these interacting systems at low $T$. In contrast, for the $1/3$ fractional quantum Hall state, ${\alpha }_{xy}$ vanishes at low $T$ with an activation gap associated with neutral collective modes rather than charged quasiparticles. Our results establish another manifestation of the non-Fermi-liquid nature of quantum Hall fluids at a finite temperature.

The cubic Kondo lattice ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$ is one of the heaviest known Fermi liquids. We have measured the low-temperature electrical resistivity $\rho \left(T\right)$, magnetic susceptibility $\chi \left(T\right)$, and heat capacity $C\left(T\right)$ in single crystals of $\mathrm{Yb}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Zn}}_{20}$ ($x\le 0.126$) and $\mathrm{Yb}{\left({\mathrm{Co}}_{1-x}{\mathrm{Fe}}_y\right)}_2{\mathrm{Zn}}_{20}$ ($y\le 0.07$). While pure ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$ displays maxima in $\rho \left(T\right)$ and $\chi \left(T\right)$, ascribed to an enhanced crystal electric field (CEF) degeneracy, the maxima are suppressed by small amounts ($\approx 6$ at.%) of substitutions of the Co atoms. This goes along with a suppression of superheavy Fermi liquid behavior as manifested in the divergence of $C/T$. We ascribe the observations to local distortions at the Yb site due to the chemical disorder in the environment, rather than to a chemical pressure effect. This fragileness to the local distortion indicates that superheavy Fermion behavior in ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$ is closely linked to an enhanced CEF degeneracy.

We discuss valley current, which is carried by quasiparticles in graphene. We show that the valley current arises owing to a peculiar term in the electron-phonon collision integral that mixes the scalar and vector gauge-field-like vertices in the electron-phonon interaction. This mixing makes collisions of phonons with electrons sensitive to their chirality, which is opposite in two valleys. As a result of collisions with phonons, electrons of the different valleys deviate in opposite directions. The valley-dependent deviation of the quasiparticle current does not request for breaking the spatial inversion symmetry. The effect exists both in pristine graphene or bilayer graphene samples, and it increases with temperature owing to a higher rate of collisions with phonons at higher temperatures. The valley current carried by quasiparticles could be detected by measuring the electric current using a nonlocal transformer of a suitable design.

The celebrated Berezinskii-Kosterlitz-Thouless (BKT) phase transition refers to a topological transition characterized, e.g., by the dissociation of vortex-antivortex pairs in two-dimensional (2D) systems. Such an unusual phase has been reported in various types of materials, but not in the class of systems made by one-unit-cell-thick (1UC) ferroelectrics (also called 2D ferroelectrics). Here, the use of a first-principles-based effective Hamiltonian method leads to the discovery of many fingerprints of a BKT phase existing in between the ferroelectric and paraelectric states of fully relaxed 1UC tin tellurium. Moreover, epitaxial strain is found to have dramatic consequences on the temperature range of such a BKT phase for 1UC SnTe. Consequently, our predictions extend the playground of BKT theory to another class of functional materials, and demonstrate that strain is an effective tool to alter the BKT characteristics there.

We propose a defect-to-edge topological quantum quench protocol that can efficiently inject electric charge from defect-core states into a chiral edge current of an induced Chern insulator. The initial state of the system is assumed to be a Mott insulator, with electrons bound to topological defects that are pinned by disorder. We show that a “critical quench” to a Chern insulator mass of the order of the Mott gap shunts charge from defects to the edge, while a second stronger quench can trap it there and boost the edge velocity, creating a controllable current. We apply this idea to a skyrmion charge in the $\nu =0$ quantum Hall antiferromagnet in graphene, where the quench into the Chern insulator could be accomplished via Floquet driving with circularly polarized light.

Here, we report a scanning tunneling microscopy (STM) and spectroscopy (STS) study in the superconducting state of $2H\text{−}{\mathrm{NbS}}_2$. We directly visualize the existence of an incommensurate charge density wave (CDW) that is pinned by atomic impurities. In strong tunneling conditions, the incommensurate CDW is depinned from impurities by the electric field from the STM tip. To explore the possible mechanism of this incommensurate CDW, we perform STM-based inelastic tunneling spectroscopy (IETS) to detect phonon excitations in $2H\text{−}{\mathrm{NbS}}_2$ and measure the influence of atomic impurities on local phonon excitations. In comparison with the calculated vibrational density of states in $2H\text{−}{\mathrm{NbS}}_2$, we find two branches of phonon excitations which correspond to the vibrations of Nb ions and S ions, and the strength of the local phonon excitations is insensitive to the atomic impurities. Our results demonstrate the coexistence of impurity-pinned incommensurate CDW and superconductivity in $2H\text{−}{\mathrm{NbS}}_2$, and open a way for detecting atomic-scale phonon excitations in transition metal dichalcogenides with STM-based IETS.