Topological phases of matter that depend for their existence on interactions are fundamentally interesting and potentially useful as platforms for future quantum computers. Despite the multitude of theoretical proposals, the only interaction-enabled topological phase experimentally observed is the fractional quantum Hall liquid. To help identify other systems that can give rise to such phases, we present in this work a detailed study of the effect of interactions on Majorana zero modes bound to vortices in a superconducting surface of a three-dimensional topological insulator. This system is of interest because, as was recently pointed out, it can be tuned into the regime of strong interactions. We start with a zero-dimensional system suggesting an experimental realization of the interaction-induced ${\mathbb{Z}}_8$ ground-state periodicity previously discussed by Fidkowski and Kitaev [Phys. Rev. B 81, 134509 (2010); Phys. Rev. B 83, 075103 (2011)] . We argue that the periodicity is experimentally observable using a tunnel probe. We then focus on interaction-enabled crystalline topological phases that can be built with the Majoranas in a vortex lattice in higher dimensions. In one dimension, we identify an interesting exactly solvable model which is related to a previously discussed one that exhibits an interaction-enabled topological phase. We study these models using analytical techniques, exact numerical diagonalization, and density matrix renormalization group. Our results confirm the existence of the interaction-enabled topological phase and clarify the nature of the quantum phase transition that leads to it. We finish with a discussion of models in dimensions 2 and 3 that produce similar interaction-enabled topological phases.

The structural, electronic, optical, and photocatalytic properties of out-of-plane and in-plane heterostructures of transition metal dichalcogenides are investigated by (hybrid) first principles calculations. The out-of-plane heterostructures are found to be indirect band gap semiconductors with type-II band alignment. Direct band gaps can be achieved by moderate tensile strain in specific cases. The excitonic peaks show blueshifts as compared to the parent monolayer systems, whereas redshifts occur when the chalcogen atoms are exchanged along the series S-Se-Te. Strong absorption from infrared to visible light as well as excellent photocatalytic properties can be achieved.

We present a theory for the large suppression of the superfluid density ${\rho }_s$ in ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right){}_2$ in the vicinity of a putative spin-density wave quantum critical point at a P doping, $x=x_c$. We argue that the transition becomes weakly first order in the vicinity of $x_c$, and disorder induces puddles of superconducting and antiferromagnetic regions at short length scales; thus, the system becomes an electronic microemulsion. We propose that frustrated Josephson couplings between the superconducting grains suppress ${\rho }_s$. In addition, the presence of “normal” quasiparticles at the interface of the frustrated Josephson junctions will give rise to a highly nontrivial feature in the low-frequency response in a narrow vicinity around $x_c$. We propose a number of experiments to test our theory.

In a combined experimental and theoretical study, we investigate the properties of ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Rh}}_x{\mathrm{O}}_4$. From the branching ratios of the $L$-edge isotropic x-ray absorption spectra, we determine that the spin-orbit coupling is remarkably independent of $x$ for both iridium and rhodium sites. $\text{DFT}+U$ calculations show that the doping is close to isoelectronic and introduces impurity bands of predominantly rhodium character close to the lower Hubbard band. Overlap of these two bands leads to metallic behavior. Since the low-energy states for $x<0.5$ have predominantly $j_{\mathrm{eff}}=\frac{1}{2}$ character, we suggest that the electronic properties of this material can be described by an inhomogeneous Hubbard model, where the on-site energies change due to local variations in the spin-orbit interaction strength combined with additional changes in binding energy.

Nuclear magnetic resonance (NMR) experiments on single crystals of ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ are presented that identify two distinct temperature-dependent spin susceptibilities: One is due to a spin component that is temperature-dependent above the critical temperature for superconductivity ($T_{\mathrm{c}}$) and reflects pseudogap behavior; the other is Fermi-liquid-like in that it is temperature independent above $T_{\mathrm{c}}$ and vanishes rapidly below $T_{\mathrm{c}}$. In addition, we demonstrate the existence of a third spin susceptibility: It is temperature independent at higher temperatures, vanishes at lower temperatures (below $T_0\ne T_{\mathrm{c}}$), and changes sign near optimal doping. This susceptibility either arises from the coupling between the two spin components, or it could be given by a distinct third spin component. Recent susceptibility data on single crystals support its presence in most cuprates.

Combining photoemission spectroscopy, Raman spectroscopy, and first-principles calculations, we characterize superconducting ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$ as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides, in ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$ it lies in the immediate vicinity of the Fermi level, giving rise to a pronounced van Hove singularity. The resulting heavy band mass resolves the apparent puzzle of a large normal-state Sommerfeld coefficient [H. Wang et al., Phys. Rev. Lett 111, 207001 (2013)] in this weakly correlated compound.

We investigate the effect of interactions on the stability of a disordered, two-dimensional topological insulator realized as an array of nanowires or chains of magnetic atoms on a superconducting substrate. The Majorana zero-energy modes present at the ends of the wires overlap, forming a dispersive edge mode with thermal conductance determined by the central charge $c$ of the low-energy effective field theory of the edge. We show numerically that, in the presence of disorder, the $c=1/2$ Majorana edge mode remains delocalized up to extremely strong attractive interactions, while repulsive interactions drive a transition to a $c=3/2$ edge phase localized by disorder. The absence of localization for strong attractive interactions is explained by a self-duality symmetry of the statistical ensemble of disorder configurations and of the edge interactions, originating from translation invariance on the length scale of the underlying mesoscopic array.

Using low-temperature scanning tunneling spectroscopy, we probe the Landau levels of the topologically protected state of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$(0001) after in situ cleavage of a single crystal. Landau levels are visible for magnetic fields $B\ge 2$ T at energies, which confirm the Dirac type dispersion including the zeroth Landau level. We find different Dirac velocities for the lower and the upper part of the Dirac cone in reasonable agreement with previous density functional theory data. The Dirac point deduced from the zeroth Landau level shifts by about 40 meV between different areas of the sample indicating long-range potential fluctuations. The local potentials are correlated to different local defect densities varying slightly stronger than expected from a statistical distribution. Moreover, the width of the Landau level peaks is analyzed. It is found to increase, mostly linearly, with the energy distance to the Fermi level. Consequently, we attribute the peak width to a dominating scattering of the hot quasiparticles by electron-electron interaction.

We study the correlated electronic structure of single-layer iridates based on structurally undistorted ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. Starting from the first-principles band structure, the interplay between local Coulomb interactions and spin-orbit coupling is investigated by means of rotational-invariant slave-boson mean-field theory. The evolution from a three-band description towards an anisotropic one-band $(J=1/2)$ picture is traced. Single-site and cluster self-energies shed light on competing Slater- and Mott-dominated correlation regimes. A nodal/antinodal Fermi-surface dichotomy is revealed at strong coupling, with an asymmetry between electron and hole doping. Electron-doped iridates show clearer tendencies of Fermi-arc formation, reminiscent of hole-doped cuprates.

We employ a combination of the ab initio band-structure methods and dynamical mean-field theory to determine the electronic structure and phase stability of paramagnetic FeO at high pressure and temperature. Our results reveal a high-spin to low-spin transition within the B1 crystal structure of FeO upon compression of the lattice volume above 73 GPa. The spin-state transition is accompanied by an orbital-selective Mott metal-insulator transition (MIT). The lattice volume is found to collapse by about 8.5% at the MIT, implying a complex interplay between electronic and lattice degrees of freedom. Our results for the electronic structure and lattice properties are in overall good agreement with experimental data.

We develop a general response theory of gapless Fermi points with nontrivial topological charges for gauge and nonlinear sigma fields, which asserts that the topological character of the Fermi points is embodied as the terms with discrete coefficients proportional to the corresponding topological charges. Applying the theory to the effective nonlinear sigma models for topological Fermi points with disorders in the framework of replica approach, we derive rigorously the Wess-Zumino terms with the topological charges being their levels in the two complex symmetry classes of A and AIII. Intriguingly, two nontrivial examples of quadratic Fermi points with the topological charge 2 are respectively illustrated for the classes A and AIII. We also address a qualitative connection of topological charges of Fermi points in the real symmetry classes to the topological terms in the nonlinear sigma models, based on the one-to-one classification correspondence.

Bilayer graphene in a perpendicular electric field can host domain walls between regions of reversed field direction or interlayer stacking. The gapless modes propagating along these domain walls, while not strictly topological, nevertheless have interesting physical properties, including valley-momentum locking. A junction where two domain walls intersect forms the analog of a quantum point contact. We study theoretically the critical behavior of this junction near the pinch-off transition, which is controlled by two separate classes of nontrivial quantum critical points. For strong interactions, the junction can host phases of unique charge and valley conductances. For weaker interactions, the low-temperature charge conductance can undergo one of two possible quantum phase transitions, each characterized by a specific critical exponent and a collapse to a universal scaling function, which we compute.

The local structures around Co and Fe in ${\mathrm{BiCo}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_3$ (BCFO) have been determined element-selectively by means of extended x-ray absorption fine structure (EXAFS) measurements at the Co and Fe $K$ edges. In the tetragonal (T) phase at $x\le 0.63$, both Fe and Co ions are trivalent and locate in the ${\mathrm{O}}_5$ pyramidal coordination. The four in-plane Fe bonds with transverse oxygen (Fe-O2) split into $2\times 2$ bonds in the monoclinic (M) phase at $0.63\le x\le 0.73$ whereas the Co-O2 bonds do not show a split exceeding the resolution of our EXAFS analysis. This result is an element-specific interpretation of the polarization rotation in the M phase around the morphotropic phase boundary. The components of thermal vibration and static disorder are evaluated separately from the temperature dependence of the Debye Waller factor from $T=8\mathrm{K}$ to 300 K. The static disorder of the Fe site is large compared to that of the Co site, thereby indicating that Fe is unstable in the ${\mathrm{O}}_5$ pyramidal coordination of the T and M phases. This study demonstrates that the instability of Fe ions plays an important role in the sequential structural distortion of the BCFO system.

We experimentally demonstrate that the ferromagnetic intramolecular $\pi -d$ interaction works between an itinerant $\pi$-electron spin and a localized $d$-electron's magnetic moment in the iron-phthalocyanine (Pc) molecular compound. The evaluation of the hidden $\pi -d$ coupling is achieved by preparing the isolated ${\mathrm{Fe}\left(\mathrm{Pc}\right)\left(\mathrm{CN}\right)}_2$ molecular solution with unpaired $\pi$- and $d$-electron spins, which is generated through the oxidization by iodine bromide (IBr). The monotonic increase of the magnetization with IBr addition and the saturation value of the Curie constant indicate the ferromagnetic $\pi -d$ coupling. Furthermore, through the magnetization measurements of the single crystals of neutral $\pi$ radical ${\mathrm{Fe}\left(\mathrm{Pc}\right)\left(\mathrm{CN}\right)}_2·2{\mathrm{CHCl}}_3$, we reveal that the on-site $\pi -d$ interaction in ${\mathrm{Fe}\left(\mathrm{Pc}\right)\left(\mathrm{CN}\right)}_2$ is extremely large ($J_{\pi \mathrm{d}}/k_{\mathrm{B}}>500\mathrm{K}$) among those in other molecular materials.

A phenomenological equation called the Landau-Lifshitz-Baryakhtar (LLBar) [Zh. Eksp. Teor. Fiz 87, 1501 (1984) [Sov. Phys. JETP 60, 863 (1984)]] equation, which could be viewed as the combination of the Landau-Lifshitz (LL) equation and an extra “exchange-damping” term, was derived by Baryakhtar using Onsager's relations. We interpret the origin of this exchange damping as nonlocal damping by linking it to the spin current pumping. The LLBar equation is investigated numerically and analytically for the spin-wave decay and domain-wall motion. Our results show that the lifetime and propagation length of short-wavelength magnons in the presence of nonlocal damping could be much smaller than those given by the LL equation. Furthermore, we find that both the domain-wall mobility and the Walker breakdown field are strongly influenced by the nonlocal damping.

We theoretically investigate spin current and magnetic response induced by the twisting phonon mode in carbon nanotubes via the spin-rotation coupling. An effective magnetic field due to the twisting mode induces both spin and orbital magnetizations. The induced spin and orbital magnetizations have both radial and axial components. We show that ac pure spin current is generated by the twisting phonon mode. The magnitude of the spin current and orbital magnetization for a (10,10) armchair nanotube is estimated as an example. We find that the ac pure spin current is detectable in magnitude when the frequency of the twisting mode is of the order of GHz, and that the orbital magnetization is found to be larger than the spin magnetization.

We have studied multiferroic ${\mathrm{YMn}}_2{\mathrm{O}}_5$ by high-pressure neutron diffraction in a large pressure range from 0.5 to 6.3 GPa. We observe a pressure-induced commensurate (PCM) phase with a propagation vector $\left(\frac{1}{2}0\frac{1}{2}\right)$ that is different from those observed at ambient pressure. It coexists with the ambient pressure phases up to the highest pressure, with an increased contribution as the pressure increases. The PCM phase, which is likely generic in the $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ family, should be taken into account to understand the strong variation of the electric polarization under pressure.

The structural transitions of the ground state of a system of repulsively interacting particles confined in a quasi-one-dimensional channel, and the effect of the interparticle interaction as well as the functional form of the confinement potential on those transitions are investigated. Although the nonsequential ordering of transitions (non-SOT), i.e., the $1-2-4-3-4-5-6-\cdots$ sequence of chain configurations with increasing density, is widely robust as predicted in a number of theoretical studies, the sequential ordering of transitions (SOT), i.e., the $1-2-3-4-5-6-\cdots$ chain, is found as the ground state for long-ranged interparticle interaction and hard-wall-like confinement potentials. We found an energy barrier between every two different phases around its transition point, which plays an important role in the preference of the system to follow either a SOT or a non-SOT. However, that preferential transition requires also the stability of the phases during the transition. Additionally, we analyze the effect of a small structural disorder on the transition between the two phases around its transition point. Our results show that a small deformation of the triangular structure changes dramatically the picture of the transition between two phases, removing in a considerable region the non-SOT in the system. This feature could explain the fact that the non-SOT is, up to now, not observed in experimental systems, and suggests a more advanced experimental setup to detect the non-SOT.

Palladium hydride ${\mathrm{PdH}}_x(0<x<1)$, which may be the most well known metal hydride, still has some unexplained phenomena. In order to elucidate the origin of the “50 K anomaly,” we have measured the heat capacity of the $\beta$ phase of ${\mathrm{PdH}}_x(x=0.638,0.725,0.782,0.829)$ and ${\mathrm{PdD}}_{0.641}$ by means of a custom-built adiabatic calorimeter. A steplike anomaly in heat capacity appeared at a temperature depending on $x$ between 50 and 75 K. An exothermic effect followed by an endothermic one, implying an enthalpy relaxation to the equilibrium state, took place around the anomaly. This anomaly is attributed to not a phase transition as has been believed so far, but a glass transition corresponding to the freezing of the jump motion of hydrogen atoms in a short-range ordered state. If the hydrogen motion is not frozen, the completely ordered state could be realized at a hypothetical transition which appears below the glass transition temperature. Interestingly, the glass transition disappeared in ${\mathrm{PdH}}_x(x=0.553,0.629)$ and ${\mathrm{PdD}}_{0.372}$ particles with a diameter of ca. 8 nm. This could be due to a quantum tunneling effect which is caused by surface and/or distortion effect of nanoparticles.

We investigate, theoretically and numerically, the spin dynamics of a two-component exciton-polariton condensate created and sustained by nonresonant spin-polarized optical pumping in a semiconductor microcavity. Using the open-dissipative mean-field model, we show that the existence of well defined phase-locked steady states of the condensate may lead to efficient switching and control of spin (polarization) states with a nonresonant excitation. Spatially inhomogeneous pulsed excitations can cause symmetry breaking in the pseudospin structure of the condensate and lead to formation of nontrivial spin textures. Our model is universally applicable to two linearly coupled polariton condensates, and therefore can also describe the behavior of condensate populations and phases in “double-well” type potentials.