The exceptionally low-symmetry crystal structures of the time-reversal symmetry-breaking superconductors ${\mathrm{LaNiC}}_2$ and ${\mathrm{LaNiGa}}_2$ lead to an internally antisymmetric nonunitary triplet state as the only possibility compatible with experiments. We argue that this state has a distinct signature: a double-peak structure in the density of states (DOS) which resolves in the spin channel in a particular way. We construct a detailed model of ${\mathrm{LaNiGa}}_2$ capturing its electronic band structure and magnetic properties ab initio. The pairing mechanism is described via a single adjustable parameter. The latter is fixed by the critical temperature $T_c$ allowing parameter-free predictions. We compute the electronic specific heat and find excellent agreement with experiment. The size of the ordered moment in the superconducting state is compatible with zero-field muon spin relaxation experiments and the predicted spin-resolved DOS suggests the spin splitting is within the reach of present experimental technology.

Topological states of matter are, generally, quantum liquids of conserved topological defects. We establish this by constructing and analyzing topological field theories which introduce gauge fields to describe the dynamics of singularities in the original field configurations. Homotopy groups are utilized to identify topologically protected singularities, and the conservation of their protected number is captured by a topological action term that unambiguously obtains from the given set of symmetries. Stable phases of these theories include quantum liquids with emergent massless Abelian and non-Abelian gauge fields, as well as topological orders with long-range quantum entanglement, fractional excitations, boundary modes, and unconventional responses to external perturbations. This paper focuses on the derivation of topological field theories and basic phenomenological characterization of topological orders associated with homotopy groups ${\pi }_n\left(S^n\right), n\ge 1$. These homotopies govern monopole and hedgehog topological defects in $d=n+1$ dimensions, and enable the generalization of both weakly interacting and fractional quantum Hall liquids of vortices to $d>2$. Hedgehogs have not been in the spotlight so far, but they are particularly important defects of magnetic moments because they can be stimulated in realistic systems with spin-orbit coupling, such as chiral magnets and $d=3$ topological materials. We predict topological orders in systems with $\mathrm{U}\left(1\right)\times \mathrm{Spin}\left(d\right)$ symmetry in which fractional electric charge attaches to hedgehogs. Monopoles, the analogous defects of charge or generic U(1) currents, may bind to hedgehogs via Zeeman effect, or effectively emerge in purely magnetic systems. The latter can lead to spin liquids with different topological orders than that of the resonant valence bond spin liquid. Charge fractionalization of quarks in atomic nuclei is also seen as possibly arising from the charge-hedgehog attachment.

We proposed that BaHgSn is a Dirac semimetal (DSM) which can host hourglasslike surface states (HSSs) as protected by nonsymmorphic glide symmetry. Compared to KHgSb, an isostructural topological crystalline insulator with the same HSSs, BaHgSn has an additional band inversion at the $\mathrm{\Gamma }$ point. This band inversion is induced by the stronger interlayer coupling among Hg-Sn honeycomb layers than that among Hg-Sb layers in KHgSb, which leads to bulk Dirac nodes in BaHgSn along the layer stacking direction $\mathrm{\Gamma }-A$. In addition, the mirror Chern number $C_i$ protected by the mirror plane ${\overline{M}}_z$ ($k_z=0$) changes from 2 in KHgSb to 3 in BaHgSn. Therefore, when a compressive uniaxial strain is applied along the $y$ axis to break the rotation symmetry protecting the DSM state, BaHgSn becomes a strong topological insulator with $Z_2$ indices of $(1;000)$ and the topological surface Dirac cone coexists with HSSs on the (010) surface. The Wilson-loop spectra have been calculated to verify these topological features. The calculated surface states, the Fermi surfaces and their quasiparticle interference patterns are ready to be compared with experimental measurements.

High-quality single crystals of ${\left(\mathrm{PbSe}\right)}_5{\left({\mathrm{Bi}}_2{\mathrm{Se}}_3\right)}_{3m}$ were grown and analyzed by nuclear magnetic resonance (NMR) spectroscopy. We report on ${}^{77}\mathrm{Se}$ and ${}^{207}\mathrm{Pb}$ NMR shifts and nuclear spin-lattice relaxation measurements in the naturally formed heterostructure homology ${\left(\mathrm{PbSe}\right)}_5{\left({\mathrm{Bi}}_2{\mathrm{Se}}_3\right)}_{3m}$ with $m=1 \left({\mathrm{Pb}}_5{\mathrm{Bi}}_6{\mathrm{Se}}_{14}\right)$ and $m=2 \left({\mathrm{Pb}}_5{\mathrm{Bi}}_{12}{\mathrm{Se}}_{23}\right)$. A distinct site-specific contribution has been detected for both nuclei as a function of temperature, which reveals an electronic changeover from a semiconducting ${\mathrm{Pb}}_5{\mathrm{Bi}}_6{\mathrm{Se}}_{14}$ to a semimetalliclike ${\mathrm{Pb}}_5{\mathrm{Bi}}_{12}{\mathrm{Se}}_{23}$ system with nontrivial band structure features near the Fermi level. The temperature dependences of the relaxation rates are dominated by significant changes in the topology of energy dispersions accompanied with band edges and crossings in the region of the Fermi surface. These results, which interrogate nuclear spin interactions from selected atomic sites, clearly expose the effects of the added ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ layer on the crystal and electronic structure of ${\mathrm{Pb}}_5{\mathrm{Bi}}_{12}{\mathrm{Se}}_{23}$. These findings provide direct microscopic insight into the unconventional and dual nature of the electronic structure of these homologous thermoelectric and topologically nontrivial compounds.

We use cotunneling spectroscopy to investigate spin, orbital, and spin-orbital Kondo transport in a strongly confined system of InAs double quantum dots that are parallel coupled to source and drain. In the one-electron transport regime, the higher-symmetry spin-orbital Kondo effect manifests at orbital degeneracy and no external magnetic field. We then proceed to show that the individual Kondo contributions can be isolated and studied separately: either by orbital detuning in the case of spin Kondo transport or by spin splitting in the case of orbital Kondo transport. By varying the interdot tunnel coupling, we show that lifting of the spin degeneracy is key to confirming the presence of an orbital degeneracy and to detecting a small orbital hybridization gap. Finally, in the two-electron regime, we show that the presence of a spin-triplet ground state results in spin Kondo transport at zero magnetic field.

An AC electric field applied to a junction comprising two spin-orbit coupled weak links connecting a quantum dot to two electronic terminals is proposed to induce a DC current and to generate a voltage drop over the junction if it is a part of an open circuit. This photovoltaic effect requires a junction in which mirror reflection symmetry is broken. Its origin lies in the different ways inelastic processes modify the reflection of electrons from the junction back into the two terminals, which leads to uncompensated DC transport. The effect can be detected by measuring the voltage drop that is built up due to that DC current. This voltage is an even function of the frequency of the AC electric field.

We conducted ${}^{195}\mathrm{Pt}$-NMR measurements on various-diameter Pt nanoparticles coated with polyvinylpyrrolidone in order to detect the quantum size effect and the discrete energy levels in the electron density of states, both of which were predicted by Kubo more than 50 years ago. We succeeded in separating the signals arising from the surface and interior regions and found that the nuclear spin-lattice relaxation rates in both regions show the metallic behavior at high temperatures. Surprisingly, the magnetic fluctuations in both regions exhibited anomalous behavior below the same temperature $T^{*}$, which points to a clear size dependence and is well scaled with ${\delta }_{\mathrm{Kubo}}$. These results suggest that a size-tunable metal-insulator transition occurs in the Pt nanoparticles as a result of the quantum size effect.

By utilizing symmetry analysis and electronic structure calculations, we investigated the low-temperature orthorhombic phase of ${\mathrm{Ag}}_2\mathrm{Se}$ in $\mathcal{SG}$ 17. In addition to the discovery of a nodal plane at $k_z=\pi$ protected by the joint operation of time reversal ($\mathcal{T}$) and the twofold screw rotation $S_{2z}$, we found 24 Weyl points mainly residing at the $k_y=0$ plane with notable Fermi arc and large quasiparticle interference pattern (QPI). Due to the absence of mirror symmetry, a pair of Weyl points with opposite chirality reside at different binding energies, which makes this system an excellent material candidate for realizing the novel chiral anomaly related phenomenon, such as the quantized circular photogalvanic and the chiral magnetic effects. Furthermore, we also reveal the striking spin textures at $k_z=\pi$ plane which demonstrates, in a large region of the surface Brillouin zone, a direction-selective spin polarization, which has a strong implication to spintronic applications.

We study the ballistic conductivity of bilayer graphene in the presence of symmetry-breaking terms in an effective Hamiltonian for low-energy excitations, such as the trigonal-warping term (${\gamma }_3$), the electron-hole symmetry-breaking interlayer hopping (${\gamma }_4$), and the staggered potential (${\delta }_{AB}$). Earlier, it was shown that for ${\gamma }_3\ne 0$, in the absence of remaining symmetry-breaking terms (i.e., ${\gamma }_4={\delta }_{AB}=0$), the conductivity ($\sigma$) approaches the value of $3{\sigma }_0$ for the system size $L\to \infty$ [with ${\sigma }_0=8e^2/\left(\pi h\right)$ being the result in the absence of trigonal warping, ${\gamma }_3=0$]. We demonstrate that ${\gamma }_4\ne 0$ leads to the divergent conductivity ($\sigma \to \infty$) if ${\gamma }_3\ne 0$, or to the vanishing conductivity ($\sigma \to 0$) if ${\gamma }_3=0$. For realistic values of the tight-binding model parameters, ${\gamma }_3=0.3\mathrm{eV},{\gamma }_4=0.15\mathrm{eV}$ (and ${\delta }_{AB}=0$), the conductivity values are in the range $\sigma /{\sigma }_0\approx 4-5$ for $100\text{nm}<L<1\mu \mathrm{m}$, in agreement with existing experimental results. The staggered potential (${\delta }_{AB}\ne 0$) suppresses zero-temperature transport, leading to $\sigma \to 0$ for $L\to \infty$. Although $\sigma =\sigma \left(L\right)$ is no longer universal, the Fano factor approaches the pseudodiffusive value ($F\to 1/3$ for $L\to \infty$) in any case with nonvanishing $\sigma$ (otherwise, $F\to 1$), signaling the transport is ruled by evanescent waves. Temperature effects are briefly discussed in terms of a phenomenological model for staggered potential ${\delta }_{AB}={\delta }_{AB}\left(T\right)$ showing that, for $0<T⩽T_c\approx 12\mathrm{K}$ and ${\delta }_{AB}\left(0\right)=1.5\mathrm{meV},\sigma \left(L\right)$ is noticeably affected by $T$ for $L\gtrsim 100$ nm.

We study Landau-Zener transitions in a fermionic dissipative environment where a two-level (up and down states) system is coupled to two metallic leads kept with different chemical potentials at zero temperature. The dynamics of the system is simulated by an iterative numerically exact influence functional path integral method. In the pure Landau-Zener problem, two kinds of transition (from up to down state and from down to up state) probability are symmetric. However, this symmetry is destroyed by coupling the system to the bath. In addition, in both kinds of transitions, there exists a nonmonotonic dependence of the transition probability on the sweep velocity; meanwhile nonmonotonic dependence of the transition probability on the system-bath coupling strength is only shown in one of them. As in the spin-boson model, these phenomena can be explained by a simple phenomenological model.

Epitaxial films evolve on time and length scales that are inaccessible to atomistic computer simulation methods like molecular dynamics (MD). To numerically predict properties for such systems, a common strategy is to employ kinetic Monte Carlo simulations, for which one needs to know the transition rates of the involved elementary steps. The main challenge is thus to formulate a consistent model for the set of transition rates and to determine its parameters. Here, we revisit a well-studied model system, the epitaxial film growth of the fullerene ${\mathrm{C}}_{60}$ on an ordered ${\mathrm{C}}_{60}$ substrate (111). We implement a systematic multiscale approach in which we determine transition rates through MD simulations of specifically designed initial configurations. These rates follow Arrhenius's law, from which we extract energy barriers and attempt rates. We discuss the issue of detailed balance for the resulting rates. Finally, we study the morphology of subatomic and multilayer film growth and compare simulation results to experiments. Our model enables further studies on multilayer growth processes of ${\mathrm{C}}_{60}$ on other substrates.

We use temperature-dependent resistivity in small-angle twisted double bilayer graphene to measure bandwidths and gaps of the bands. This electron-hole asymmetric system has one set of nondispersing bands that splits into two flat bands with the electric field—distinct from the twisted bilayer system. With electric field, the gap between two emergent flat bands increases monotonically and bandwidth is tuned from 1 to 15 meV. These two flat bands with gap result in a series of thermally induced insulator to metal transitions—we use a model, at charge neutrality, to measure the bandwidth using only transport measurements. Having two flat bands with tunable gap and bandwidth offers an opportunity to probe the emergence of correlations.

The magnetic flux periodicity of $\frac{hc}{2e}$ is a well-known manifestation of Cooper pairing in typical $s$-wave superconductors. In this paper we theoretically show that the flux periodicity of a two-dimensional second order topological superconductor, which features zero-energy Majorana modes localized at the corners of the sample, is $\frac{hc}{e}$ instead. We further show that the periodicity changes back to $\frac{hc}{2e}$ at the transition to a topologically trivial superconductor, where the Majorana modes hybridize with the bulk states, demonstrating that the doubling of periodicity is a manifestation of the nontrivial topology of the state.

A parametrized tight-binding (TB) model based on the first-principles calculations is developed for monolayer tin diselenide (${\mathrm{SnSe}}_2$). The truncated model is derived from six maximally localized Wannier orbitals, which accurately describe the quasiparticle electronic states of ${\mathrm{SnSe}}_2$ over a wide energy range. Based on this TB model, we investigate the effects of doping, magnetic field, and point defects on the electronic and optical properties of ${\mathrm{SnSe}}_2$. Our numerical calculation shows that the ambipolar optical absorption can be effectively modulated by the type of injected carriers. The plasmonic spectrum of doped ${\mathrm{SnSe}}_2$ consists of standard intraband two-dimensional excitation and interband transition modes, which damp into electron-hole pairs with different speed. In the presence of perpendicular magnetic field, monolayer ${\mathrm{SnSe}}_2$ behaves like traditional two-dimensional electron gas with equally spaced Landau-level spectrums and magneto-optical conductivity. Moreover, the appearance of point defects in the sample would generate sharp midgap states, activating new transitions in the optical spectrum. Our proposed TB model can be used for further exploring the electronic and optical properties of large-scale ${\mathrm{SnSe}}_2$ with complex structures beyond the first-principles calculations.

In this study, the problem of strong Coulomb interactions in topological superconducting wire is analyzed by means of the density-matrix-renormalization-group (DMRG) approach. To analyze properties of edge states in the BDI-class structure, a quantity called Majorana polarization is used. From its dependence on wire length and an entanglement-spectrum degeneracy, topological phase diagrams are obtained. The DMRG calculations for the Shubin-Vonsovsky–type model of the wire show the transformation of phases with Majorana single and double modes (MSMs and MDMs, respectively) under the increase of onsite and intersite correlations. In particular, we demonstrate different scenarios including the possibilities of both induction and suppression of the MSMs and MDMs. It is shown that in the strongly correlated regime, the contributions of single-particle excitations to the Majorana-type states significantly decrease at low magnetic fxields. Moreover, the $t-J^{*}-V$ model is derived allowing to study the effective interactions and improve the DMRG numerics. It is found out that in the limiting case of the effective Hamiltonian with infinitely strong onsite repulsion, $t$ model, the topological phases are destroyed. Finally, the ways to probe the MSMs and MDMs via the features of caloric functions are discussed.

$\left[\mathrm{Ni}({\mathrm{HF}}_2\right)(3-\text{Clpyridine}){}_4]{\mathrm{BF}}_4$ (NBCT) is a one-dimensional, $S=1$ spin chain material that shows no magnetic neutron Bragg peaks down to temperatures of 0.1 K. Previous work identified NBCT as being in the Haldane phase and near a quantum phase transition as a function of $D/J$ to the large-$D$ quantum paramagnet phase (QPM), where $D$ is the axial single-ion anisotropy and $J$ is the intrachain superexchange. Herein, inelastic neutron scattering results are presented on partially deuterated, ${}^{11}\mathrm{B}$-enriched NBCT polycrystalline samples in zero magnetic field and down to temperatures of 0.3 K. Comparison to density matrix renormalization group calculations yields $D/J=1.51$ and a significant rhombic single-ion anisotropy $E$ ($E/D\approx 0.03, E/J\approx 0.05$). These $D, J$, and $E$ values place NBCT in the large-$D$ QPM phase but precipitously near a quantum phase transition to a long-range ordered phase.

We analyze the paradigmatic competition between intraband and crossband Cooper-pair formation in two-band superconductors, neglected in most works to date. We derive the phase-sensitive gap equations and describe the crossover between the intraband-dominated and the crossband-dominated regimes, delimited by a “gapless” state. Experimental signatures of crosspairing comprise notable gap splitting in the excitation spectrum, non-BCS behavior of gaps versus temperature, as well as changes in the pairing symmetry as a function of temperature. The consequences of these findings are illustrated on the examples of ${\mathrm{MgB}}_2$ and ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$.

Two-dimensional superconductors exfoliated from layered materials harbor novel superconductivity and exotic correlated phases, often concomitantly, but their discovery has been few and far between. Employing the anisotropic Migdal-Eliashberg formalism based on ab initio calculations, we find monolayer ${\mathrm{NiTe}}_2$ to be an intrinsic superconductor with a $T_{\text{c}}\sim 5.7$ K, although the bulk crystal is not known to superconduct. Remarkably, bilayer ${\mathrm{NiTe}}_2$ intercalated with lithium is found to display two-gap superconductivity with a critical temperature $T_{\text{c}}\sim 11.3$ K and a superconducting gap of $\sim 3.1$ meV, arising from a synergy of electronic and phononic effects. As monolayer and bilayer ${\mathrm{NiTe}}_2$ have been recently isolated experimentally, and lithium can be inserted into the bilayer via ionic liquid gating, the comparatively high $T_{\text{c}}$, substrate independence, and proximity tunability will make these superconductors ideal platforms for exploring intriguing correlation effects and quantum criticality associated two-dimensional superconductivity.

Recent progress in topological mechanics has revealed a family of Maxwell lattices that exhibit topologically protected floppy edge modes. These modes lead to a strongly asymmetric elastic wave response. In this paper, we show how topological Maxwell lattices can be used to realize nonreciprocal transmission of elastic waves. Our design leverages the asymmetry associated with the availability of topological floppy edge modes and the geometric nonlinearity built into the mechanical systems' response to achieve the desired nonreciprocal behavior, which can be further utilized to form a phonon diode via the addition of on-site pinning potentials that blocks the linear transmission and only allows the signal to transmit in one way. Finally, the nonreciprocal wave transmission can be switched on and off via topological phase transitions, paving the way to the design of cellular metamaterials that can serve as tunable topologically protected phonon diodes.