Quasi-one-dimensional $A_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ (with $A=\text{K}$, Cs, Rb) is an intriguing new family of superconductors which exhibit many similar features to the cuprate and iron-based unconventional superconductor families. Yet, in contrast to these systems, no charge or magnetic ordering has been observed which could provide the electronic correlations presumed necessary for an unconventional superconducting pairing mechanism—an absence which defies predictions of first-principles models. We report the results of neutron scattering experiments on polycrystalline ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ ($T_c\sim 7\text{K}$) which probed the low-temperature dynamics near $T_c$. Neutron diffraction data evidence a subtle response of the nuclear lattice to the onset of superconductivity while inelastic scattering reveals a highly dispersive column of intensity at the commensurate wave vector $q=\left(00\frac{1}{2}\right)$ which loses intensity beneath $T_c$—indicative of short-range magnetic fluctuations. Using linear spin-wave theory, we model the observed scattering and suggest a possible structure to the short-range magnetic order. These observations suggest that ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ is in close proximity to a magnetic instability and that the incipient magnetic order both couples strongly to the lattice and competes with superconductivity, in direct analogy with the iron-based superconductors.

We present an extended Tersoff potential for boron nitride (BN-ExTeP) for application in large scale atomistic simulations. BN-ExTeP accurately describes the main low energy B, N, and BN structures and yields quantitatively correct trends in the bonding as a function of coordination. The proposed extension of the bond order, added to improve the dependence of bonding on the chemical environment, leads to an accurate description of point defects in hexagonal BN $(h$-BN) and cubic BN $(c$-BN). We have implemented this potential in the molecular dynamics LAMMPS code and used it to determine some basic properties of pristine 2D $h$-BN and the elastic properties of defective $h$-BN as a function of defect density at zero temperature. Our results show that there is a strong correlation between the size of the static corrugation induced by the defects and the weakening of the in-plane elastic moduli.

We report the observation of magnetic-field-induced decrease of the spin Peltier effect (SPE) in a junction of a paramagnetic metal Pt and a ferrimagnetic insulator ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG) at room temperature. For driving the SPE, spin currents are generated via the spin Hall effect from applied charge currents in the Pt layer, and injected into the adjacent thick YIG film. The resultant temperature modulation is detected by a commonly used thermocouple attached to the Pt/YIG junction. The output of the thermocouple shows sign reversal when the magnetization is reversed and linearly increases with the applied current, demonstrating the detection of the SPE signal. We found that the SPE signal decreases with the magnetic field. The observed decreasing rate was found to be comparable to that of the spin Seebeck effect (SSE), suggesting the dominant and similar contribution of the low-energy magnons in the SPE as in the SSE.

We investigate the magnetic torque and magnetoresistance (MR) responses in oblique magnetic fields in micrometer-sized specimens of the hexagonal chiral magnetic crystal ${\mathrm{CrNb}}_3{\mathrm{S}}_6$. The results exhibit hysteresis over a wide range of applied field angles, while reversible behavior appears only when the magnetic field is closely aligned to the helical axis of the crystal. Stepwise changes of the magnetic torque and MR detected in the hysteresis region indicate the existence of chiral solitons in the oblique magnetic fields. A magnetic phase diagram is derived from the experimental results, and the stability of the chiral magnetic phases, such as the chiral soliton lattice and chiral conical phase, and the nature of the phase transition between them are discussed.

Recent developments in the techniques of ultrafast pump-probe photoemission have made possible the search for collective modes in strongly correlated systems out of equilibrium. Including inelastic scattering processes and a retarded interaction, we simulate time- and angle-resolved photoemission spectroscopy (trARPES) to study the amplitude mode of a $d$-wave superconductor, a collective mode excited through the nonlinear light-matter coupling to the pump pulse. We find that the amplitude mode oscillations of the $d$-wave order parameter occur in phase at a single frequency that is twice the quasi-steady-state maximum gap size after pumping. We comment on the necessary conditions for detecting the amplitude mode in trARPES experiments.

We study the effect of isoelectronic doping and external pressure in tuning the ground state of the honeycomb iridate ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ by combining optical spectroscopy with synchrotron x-ray diffraction measurements on single crystals. The obtained optical conductivity of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is discussed in terms of a Mott-insulating picture versus the formation of quasimolecular orbitals and in terms of Kitaev interactions. With increasing Li content $x$, $\left({\mathrm{Na}}_{1-x}{\mathrm{Li}}_x\right){}_2{\mathrm{IrO}}_3$ moves deeper into the Mott-insulating regime, and there are indications that up to a doping level of 24% the compound comes closer to the Kitaev limit. The optical conductivity spectrum of single-crystalline $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ does not follow the trends observed for the series up to $x=0.24$. There are strong indications that $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ is not as close to the Kitaev limit as ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and lies closer to the quasimolecular orbital picture instead. Except for the pressure-induced hardening of the phonon modes, the optical properties of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ seem to be robust against external pressure. Possible explanations of the unexpected evolution of the optical conductivity with isolectronic doping and the drastic change between $x=0.24$ and $x=1$ are given by comparing the pressure-induced changes of lattice parameters and the optical conductivity with the corresponding changes induced by doping.

We use determinant quantum Monte Carlo (DQMC), in combination with the principal component analysis (PCA) approach to unsupervised learning, to extract information about phase transitions in several of the most fundamental Hamiltonians describing strongly correlated materials. We first explore the zero-temperature antiferromagnet to singlet transition in the periodic Anderson model, the Mott insulating transition in the Hubbard model on a honeycomb lattice, and the magnetic transition in the 1/6-filled Lieb lattice. We then discuss the prospects for learning finite temperature superconducting transitions in the attractive Hubbard model, for which there is no sign problem. Finally, we investigate finite temperature charge density wave (CDW) transitions in the Holstein model, where the electrons are coupled to phonon degrees of freedom, and carry out a finite size scaling analysis to determine $T_c$. We examine the different behaviors associated with Hubbard-Stratonovich auxiliary field configurations on both the entire space-time lattice and on a single imaginary time slice, or other quantities, such as equal-time Green's and pair-pair correlation functions.

We propose a quantum field theory description of the X-cube model of fracton topological order. The field theory is not (and cannot be) a topological quantum field theory (TQFT) since, unlike the X-cube model, TQFTs are invariant (i.e., symmetric) under continuous space-time transformations. However, the theory is instead invariant under a certain subgroup of the conformal group. We describe how braiding statistics and ground-state degeneracy are reproduced by the field theory, and how the the X-cube Hamiltonian and field theory can be minimally coupled to matter fields. We also show that even on a manifold with trivial topology, spatial curvature can induce a ground-state degeneracy that is stable to arbitrary local perturbations! Our formalism may allow for the description of other fracton field theories, where the only necessary input is an equation of motion for a charge density.

We analytically derived the effective two-body interaction for a finite thickness quantum Hall system with a harmonic perpendicular confinement and an in-plane magnetic field. The anisotropic effective interaction in the lowest Landau level (LLL) and first Landau level (1LL) are expanded in the basis of the generalized pseudopotentials (PPs), and we analyze how the coefficients of some prominent isotropic and anisotropic PPs depend on the thickness of the sample and the strength of the in-plane magnetic field. We also investigate the stability of the topological quantum Hall states, especially the Laughlin state and its emergent guiding center metric, which we can now compute analytically. An interesting reorientation of the anisotropy direction of the Laughlin state in the 1LL is revealed, and we also discuss various possible experimental ramifications for this quantum Hall system with broken rotational symmetry.

We present a quantitative analysis of the energy levels and wave functions of carriers in a cubic quantum dot of GaAs embedded in ${\mathrm{Ga}}_{(1-x)}{\mathrm{Al}}_x\mathrm{As}$ with finite confining potential barriers at their interfaces. The energy spectrum has substantially reduced degeneracies compared to the analytically determined energy levels of the infinite-barrier quantum box of the same dimensions. The level degeneracy of states has been explained by group representations of the point group $O_h$. Projection operators for the irreducible representations provide a way to obtain the linear combinations of the degenerate wave functions which form a basis set for the representations. Energy-level splittings in the presence of externally applied electric and magnetic fields are also studied.

A metastable trigonal phase, existing only as small patches on a chemically exfoliated few-layered, thermodynamically stable $1H$ phase of $\mathrm{Mo}{\mathrm{S}}_2$, is believed to critically influence the properties of $\mathrm{Mo}{\mathrm{S}}_2$-based devices. The electronic structure of this metastable phase is little understood in the absence of a direct experimental investigation of its electronic properties, complicated further by conflicting claims from theoretical investigations. We address this issue by investigating the electronic structure of this minority phase in chemically exfoliated $\mathrm{Mo}{\mathrm{S}}_2$ few-layered systems by enhancing its contributions with the use of highly spatially resolved ($\le 120$ nm resolution) photoemission spectroscopy and Raman spectroscopy in conjunction with state-of-the-art electronic structure calculations. Based on these results, we establish that the ground state of this phase, arrived at by the chemical exfoliation of $\mathrm{Mo}{\mathrm{S}}_2$ using the usual Li intercalation technique, is a small gap ($\sim 90\pm 40\mathrm{meV}$) semiconductor in contrast to most claims in the literature; we also identify the specific trigonal structure it has among many suggested ones.

The quantum Hall effect is studied in the topological insulator ${\mathrm{BiSbTeSe}}_2$. By employing top- and back-gate electric fields at high magnetic field, the Landau levels of the Dirac cones in the top and bottom topological surface states can be tuned independently. When one surface is tuned to the electron-doped side of the Dirac cone and the other surface to the hole-doped side, the quantum Hall edge channels are counter-propagating. The opposite edge mode direction, combined with the opposite helicities of top and bottom surfaces, allows for scattering between these counter-propagating edge modes. The total Hall conductance is expected to be integer valued only when the scattering is strong. For weaker interaction, a noninteger quantum Hall effect is expected and indications for this effect are measured.

We show that spin-orbit coupling (SOC) in InSe enables the optical transition across the principal band gap to couple with in-plane polarized light. This transition, enabled by $p_{x,y}\leftrightarrow p_z$ hybridization due to intra-atomic SOC in both In and Se, can be viewed as a transition between two dominantly $s$- and $p_z$-orbital based bands, accompanied by an electron spin-flip. Having parametrized $\mathbf{k}·\mathbf{p}$ theory using first-principles density functional theory we estimate the absorption for ${\sigma }^{\pm }$ circularly polarized photons in the monolayer as $\sim 1.5$%, which saturates to $\sim 0.3$% in thicker films (3–5 layers). Circularly polarized light can be used to selectively excite electrons into spin-polarized states in the conduction band, which permits optical pumping of the spin polarization of In nuclei through the hyperfine interaction.

We study topological crystalline insulators doped with magnetic impurities, in which ferromagnetism at the surface lowers the electronic energy by spontaneous breaking of a crystalline symmetry. The number of energetically equivalent ground states is sensitive to the crystalline symmetry of the surface, as well as the precise density of electrons at the surface. We show that for a SnTe model in the topological state, magnetic states can have twofold symmetry, sixfold symmetry, or eightfold degenerate minima. We compute spin stiffnesses within the model to demonstrate the stability of ferromagnetic states, and consider their ramifications for thermal disordering. Possible experimental consequences of the surface magnetism are discussed.

Topological nodal-line semimetals are characterized by one-dimensional lines of band crossing in the Brillouin zone. In contrast to nodal points, nodal lines can be in topologically nontrivial configurations. In this Rapid Communication, we introduce the concept of “nodal-knot semimetals,” whose nodal lines form topologically nontrivial knots in the Brillouin zone. We introduce a generic construction of nodal-knot semimetals, which yields the simplest trefoil nodal knot and other more complicated nodal knots. The knotted-unknotted transitions by nodal-line reconnections are also studied. Our work brings the knot theory to the subject of topological semimetals.

We examine the ballistic conduction through Au-NiMnSb-Au heterostructures consisting of up to four units of the half-metallic NiMnSb in the scattering region, using density functional theory (DFT) methods. For a single NiMnSb unit the transmission function displays a spin polarization of around $50\%$ in a window of $1\mathrm{eV}$ centered around the Fermi level. By increasing the number of layers, an almost complete spin polarization of the transmission is obtained in this energy range. Supplementing the DFT calculations with local electronic interactions, of Hubbard-type on the Mn sites, leads to a hybridization between the interface and many-body states. The significant reduction of the spin polarization seen in the density of states is not apparent in the spin polarization of the conduction electron transmission, which suggests that the hybridized interface and many-body induced states are localized.

Charge transport in disordered organic semiconductors, which is governed by incoherent hopping between localized molecular states, is frequently studied using a mean-field approach. However, such an approach only considers the time-averaged occupation of sites and neglects the correlation effect resulting from the Coulomb interaction between charge carriers. Here, we study the charge transport in unipolar organic devices using kinetic Monte Carlo simulations and show that the effect of Coulomb correlation is already important when the charge-carrier concentration is above ${10}^{-3}$ per molecular site and the electric field is smaller than ${10}^8$ V/m. The mean-field approach is then no longer valid, and neglecting the effect can result in significant errors in device modeling. This finding is supported by experimental current density-voltage characteristics of ultrathin sandwich-type unipolar poly(3-hexylthiophene) (P3HT) devices, where high carrier concentrations are reached.

We study the spin-charge conversion mechanisms in a two-dimensional gas of electrons moving in a smooth disorder potential by accounting for both Rashba-type and Mott's skew scattering contributions. We find that the quantum interference effects between spin-flip and skew scattering give rise to anisotropic spin precession scattering (ASP), a direct spin-charge conversion mechanism that was discovered in an earlier study of graphene decorated with adatoms [Huang et al., Phys. Rev. B 94, 085414 (2016)]. Our findings suggest that, together with other spin-charge conversion mechanisms such as the inverse galvanic effect, ASP is a fairly universal phenomenon that should be present in disordered two-dimensional systems lacking inversion symmetry.