We report studies of thermal conductivity as functions of magnetic field and temperature in the helimagnetic insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ that reveal novel features of the spin-phase transitions as probed by magnon heat conduction. The tilted conical spiral and low-temperature skyrmion phases, recently identified in small-angle neutron scattering studies, are clearly identified by sharp signatures in the magnon thermal conductivity. Magnon scattering associated with the presence of domain boundaries in the tilted conical phase and regions of skyrmion and conical-phase coexistence are identified.

The magnon-polaron induced anomaly has recently been observed in the magnetic field dependence of the spin seebeck effect in ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ [T. Kikkawa et al., Phys. Rev. Lett. 117, 207203 (2016)]. In the present theoretical work, we show that such an anomaly in ${\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ depends strongly on the temperature, and it exists only at high temperature but disappears at low temperature. The finite-temperature magnon-polaron anomaly results from the distortion of the magnon dispersion driven by thermal magnon excitation. In contrast to the scenario in ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, the anomaly in ${\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ can present either single-peak or double-peak features at different temperatures.

I propose a method to directly measure the space and time dependence of the pair field correlator of a pair density wave. The method is based on two separate ideas. First, I adopt a recently proposed solenoid insertion method to provide the momentum in a tunnel junction. Second, I suggest the use of optimal or overdoped Bi-2201 films as a tunneling electrode with a known charge ordering wave vector which can match the expected pair density wave wave vectors I wish to study. The method is applicable to both fluctuating and ordered states. Potential applications are to the proposed stripe pair density wave order in LBCO and the possible pair density wave fluctuations at finite temperature or in high magnetic field in underdoped YBCO as well as other members of the cuprate family.

Motivated by recent experiments, we use the $+U$ extension of the generalized gradient approximation to density functional theory to study superlattices composed of alternating layers of ${\mathrm{LaNiO}}_3$ and ${\mathrm{LaMnO}}_3$. For comparison we also study a rocksalt [(111) double perovskite] structure and bulk ${\mathrm{LaNiO}}_3$ and ${\mathrm{LaMnO}}_3$. A Wannier function analysis indicates that band parameters are transferable from bulk to superlattice situations with the exception of the transition-metal $d$-level energy, which has a contribution from the change in $d$-shell occupancy. The charge transfer from Mn to Ni is found to be moderate in the superlattice, indicating metallic behavior, in contrast to the insulating behavior found in recent experiments, while the rocksalt structure is found to be insulating with a large Mn-Ni charge transfer. We suggest a high density of cation antisite defects may account for the insulating behavior experimentally observed in short-period superlattices.

We consider a one-dimensional commensurate Peierls insulator in the presence of spin-dependent sign-alternating potentials. In a continuum description, the latter supply the fermions with spin-dependent relativistic masses $m_{\uparrow ,\downarrow }$. The ground-state phase diagram describes three gapped phases: the charge-density-wave (CDW) and spin-density-wave (SDW)-like band insulator phases sandwiched by a mixed phase in which the CDW and SDW superstructures coexist with a nonzero spontaneous dimerization (SD). The critical lines separating the massive phases belong to the Ising universality class. The Ising criticality is accompanied by the Kohn anomaly in the renormalized phonon spectrum. We derive a Ginzburg criterion which specifies a narrow region around the critical point where quantum fluctuations play a dominant role, rendering the adiabatic (or mean-field) approximation inapplicable. A full account of quantum effects is achieved in the antiadiabatic limit where the effective low-energy theory represents a massive version the $N=4$ Gross-Neveu model. Using Abelian bosonization, we demonstrate that the description of the SD phase, including its critical boundaries, is well approximated by a sum of two effective double-frequency sine-Gordon (DSG) models subject to self-consistency conditions that couple the charge and spin sectors. Using the well-known critical properties of the DSG model, we obtain the singular parts of the dimerization order parameter and staggered charge and spin susceptibilities near the Ising critical lines. We show that, in the antiadiabatic limit, on the line $m_{\downarrow }=0$ there exists of a Berezinskii-Kosterlitz-Thouless critical point separating a Luttinger-liquid gapless phase from the spontaneously dimerized one. We also discuss topological excitations of the model carrying fractional charge and spin.

In this paper, we examine the viscoelastic properties of integer quantum Hall (IQH) states in a tilted magnetic field. In particular, we explore to what extent the tilted-field system behaves like a two-dimensional electron gas with anisotropic mass in the presence of strain deformations. We first review the Kubo formalism for viscosity in an external magnetic field, paying particular attention to the role of rotational symmetry and contact terms. Next, we compute the conductivity, stress, and viscosity tensors for IQH states in the presence of a tilted field and vertical confining potential. By comparing our results with the recently developed bimetric formalism, we show that, at the level of the contracted Hall viscosity tensor, the mapping between tilted field and effective mass anisotropy holds only if we simultaneously modify the background perpendicular magnetic field; in other words, a simultaneous measurement of the density, contracted Hall viscosity, and Hall conductivity at fixed particle number can distinguish between tilted field and effective mass anisotropy. Additionally, we show that in the presence of a tilted magnetic field, the stress tensor acquires an unusual anisotropic ground state average, leading to anomalous elastic response functions. We develop a formalism for projecting a three-dimensional Hamiltonian with confining potential and magnetic field to a two-dimensional Hamiltonian in order to further address the phenomenology of the tilted-field IQH fluid. We find that the projected fluid couples nonminimally to geometric deformations, indicating the presence of internal geometric degrees of freedom.

${\mathrm{LiV}}_2{\mathrm{O}}_4$ spinel is known to exhibit heavy-fermion-like behavior below a characteristic temperature $T_K\simeq 20$ K, while it preserves a paramagnetic state down to $T\sim {10}^{-2}$ K due to geometrical frustration. Here, it is shown that the dynamical spin susceptibility $\chi (\mathbf{q},\omega )$ in ${\mathrm{LiV}}_2{\mathrm{O}}_4$ exhibits an anomalous duality which is modeled as a sum of itinerant (${\chi }_{\mathrm{F}}$) and local (${\chi }_{\mathrm{L}}$) components, and that the local spin dynamics inferred from ${\chi }_{\mathrm{L}}(\mathbf{q},\omega )$ is qualitatively different from that expected from time-averaged bulk properties. The anomaly coexists with the marginal Fermi-liquid behavior inferred from the $-lnT$ dependence of the electronic specific heat over a wide temperature range below $T_K$. We argue that such unusual properties of ${\mathrm{LiV}}_2{\mathrm{O}}_4$ can be attributed to the putative metallic spin-liquid state emerging near the quantum critical point between spin-glass and Fermi-liquid states.

A canonical model for many-body localization (MBL) is studied, of interacting spinless fermions on a lattice with uncorrelated quenched site disorder. The model maps onto a tight-binding model on a ‘Fock-space (FS) lattice’ of many-body states, with an extensive local connectivity. We seek to understand some aspects of MBL from this perspective, via local propagators for the FS lattice and their self-energies (SE's), focusing on the SE probability distributions, over disorder and FS sites. A probabilistic mean-field theory (MFT) is first developed, centered on self-consistent determination of the geometric mean of the distribution. Despite its simplicity this captures some key features of the problem, including recovery of an MBL transition, and predictions for the forms of the SE distributions. The problem is then studied numerically in $1d$ by exact diagonalization, free from MFT assumptions. The geometric mean indeed appears to act as a suitable order parameter for the transition. Throughout the MBL phase the appropriate SE distribution is confirmed to have a universal form, with long-tailed Lévy behavior as predicted by MFT. In the delocalized phase for weak disorder, SE distributions are clearly log-normal, while on approaching the transition they acquire an intermediate Lévy-tail regime, indicative of the incipient MBL phase.

We theoretically investigate the role of the interfacial Rashba spin-orbit coupling in spin pumping based on a nonperturbative calculation. A nonmonotonic behavior is predicted in the Rashba-strength dependence of the Gilbert damping coefficients. We show that the in-plane damping component can exceed the Sharvin limit thanks to the Rashba-field-induced torque. Nevertheless, the pumped spin current remains below the Sharvin limit and satisfies the Onsager reciprocity relations with the spin-current-induced spin-transfer torque.

We consider a qubit that is driven along its logical $z$ axis, with noise along the $z$ axis in the driving field $\mathrm{\Omega }$ proportional to some function $f\left(\mathrm{\Omega }\right)$, as well as noise along the logical $x$ axis. We establish that whether or not errors due to both types of noise can be canceled out, even approximately, depends on the explicit functional form of $f\left(\mathrm{\Omega }\right)$ by considering a power-law form, $f\left(\mathrm{\Omega }\right)\propto {\mathrm{\Omega }}^k$. In particular, we show that such cancellation is impossible for $k=0,1$, or any even integer. However, any other odd integer value of $k$ besides 1 does permit cancellation; in fact, we show that both types of errors can be corrected with a sequence of four square pulses of equal duration. We provide sets of parameters that correct for errors for various rotations and evaluate the error, measured by the infidelity, for the corrected rotations versus the naïve rotations, i.e., the operations that, in the complete absence of noise, would produce the desired rotations (in this case a single pulse of appropriate duration and magnitude). We also consider a train of four trapezoidal pulses, which take into account the fact that there will be, in real experimental systems, a finite rise time, again providing parameters for error-corrected rotations that employ such pulse sequences. Our dynamical decoupling error correction scheme works for any qubit platform as long as the errors are quasistatic.

Using ab initio methods based on density functional theory, we show that rotated graphene bilayers at angles different from the magic ones can have an electronic spectrum similar to those by applying moderate external pressures. We find that for an angle of $5.{08}^{\circ }$ and a pressure of 2.19 GPa, the spin-restricted spectrum displays a flat band at the Fermi level similar to the one found at magic angles. In addition, the spin-unrestricted calculations show a correlated ferromagnetic ground state with a total magnetic moment of $3.7{\mu }_B$ per unit cell being mostly localized in the AA stacking region of the Moiré pattern. The possibility of antiferromagnetic order is considered but not calculated. Doping the system with either electrons or holes destroys the magnetic moments.

An effective semianalytical method for analyzing the Cartesian multipole contributions in light transmission and reflection spectra of flat metasurfaces composed of identical nanoparticles is developed and demonstrated. The method combines numerical calculation of metasurface reflection and transmission coefficients with their multipole decompositions. The developed method is applied for the multipole analysis of reflection and transmission spectra of metasurfaces composed of silicon nanocubes or nanocones. In the case of nanocubes, we numerically demonstrate a “lattice invisibility effect,” when light goes through the metasurface almost without amplitude and phase perturbations with the simultaneous excitation of nanoparticles' multipole moments. The effect is realized due to destructive interference between the fields generated by the basic multipole moments of nanoparticles in the backward and forward directions. For metasurfaces composed of conical nanoparticles, we show that their transmission coefficient does not depend on illumination direction. In contrast, the reflection and absorption can be different for the illumination from different metasurface sides, which is associated with the excitation of different multipoles. We believe our results could be useful for analysis and understanding of the electromagnetic properties of nanoparticle arrays and pave the way for the design of novel metasurfaces for various optical applications.

While long-theorized, the direct observation of multiple highly dispersive ${\mathrm{C}}_{60}$ valence bands has eluded researchers for more than two decades due to a variety of intrinsic and extrinsic factors. Here we report a realization of multiple highly dispersive (330–520 meV) valence bands in pure thin film ${\mathrm{C}}_{60}$ on a novel substrate—the three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$—through the use of angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. The effects of this novel substrate reducing ${\mathrm{C}}_{60}$ rotational disorder are discussed. Our results provide important considerations for past and future band structure studies as well as the increasingly popular ${\mathrm{C}}_{60}$ electronic device applications, especially those making use of heterostructures.

Weyl semimetals are a three-dimensional topological phase of matter with linearly dispersed Weyl points which appear in pairs and carry opposite chirality. The separation of paired Weyl points allows charge transfer between them in the presence of parallel electric and magnetic fields, which is known as the chiral anomaly. In this paper, we theoretically study the influence of the chiral anomaly induced chiral charge imbalance on the Josephson current in a Weyl superconductor–Weyl semimetal–Weyl superconductor junction. In Weyl superconductors, two types of pairings are considered, namely, zero momentum BCS-like pairing and finite momentum Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like pairing. For BCS-like pairing, we find that the Josephson current exhibits $0\text{−}\pi$ transitions and oscillates as a function of ${\lambda }_{0}L$, where ${\lambda }_0$ is the chirality imbalance induced by parallel electric and magnetic fields and $L$ is the length of the Weyl semimetal. The amplitude of the Josephson current also depends on the angle $\beta$ between the line connecting two paired Weyl points and the transport direction along the junction. For FFLO-like pairing, chirality imbalance induced periodic oscillations are absent and the Josephson current is also independent of the angle $\beta$. These findings are useful in detecting the chiral anomaly and distinguishing the superconducting pairing mechanism of Weyl semimetals.

In order to develop computational methods that can simulate thermodynamic properties of disordered materials at a first-principles level, we investigate the use of a random set of configurations to evaluate the canonical partition function of lattice-based disordered systems. Testing the sampling method on the one- and two-dimensional Ising models indicates that for the ordered system at low temperature, convergence is achieved when the number of samples $\mathcal{S}$ is comparable to or larger than the number of configurations $\mathrm{\Omega }$, while for the partially disordered system at high temperature, convergence is achieved for smaller sample sizes as low as $\mathcal{S}\approx \mathrm{\Omega }/100$ or $\mathcal{S}\approx \mathrm{\Omega }/1000$. The sampling method is combined with first-principles calculations to examine the ordered $\leftrightarrow$ disordered phase transition for the Li ion electrolyte materials ${\mathrm{Li}}_2\mathrm{OHCl}$ and ${\mathrm{Li}}_2\mathrm{OHBr}$. Static-lattice internal energies and harmonic-phonon free energies were incorporated into the evaluation of the partition function. The evaluation of the partition function depends on the value of $\mathrm{\Omega }$ corresponding to the number of metastable states of the system. Accordingly, we developed a method of approximating $\mathrm{\Omega }$ using the properties of the sampled configurations. The results of the calculations are consistent with the experimental observation that the transition temperature for the orthorhombic $\leftrightarrow$ cubic phase transition is higher for ${\mathrm{Li}}_2\mathrm{OHCl}$ than for ${\mathrm{Li}}_2\mathrm{OHBr}$. We expect the sampling method to be generally useful for investigating the thermodynamic properties of other disordered-lattice systems. We also investigate a “disordered-subspace function” which is shown to satisfy inequality relationships with respect to the Helmholtz free energy.

We present an experimental study of the structural and magnetic phase transitions of the quadruple perovskite ${\mathrm{YMn}}_7{\mathrm{O}}_{12}$. On cooling below 640 K a displacive structural phase transition occurs, lowering the crystal symmetry from cubic $\left(Im\overline{3}\right)$ to monoclinic $\left(I2/m\right)$, driven by long range B site ${\mathrm{Mn}}^{3+}$ orbital order. A second, unconventional structural phase transition takes place on further cooling below 200 K, which is characterized by static atomic displacements $\left(P{2}_1/n\right)$ as well as a dipolar-glass-like state of yttrium. This low temperature structural transition is driven by the underbonding of yttrium, which is present due to the rigid framework of octahedra tilts unique to the quadruple perovskites that primarily accommodates A site manganese ions. A ground state ferrimagnetic structure was also determined, similar to that found in other $R{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ compounds that do not present a low temperature structural phase transition.

The electron spin dynamics of $\mathrm{C}{\mathrm{e}}^{3+}$ ions in YAG crystals is studied by time-resolved photoluminescence spectroscopy with an alternating left and right circularly polarized continuous-wave laser modulation technique. The electron spin relaxation due to hyperfine interaction with randomly oriented nuclear spins can be efficiently suppressed by a weak external longitudinal magnetic field. The suppression of the hyperfine-induced spin relaxation makes the electron spin polarization increase from 0.001 to 0.016 for the lowest $4f$ state, and from 0.08 to 0.32 for the lowest $5d$ state. The suppression magnetic fields for the $4f$ electrons are several times weaker than that for the $5d$ electrons, dependent on the crystal orientation. The dispersion of the local hyperfine field distribution $\mathrm{\Delta }B$ is isotropic and equal to 4.0 mT for the $5d$ electrons, and anisotropic for the $4f$ electrons with $\mathrm{\Delta }{B}_{\left[001\right]}=0.8\mathrm{mT},\mathrm{\Delta }{B}_{\left[110\right]}=1.1\mathrm{mT},\mathrm{\Delta }{B}_{\left[1\overline{1}0\right]}=2.4\mathrm{mT}$. The hyperfine coupling strength for the $4f$ electrons is ∼4 times weaker than that for the $5d$ electrons.

The coupled-wire construction provides a useful way to obtain microscopic Hamiltonians for various two-dimensional topological phases, among which fractional quantum Hall states are paradigmatic examples. Using the recently introduced flux attachment and vortex duality transformations for coupled wires, we show that this construction is remarkably versatile to encapsulate phenomenologies of hierarchical quantum Hall states: the Jain-type hierarchy states of composite fermions filling Landau levels and the Haldane-Halperin hierarchy states of quasiparticle condensation. The particle-hole conjugate transformation for coupled-wire models is also given as a special case of the hierarchy construction. We also propose coupled-wire models for the composite Fermi liquid, which turn out to be compatible with a sort of the particle-hole symmetry implemented in a nonlocal way at $\nu =\frac{1}{2}$. Furthermore, our approach shows explicitly the connection between the Moore-Read Pfaffian state and a chiral $p$-wave pairing of the composite fermions. This composite-fermion picture is also generalized to a family of the Pfaffian state, including the anti-Pfaffian state and Bonderson-Slingerland hierarchy states.