Realization of topological superconductors is one of the most important goals in studies of topological phases in quantum materials. In this work we theoretically propose a way to attain topological superconductors with nontrivial Fermi surfaces of Bogoliubov quasiparticles. Considering the interacting Luttinger model with $j=3/2$ electrons, we investigate the dominant superconducting channels for a multiorbital quadratic band touching system with finite chemical potential, which breaks the particle-hole symmetry in the normal state. Notably, while the system generally favors $d$-wave pairing, the absence of the particle-hole symmetry necessarily induces $s$-wave pairing and such emergence of $s$-wave pairing leads to new types of topological $d+s$ wave superconductors and selects particular $d$-wave pairing channels. Based on the Landau theory with SO(3) symmetry, we demonstrate that two kinds of topological superconductors are energetically favored; uniaxial nematic phase with secondary $s$-wave pairing ($d_{3z^2-r^2}+s$) and time-reversal-symmetry broken phase with secondary $s$-wave pairing $(d_{3z^2-r^2}+d_{xy}+i{d}_{x^2-y^2}+s)$. These superconductors contain either nodal lines or Fermi pockets of gapless Bogoliubov quasiparticles and moreover exhibit topological invariants, leading to nontrivial surface states such as drumheadlike surface states or Fermi arcs. We discuss applications of our theory to relevant families of materials, especially half-Heusler compound YPtBi, and suggest possible future experiments.

A combination of strong Cooper pairing and weak superconducting fluctuations is crucial to achieve and stabilize high-$T_c$ superconductivity. We demonstrate that a coexistence of a shallow carrier band with strong pairing and a deep band with weak pairing, together with the Josephson-like pair transfer between the bands to couple the two condensates, realizes an optimal multicomponent superconductivity regime: it preserves strong pairing to generate large gaps and a very high critical temperature but screens the detrimental superconducting fluctuations, thereby suppressing the pseudogap state. Surprisingly, we find that the screening is very efficient even when the interband coupling is very small. Thus, a multiband superconductor with a coherent mixture of condensates in the BCS regime (deep band) and in the BCS-BEC crossover regime (shallow band) offers a promising route to higher critical temperatures.

We consider a two-dimensional model of particles interacting in a Landau level. We work in a finite disk geometry and take the particles to interact with a linearly decreasing two-body Haldane pseudopotential. We show that the ground-state subspace of this model is spanned by the wave functions that can be written as polynomial conformal blocks (of an arbitrary conformal field theory) consistent with the filling fraction (scaling dimension). To remove degeneracies, we then add a quadratic perturbation to the Hamiltonian and show that (1) conformal blocks constructed using the Moore-Read construction (e.g., Laughlin, Pfaffian, and Read-Rezayi states) remain exact eigenstates of this model in the thermodynamic limit, and (2) by tuning an externally imposed single-body $-L_z^2$ potential we can enforce Moore-Read conformal blocks to become exact ground states of this model in the thermodynamic limit. We cannot rule out the possibility of residual degeneracies in this limit. This model has no filling dependence and is comprised only from two-body long-range interactions and external single-body potentials. Our results provide insight into how conformal block wave functions can emerge in a Landau level.

We compute dynamic spin susceptibilities in the two-dimensional Hubbard model using the method of dual fermions, and we provide a comparison to lattice Monte Carlo and cluster dynamical mean-field theory. We examine the energy dispersion identified by peaks in $\mathrm{Im}\chi (\omega ,q)$, which define spin modes, and we compare the exchange scale and magnon dispersion to neutron experiments on the parent ${\mathrm{La}}_2{\mathrm{CuO}}_4$ cuprate. We present the evolution of the spin excitations as a function of Hubbard interaction strengths and doping, and we explore the particle-hole asymmetry of the spin excitations. We also study the correlation lengths and the spin excitation dispersion peak structure, and we find a Y-shaped dispersion similar to neutron results on doped ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$.

The ground-state density of states of the half-filled Falicov-Kimball model contains a charge-density-wave gap. At finite temperature, this gap is not immediately closed, but is rather filled in by subgap states. For a specific combination of parameters, this leads to a stable phase where the system is in an ordered charge-density-wave phase, but there is high density of states at the Fermi level. We show that this property can be, in finite dimensions, traced to a crossing of sharp states resulting from the single particle excitations of the localized subsystem. The analysis of the inverse participation ratio points to a strong localization in the discussed regime. However, the pronounced subgap density of states can still lead to a notable increase of charge transport through a finite size system. We show this by focusing on the transmission in heterostructures where a Falicov-Kimball system is sandwiched between two metallic leads.

The pyrochlore materials have long been predicted to harbor a quantum spin liquid, an intrinsic long-range-entangled state supporting fractionalized excitations. Existing pyrochlore experiments, on the other hand, have discovered several weakly ordered states and a tendency of close competition amongst them. Motivated by these facts, we give a complete classification of spin-orbit-coupled ${\mathbb{Z}}_2$ spin-liquid states on the pyrochlore lattice by using the projective symmetry group (PSG) approach for bosonic spinons. For each spin liquid, we construct a mean-field Hamiltonian that can be used to describe phase transitions out of the spin liquid via spinon condensation. Studying these phase transitions, we establish phase diagrams for our mean-field Hamiltonians that link magnetic orders to specific spin liquids. In general, we find that seemingly unrelated magnetic orders are intertwined with each other and that the conventional spin orders seen in the experiments are accompanied by more exotic hidden orders. Our critical theories are categorized into $z=1$ and $z=2$ types, based on their spinon dispersion and Hamiltonian diagonalizability, and are shown to give distinct signatures in the heat capacity and the spin structure factor. This study provides a clear map of pyrochlore phases for future experiments and variational Monte Carlo studies in pyrochlore materials.

In the presence of symmetries, one-dimensional quantum systems can exhibit topological order, which in many cases can be characterized by a quantized value of the many-body geometric Zak or Berry phase. We establish that this topological Zak phase is directly related to the Zak phase of an elementary quasiparticle excitation in the system. By considering various systems, we establish this connection for a number of different interacting phases including the Su-Schrieffer-Heeger model, $p$-wave topological superconductors, and the Haldane chain. Crucially, in contrast to the bulk many-body Zak phase associated with the ground state of such systems, the topological invariant associated with quasiparticle excitations (above this ground state) exhibits a more natural route for direct experimental detection. To this end, we build upon recent work [F. Grusdt, et al., Nat. Commun. 7, 11994 (2016)] and demonstrate that mobile quantum impurities can be used, in combination with Ramsey interferometry and Bloch oscillations, to directly measure these quasiparticle topological invariants. Finally, a concrete experimental realization of our protocol for dimerized Mott insulators in ultracold atomic systems is discussed and analyzed.

We study the in-plane acoustic phonons in twisted bilayer graphenes using the effective continuum approach. We calculate the phonon modes by solving the continuum equation of motion for infinitesimal vibration around the static relaxed state with triangular domain structure. We find that the moiré interlayer potential only affects the in-plane asymmetric modes, where the original linear dispersion is broken down into miniphonon bands separated by gaps, while the in-plane symmetric modes with their linear dispersion are hardly affected. The phonon wave functions of asymmetric modes are regarded as collective vibrations of the domain-wall network, and the low-energy phonon band structure can be qualitatively described by an effective moiré-scale lattice model.

Out-of-plane vibrations are considered as the dominant factor limiting the intrinsic carrier mobility of suspended two-dimensional materials at low carrier concentrations. Anharmonic coupling between in-plane and flexural phonon modes is usually excluded from the consideration. Here we present a theory for electron-phonon scattering, in which the anharmonic coupling between acoustic phonons is systematically taken into account. Our theory is applied to the typical group V two-dimensional semiconductors: hexagonal phosphorus, arsenic, and antimony. We find that the role of the flexural modes is essentially suppressed by their coupling with in-plane modes. At dopings lower than ${10}^{12}{\mathrm{cm}}^{-2}$ the mobility reduction does not exceed 30%, being almost independent of the concentration. Our findings suggest that compared to in-plane phonons, flexural phonons are considerably less important in the electronic transport of two-dimensional semiconductors, even at low carrier concentrations.

We report a detailed experimental study of the band structure of the recently discovered topological material ${\mathrm{Hf}}_2{\mathrm{Te}}_2\mathrm{P}$. Using the combination of scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy with surface K doping, we probe the band structure of ${\mathrm{Hf}}_2{\mathrm{Te}}_2\mathrm{P}$ with energy and momentum resolution above the Fermi level. Our experiments show the presence of multiple surface states with a linear Dirac-like dispersion, consistent with the predictions from previously reported band-structure calculations. In particular, scanning tunneling spectroscopy measurements provide experimental evidence for the strong topological surface state predicted at $460\mathrm{meV}$, which stems from the band inversion between Hf-$d$ and Te-$p$ orbitals. This band inversion comprised of more localized $d$ states could result in a better surface-to-bulk conductance ratio relative to more traditional topological insulators.

We provide here a direct and exact formalism to describe the formation of edge or surface states, as well as to calculate boundary Green's functions. Modeling the boundary as an impurity potential, we show via the T-matrix formalism that the impurity states evolve into boundary modes when the impurity potential goes to infinity. We apply this technique to obtain Majorana states in one- (1D) and two-dimensional Kitaev systems. For the 1D case we also calculate the corresponding boundary Green's functions. We argue that our formalism can be applied to other topological models, as well as to any model exhibiting edge states.

We report the synthesis and detailed investigation of the single crystal ${\mathrm{CePd}}_2{\mathrm{Al}}_8$. Resistivity measurements reveal typical metallic behavior at high temperatures and a plateau at about 15 K followed by a rapid drop due to successive antiferromagnetic (AFM) and ferromagnetic (FM) transitions. The specific heat and magnetic susceptibility present obvious anomalies related to both transitions. We find an extremely large magnetic anisotropy with ${\chi }_a/{\chi }_b>2000$ at 2 K. With increasing magnetic field, the AFM transition temperature is gradually suppressed and only the ferromagnetic phase remains above 500 Oe for $H\parallel a$, with an ordered moment of $1.43{\mu }_B$, while for $H\parallel b$ the FM transition turns into an AFM-like transition. With pressure, the two phases merge at 12.5 GPa into a possibly ferromagnetic phase, pointing to a possible critical point at higher pressures. We construct the temperature-magnetic field and temperature-pressure phase diagrams for the single crystal ${\mathrm{CePd}}_2{\mathrm{Al}}_8$.

We adopt a high-throughput search strategy that begins with electronic structure features to screen a set of half-Heusler compounds for thermoelectric performance. This is motivated by the contradictory electrical transport requirements, specifically high electrical conductivity $\sigma$ and high thermopower $S$ for obtaining high figure of merit $ZT$. We use an electronic fitness function that measures the extent to which a specific band structure decouples $\sigma$ and $S$ for this purpose. We then perform detailed, more costly, calculations of thermal conductivity and electrical properties for those compounds that have a high electronic fitness. This provides an efficient method for identifying promising compounds from a set of candidates. We apply this to a set of 75 previously studied half-Heusler compounds. The approach identifies several compounds as having potential as thermoelectric materials. Importantly, these include not only some previously identified candidates, but also some other compounds that had not been identified previously using other screening methods.

Combining first-principles calculations with Landauer-Büttiker formalism, ballistic thermoelectric transport properties of semiconducting two-dimensional transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely ${\mathrm{MX}}_2$ with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are investigated in their 2H and 1T phases. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we report the thermoelectric properties of the semiconducting ones. We find that 2H phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T phases. The maximum room temperature $p$-type thermoelectric figure of merit ($ZT$) of 1.57 is obtained for $2\mathrm{H}-{\mathrm{HfSe}}_2$, which can be as high as 3.30 at $T=800$ K. Additionally, $2\mathrm{H}-{\mathrm{ZrSe}}_2, 2\mathrm{H}-{\mathrm{ZrTe}}_2$, and $2\mathrm{H}-{\mathrm{HfS}}_2$ have considerable $ZT$ values (both $n$- and $p$-type), that are above 1 at room temperature. The 1T phases of Zr and Hf-based oxides possess relatively high power factors, however their high lattice thermal conductance values limit their $ZT$ values to below 1 at room temperature.

Mono- and few-layer transition-metal dichalcogenides (TMDCs) provide opportunities for ideal two-dimensional semiconductors for electronic and optoelectronic devices. For electronic devices on TMDCs, it is essential to incorporate $n$- and/or $p$-type dopants which are stable in positions after patterned doping. Here we investigate hydrogen doping for TMDC ($M{X}_2$ with $M=\mathrm{Mo}$, W and $X=\mathrm{S}$, Se, Te) nanosheets by first-principles calculations to address diffusion and doping properties. We find that adsorbed hydrogen atoms in TMDCs are energetically most stable at the interstitial site right on the Mo or W plane and have substantial energy barriers against diffusion that increase in the order of sulfides, selenides, and tellurides. Located at the most stable interstitial site on the Mo or W plane, the hydrogen atoms produce electrons in the conduction bands in the extremely high rate of one electron per hydrogen atom, without any defect state inside the band gap remarkably. We analyze the chemical bonding character around the dopant and the mechanism for such high efficiency of electron doping. We also consider properties of hydrogen molecules and Te vacancies for comparison. Our work shows that hydrogen doping is the promising pathway to development of highly integrated electronic devices on TMDCs.

Recent advances in ultrashort laser pulse techniques have opened up a wide variety of applications in both fundamental physics and industrial fields. In this work, $abinitio$ molecular dynamics simulations based on time-dependent density functional theory revealed a steady deceleration of lattice distortion propagation in an aluminum slab with increasing laser pulse intensity. Analyzing the interatomic force revealed a significant reduction in the harmonic terms and nonmonotonic growth of anharmonicity. This is characterized by spatially nonuniform force screening by plasmon under a nonperturbative excitation condition that is not captured in Born–Oppenheimer molecular dynamics, and is consistent with the current interpretation of laser-induced periodic structure patterning. This work highlights the significance of collective electronic excitations for modeling the structure formation in such a system.

We investigate the transport properties of a superconducting quantum point contact in the presence of an arbitrary periodic drive. In particular, we calculate the DC current and noise in the tunnel limit, obtaining general expressions in terms of photoassisted probabilities. Interesting features can be observed when the frequency is comparable to the gap. Here, we show that quantized Lorentzian pulses minimize the excess noise, further strengthening the hierarchy among different periodic drives observed in the electron quantum optics domain. In this regime, the excess noise is directly connected to the overlap between electron and hole energy distributions driven out of equilibrium by the applied voltage. In the adiabatic limit, where the frequency of the drive is very small compared to the superconducting gap, we recover the conventional Shapiro-spikes physics in the supercurrent.