Stacking atomically thin two-dimensional materials with a rotational misalignment between their layers produces a van der Waals bilayer in which all mirror symmetries can be broken. A fundamental experimental signature of a twisted multilayer is circular dichroism (CD): the conversion of a linearly polarized incident optical field to an elliptically polarized field in transmission. This work investigates the relations between CD, the twist angle, and interlayer slip. In experiments where bilayers are formed by contacting one layer with another, the relative rotation angle between the symmetry axes of each sheet are controlled, but the lateral shifts between the layers are not. Accounting for the lateral shift between the layers expands the configuration space of twisted bilayer structures. Here we demonstrate how the symmetry constraints in this expanded configuration space are expressed in the CD spectrum of twisted bilayer graphene as a function of the rotation angle and interlayer slip.

In high-quality conductors, the hydrodynamic or the ballistic regimes of heat and charge transport can be realized at low temperatures. We study the Hall effect in a system of interacting two-dimensional charged particles in a narrow ballistic sample in a weak magnetic field. In such a system, the Hall electric field is mainly caused by redistribution of the particles moving between the longitudinal sample edges along the ballistic collisionless trajectories. Our consideration is based on solving the classical kinetic equation in the ballistic regime with taking into account the interparticle scattering and the magnetic force by the perturbation theory. In a one-component (electron or hole) system, the Hall coefficient turns out to be one-half of the Hall coefficient of conventional wide Ohmic samples. We argue that this result is apparently consistent with the recent measurements of the Hall resistance in ultra-high-mobility GaAs quantum wells. In a two-component electron-hole system, the Hall coefficient depends linearly on the difference $n_0^e-n_0^h$ between the electron and the hole equilibrium densities $n_0^e$ and $n_0^h$ near the charge neutrality point $\left(n_0^e=n_0^h\right)$ and saturates to the Hall coefficient of a one-component system far from the charge neutrality point $(n_0^e\gg n_0^h$ or $n_0^e\ll n_0^h)$. We also studied the corrections from the interparticle scattering to magnetoresistance and the Hall effect, being the precursors of forming the viscous or the Ohmic flows.

We use exact techniques to demonstrate theoretically the pumping of fractional charges in a single-level noninteracting quantum dot, when the dot-reservoir coupling is adiabatically driven from weak to strong coupling. The pumped charge averaged over many cycles is quantized at a fraction of an electron per cycle, determined by the ratio of Lamb shift to level broadening; this ratio is imposed by the reservoir band structure. For uniform density of states, half an electron is pumped per cycle. We call this adiabatic almost-topological pumping, because the pumping's Berry curvature is sharply peaked in the parameter space. Hence, so long as the pumping contour does not touch the peak, the pumped charge depends only on how many times the contour winds around the peak (up to exponentially small corrections). However, the topology does not protect against nonadiabatic corrections, which grow linearly with pump speed. In one limit, the peak becomes a delta function, so the adiabatic pumping of fractional charges becomes entirely topological. Our results show that quantization of the adiabatic pumped charge at a fraction of an electron does not require fractional particles or other exotic physics.

The crystal structure of the ${\mathrm{MAPbI}}_3$ hybrid perovskite forms an intricate electrostatic puzzle with different ordering patterns of the MA molecules at elevated temperatures. For this perovskite three published model Hamiltonians based on the point-dipole (pd) approximation combined with short-range effective interactions are compared to a recently developed machine-learning force field. A molecular order parameter is used to consistently compare the transformation of the antiferroelectric ordering in the orthorhombic phase upon raising the temperature. We show that the ground states and the order-disorder transition of the three models are completely different. Our analysis indicates that the long-range order in the low-temperature orthorhombic phase can be captured by pd-based models with a short cutoff radius, including the nearest and next-nearest neighbor molecules. By constructing effective atomic interactions the ordering can already be described within a 6 Å radius. By extracting the coupling energetics of the molecules from density functional theory calculations on ${\mathrm{MA}}_x{\mathrm{Cs}}_{1-x}{\mathrm{PbI}}_3$ test systems, we show that the pd approximation holds at least for static structures. To improve the accuracy of the pd interaction an Ewald summation is applied combined with a distance dependent electronic screening function.

Chiral phonons have been theoretically proposed [Zhang and Niu, Phys. Rev. Lett. 115, 115502 (2015)] and experimentally verified [Zhu et al., Science 359, 579 (2018)] in materials with hexagonal lattice. Here, we theoretically predict and investigate the chiral phonons in the kagome lattice. We show that with broken inversion symmetry, chiral phonons with large circular polarization can emerge at the $K$ and $K^{\prime }$ high symmetry points. Different from the hexagonal lattice, these chiral phonons have all three sublattices vibrating with the same chirality, and the vibration orbit can take an elliptical shape. They possess valley-contrasting Berry curvatures sharply peaked at the $K$ and $K^{\prime }$ valleys, which can be used to separate the chiral phonons according to their valley label via the valley phonon Hall effect. The pseudoangular momentum for these chiral phonons has also been studied, which determines the selection rules of electron-phonon interaction in the intervalley scattering process. Furthermore, we show that by tuning the relative bond strength, there can be a change in phonon chirality associated with a phonon branch switching. Our work enriches the studies on chiral phonons and offers a probe for many fascinating quantum materials which possess the kagome type lattice.

A charge current on the surface of a topological insulator (TI) produces a surface spin polarization that can be measured experimentally using a ferromagnetic (FM) tunnel contact either in a three-terminal or a four-terminal potentiometric measurement. The potential measured on the FM contact depends on the direction and the magnitude of the surface charge current, as well as the FM magnetization direction relative to the spin polarization on the surface of the TI. In such a measurement, the resistance always obeys Onsager reciprocity, i.e., $R_{ab,cd}(+\vec{M})=R_{cd,ab}(-\vec{M})$, where $R_{ab,cd}$ is the resistance measured with current injected between contacts $a$ and $b$, voltage measured between contacts $c$ and $d$, and the FM having magnetization $\vec{M}$. In a two-terminal measurement in which the current and the voltage contacts are the same, Onsager reciprocity dictates that the resistance remains the same even after the magnetization of the FM is reversed, i.e., $R_{ab,ab}(+\vec{M})=R_{ab,ab}(-\vec{M})$. However, previous theories [Phys. Rev. Lett. 105, 066802 (2010), Europhys. Lett. 93, 67004 (2011)] claimed that change of resistance in such two-terminal measurement on the surface of a diffusive TI is possible upon reversing the FM magnetization direction. Here, we resolve this conflicting issue by showing that the Onsager reciprocity relation remains valid even in a two-terminal measurement on the surface of a diffusive TI. We consider the modifications in both the continuity equation of the charge density and the charge current density on the surface of the TI due to the effect of tunneling of electrons from the FM tunnel contact. We derive the transport equations on the surface of the TI from full quantum mechanical kinetic equation based on Keldysh Green's function, and obtain the resistance measured in a two-terminal or a multiterminal measurement after solving the transport equations analytically. We establish the validity of the Onsager reciprocity relation in both the two-terminal and the multiterminal measurements and also show the crucial importance of the tunnel contact in such spin detection experiments.

We study magnetic behavior of the ${\mathrm{Yb}}^{3+}$ ions on a frustrated pyrochlore lattice in the spinel ${\mathrm{CdYb}}_2{\mathrm{Se}}_4$. The crystal-electric-field parameters deduced from high-energy inelastic neutron scattering reveal a well-isolated ytterbium ground-state doublet with a weakly Ising character. Magnetic order studied by powder neutron diffraction evolves from the XY-type antiferromagnetic ${\mathrm{\Gamma }}_5$ state to a splayed icelike ferromagnet (both with $\mathbf{k}=0$) in applied magnetic field with $B_c=3\mathrm{T}$. Low-energy inelastic neutron scattering identifies weakly dispersive magnetic bands around 0.72 meV starting at $\left|\mathbf{Q}\right|=1.1{\AA }^{-1}$ at zero field, which diminish with field and vanish above 3 T. We explain the observed magnetic behavior in framework of the nearest-neighbor anisotropic exchange model for effective $S=\frac{1}{2}$ Kramers doublets on the pyrochlore lattice. The estimated exchanges position the ${\mathrm{CdYb}}_2{\mathrm{Se}}_4$ spinel close to the phase boundary between the ${\mathrm{\Gamma }}_5$ and splayed ferromagnet states, similar to the Yb pyrochlores, suggesting an important role of the competition between these phases.

We report a high-magnetic-field study of the itinerant ferromagnet URhSi. Magnetization and electrical resistivity were measured under magnetic fields ${\mu }_{0}H$ up to 58 T applied along the directions $\mathbf{a}$, $\mathbf{b}$, and $\mathbf{c}$ of the orthorhombic structure and temperatures $T$ ranging from 1.5 to 50 K. For $\mathbf{H}\parallel \mathbf{b}$, pseudo-metamagnetism at ${\mu }_0{H}_m\simeq 30\text{–}40\mathrm{T}$ is associated with a broad step in the magnetization and a maximum in the resistivity. The properties of URhSi are discussed and compared with those of the isostructural superconducting ferromagnets URhGe and UCoGe and of the superconducting paramagnet ${\mathrm{UTe}}_2$.

Motivated by recent measurements of strain effects on the transition temperature ($T_c$) of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, we study the strain response of a two-dimensional chiral $p$-wave superconductor. We focus on the effects of inhomogeneous strain fields, which are always present in any such experiment, and which have been neglected in previous theoretical treatments. We show that the response of $T_c$ of a chiral superconductor to strain changes from being linear without inhomogeneity to being quadratic in the presence of inhomogeneity. We discuss our results in the context of the ongoing debate of superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

In this paper we extend the Falicov-Kimball model (FKM) to the case where the “quasiparticles” entering the FKM are not “ordinary” fermions. As an example we first discuss how the FKM can be generalized to the case with spin-dependent hopping. Afterward, we discuss several cases where the quasiparticles entering the FKM are Majorana fermions [extended Majorana-Falicov-Kimball model (MFKM)]. Two examples of the extended MFKM are discussed in detail: (i) a $p$-wave BCS superconductor on a bipartite lattice and (ii) a BCS-Anderson model. We also discuss the most general forms of extended MFKM, including a brief discussion of the case where the Majorana fermions represent spins but not real fermion particles.

We present an approach that enables an efficient and accurate study of dynamically unstable crystals over the full temperature range. The approach is based on an interatomic potential fitted to ab initio molecular dynamics energies for both the high- and low-temperature stable phases. We verify by comparison to explicit ab initio simulations that such a bespoke potential, for which we use here the functional form of the embedded atom method, provides accurate transformation temperatures and atomistic features of the transformation. The accuracy of the potential makes it an ideal tool to study the important impact of finite size and finite time effects. We apply our approach to the dynamically unstable $\beta$ (bcc) titanium phase and study in detail the transformation to the low-temperature stable hexagonal $\omega$ phase. We find a large set of previously unreported linear-chain disordered (LCD) structures made up of three types of ${\left[111\right]}_{\beta }$ linear-chain defects that exhibit randomly disordered arrangements in the ${\left(111\right)}_{\beta }$ plane.

We present a first-principles theoretical approach to calculate temperature dependent phonon dispersions in bcc Fe, which captures finite temperature spin-lattice coupling by treating thermal disorder in both the spin and lattice systems simultaneously. With increasing temperature, thermal atomic displacements are found to induce increasingly large fluctuations in local magnetic moment magnitudes. The calculated phonon dispersions of bcc Fe show excellent agreement with measured data over a wide range of temperatures both above and below the magnetic and structural transition temperatures, suggesting the applicability of the developed approach to other magnetic materials.

We report inelastic neutron scattering experiments in ${\mathrm{Ca}}_2{\mathrm{Y}}_2{\mathrm{Cu}}_5{\mathrm{O}}_{10}$ and map out the full one-magnon dispersion which extends up to a record value of 53 meV for frustrated ferromagnetic (FM) edge-sharing ${\mathrm{CuO}}_2$ chain (FFESC) cuprates. A homogeneous spin-1/2 chain model with a FM nearest-neighbor (NN), an antiferromagnetic (AFM) next-nearest-neighbor (NNN) inchain, and two diagonal AFM interchain couplings (ICs) analyzed within linear spin-wave theory (LSWT) reproduces well the observed strong dispersion along the chains and a weak one perpendicularly. The ratio $\alpha =|J_{a2}/J_{a1}|$ of the FM NN and the AFM NNN couplings is found as $\sim 0.23$, close to the critical point ${\alpha }_c=1/4$ which separates ferromagnetically and antiferromagnetically correlated spiral magnetic ground states in single chains, whereas ${\alpha }_c>0.25$ for coupled chains is considerably upshifted even for relatively weak IC. Although the measured dispersion can be described by homogeneous LSWT, the scattering intensity appears to be considerably reduced at $\sim 11.5$ and $\sim 28\mathrm{meV}$. The gaplike feature at 11.5 meV is attributed to magnon-phonon coupling whereas based on density matrix renormalization group simulations of the dynamical structure factor the gap at 28 meV is considered to stem partly from quantum effects due to the AFM IC. Another contribution to that gap is ascribed to the intrinsic superstructure from the distorting incommensurate pattern of CaY cationic chains adjacent to the ${\mathrm{CuO}}_2$ ones. It gives rise to nonequivalent ${\mathrm{CuO}}_4$ units and Cu-O-Cu bond angles $\mathrm{\Phi }$ and a resulting distribution of all exchange integrals. The $J$'s fitted by homogeneous LSWT are regarded as average values. The record value of the FM NN integral $J_1=24\mathrm{meV}$ among FFESC cuprates can be explained by a nonuniversal $\mathrm{\Phi }(\ne {90}^{\circ })$ and Cu-O bond length dependent anisotropic mean direct FM Cu-O exchange ${\overline{K}}_{pd}\sim 120\mathrm{meV}$, similar to a value of 105 meV for ${\mathrm{Li}}_2{\mathrm{CuO}}_2$, in accord with larger values for ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and ${\mathrm{CuGeO}}_3$ $(\sim 110\mathrm{meV})$ reported by Braden et al. [Phys. Rev. B 54, 1105 (1996)] phenomenologically. Enhanced $K_{pd}$ values are also needed to compensate a significant AFM $J_{dd}\ge$ 6 meV from the $dd$ channel, generic for FFESC cuprates but ignored so far.

A new type of magnetoelectric (ME) effect was discovered in ${\mathrm{Tb}}_{1-x}{\mathrm{Gd}}_x{\mathrm{Mn}}_2{\mathrm{O}}_5$ in which Gd ions are substituted into ${\mathrm{TbMn}}_2{\mathrm{O}}_5$. The substitution of Gd ions increased to $x=0.5$ was found to completely suppress increases in electric polarization below $T\sim 14\mathrm{K}$, which is clearly observed in ${\mathrm{TbMn}}_2{\mathrm{O}}_5$. The electric polarization of ${\mathrm{Tb}}_{0.5}{\mathrm{Gd}}_{0.5}{\mathrm{Mn}}_2{\mathrm{O}}_5$ reappears below $T\sim 14\mathrm{K}$ in response to an external magnetic field applied along each crystallographic axis. This isotropic ME effect is in distinction from those exhibited by the $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ family and by other multiferroic compounds. Resonant x-ray scattering was used to investigate the mechanism of the ME effect, and the data clearly demonstrate that this phenomenon can be attributed to competition between two different magnetic orderings. Furthermore, the magnetic-field dependence of the resonant intensity around the O $K$ edge implies that charge transfer between O and Mn ions microscopically contributes to the ME effects. These findings indicate potential new applications for multiferroic materials.

We report the discovery of linear magnetoelectric effect in the well-known green-phase compound, ${\mathrm{Sm}}_2{\mathrm{BaCuO}}_5$, which crystallizes in the centrosymmetric orthorhombic (Pnma) structure. Magnetization and specific heat measurements reveal the long-range antiferromagnetic ordering of $\mathrm{C}{\mathrm{u}}^{2+}-$ and ${\mathrm{Sm}}^{3+}$ ions moments at $T_{\mathrm{N}1}=23\mathrm{K}$ and $T_{\mathrm{N}2}=5\mathrm{K}$, respectively. Applied magnetic field induces dielectric anomaly at $T_{\mathrm{N}1}$ whose magnitude increases with field, which results in significant (1.7%) magnetocapacitance effect. On the other hand, the dielectric anomaly observed in zero-applied magnetic field at $T_{\mathrm{N}2}$ shows a small (0.4%) magnetocapacitance effect. Interestingly, applied magnetic field induces an electric polarizaiton, below $T_{\mathrm{N}1}$, that varies linearly up to the maximum applied field of 9 T with the magnetoelectric coefficient α ∼ 4.4 ps/m, demonstrating high magnetoelectric coupling. Below $T_{\mathrm{N}2}$, the electric polarization decreases from 35 to $29\mu \mathrm{C}/{\mathrm{m}}^2$ at 2 K and 9 T due to ordering of Sm sublattice. The observed linear magnetoelectricity in ${\mathrm{Sm}}_2{\mathrm{BaCuO}}_5$ is explained using symmetry analysis.

We theoretically predict proximity-induced odd-frequency $\left(\mathrm{odd}-\omega \right)$ pairing on the surface of a Weyl nodal loop semimetal, characterized by a nodal loop Fermi surface and drumheadlike surface states (DSSs), attached to conventional spin-singlet $s$-wave superconducting leads. Due to the complete spin polarization of the DSS, only $\mathrm{odd}-\omega$ equal-spin triplet pairing is present, and we show that it gives rise to a finite Josephson current. Placing an additional ferromagnet in the junction can also generate $\mathrm{odd}-\omega$ mixed-spin triplet pairing, but the pairing and current are not affected if the magnetization is orthogonal to the DSS spin polarization, which further confirms the equal-spin structure of the pairing.

We derive a low-energy effective field theory for chiral superfluids, which accounts for both spontaneous symmetry breaking and fermionic ground-state topology. Using the theory, we show that the odd (or Hall) viscosity tensor, at small wave vector, contains a dependence on the chiral central charge $c$ of the boundary degrees of freedom, as well as additional nonuniversal contributions. We identify related bulk observables which allow for a bulk measurement of $c$. In Galilean invariant superfluids, only the particle current and density responses to strain and electromagnetic fields are required. To complement our results, the effective theory is benchmarked against a perturbative computation within a canonical microscopic model.