We report ${}^{133}\mathrm{Cs}$ NMR and ${}^{75}\mathrm{As}$ nuclear quadrupole resonance (NQR) measurements on the normal metallic state above $T_c$ of a quasi-one-dimensional superconductor ${\mathrm{Cs}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ ($T_c<1.6$ K). From the ${}^{133}\mathrm{Cs}$ NMR Knight shift ${}^{133}K$ measured at the Cs1 site, we show that the uniform spin susceptibility ${\chi }_{\mathrm{spin}}$ increases from 295 K to $\sim 60$ K, followed by a mild suppression; ${\chi }_{\mathrm{spin}}$ then levels off below $\sim 10$ K. In contrast, a vanishingly small magnitude of ${}^{133}K$ indicates that Cs2 sites contribute very little to electrical conduction and the exchange interactions between 3d electrons at Cr sites. Low frequency Cr spin dynamics, reflected on ${}^{75}\mathrm{As}1/T_{1}T$ (the nuclear spin-lattice relaxation rate $1/T_1$ divided by temperature $T$), shows an analogous trend as ${\chi }_{\mathrm{spin}}$. Comparison with the results of $1/T_{1}T$ near $T_c$ with ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ ($T_c=6.1$ K) and ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ ($T_c=4.8$ K) establishes a systematic trend that substitution of ${\mathrm{K}}^{+}$ ions with larger alkali ions progressively suppresses Cr spin fluctuations together with $T_c$.

The optical magnetoelectric (OME) effect based on electromagnon resonance has been investigated in a multiferroic helimagnet ${\mathrm{MnWO}}_4$ with the canted spin-spiral plane by the terahertz spectroscopy. Versatile OME effects emerge from the canted spin-spiral state, including the magnetochiral effect as manifested by the nonreciprocal directional dichroism. In addition to the transverse fluctuation mode of the spin-driven ferroelectric polarization, the electromagnon with a character of the longitudinal amplitude mode was found to cause the OME effect characteristic of the canted spin-spiral plane.

Using first-principles calculations based on density functional theory, we have studied the mechanical, electronic, and magnetic properties of Heusler alloys, namely, ${\mathrm{Ni}}_{2}BC$ and ${\mathrm{Co}}_{2}BC$ ($B$ = Sc, Ti, V, Cr, and Mn as well as Y, Zr, Nb, Mo, and Tc; $C$ = Ga and Sn). On the basis of electronic structure (density of states) and mechanical properties (tetragonal shear constant), as well as magnetic interactions (Heisenberg exchange coupling parameters), we probe the properties of these materials in detail. We calculate the formation energy of these alloys in the (face-centered) cubic austenite structure to probe the stability of all these materials. From the energetic point of view, we have studied the possibility of the electronically stable alloys having a tetragonal phase lower in energy compared to the respective cubic phase. A large number of the magnetic alloys is found to have the cubic phase as their ground state. On the other hand, for another class of alloys, the tetragonal phase has been found to have lower energy compared to the cubic phase. Further, we find that the values of tetragonal shear constant show a consistent trend: a high positive value for materials not prone to tetragonal transition and low or negative for others. In the literature, materials which have been seen to undergo the martensite transition are found to be metallic in nature. We probe here if there is any Heusler alloy which has a tendency to undergo a tetragonal transition and at the same time possesses a high spin polarization at the Fermi level. From our study, it is found that out of the four materials which exhibit a martensite phase as their ground state, three of these, namely, ${\mathrm{Ni}}_2\mathrm{MnGa}, {\mathrm{Ni}}_2\mathrm{MoGa}$, and ${\mathrm{Co}}_2\mathrm{NbSn}$ have a metallic nature; on the contrary, ${\mathrm{Co}}_2\mathrm{MoGa}$ exhibits a high spin polarization.

Both topological crystalline insulator surfaces and graphene host multivalley massless Dirac fermions which are not pinned to a high-symmetry point of the Brillouin zone. Strain couples to the low-energy electrons as a time-reversal-invariant gauge field, leading to the formation of pseudo-Landau-levels (PLLs). Here we study periodic pseudomagnetic fields originating from strain superlattices. We study the low-energy Dirac PLL spectrum induced by the strain superlattice and analyze the effect of various polarized states. Through self-consistent Hartree-Fock calculations we establish that, due to the strain superlattice and PLL electronic structure, a valley-ordered state spontaneously breaking time reversal and realizing a quantum Hall phase is favored, while others are suppressed. Our analysis applies to both topological crystalline insulators and graphene.

It is well known that in a quantum phase transition (QPT), entanglement remains short ranged [Osterloh et al., Nature (London) 416, 608 (2005)]. We ask if there is a quantum property entailing the whole system which diverges near this point. Using the recently proposed measures of quantum macroscopicity, we show that near a quantum critical point, it is the effective size of macroscopic superposition between the two symmetry breaking states which grows to the scale of system size, and its derivative with respect to the coupling shows both singular behavior and scaling properties.

We suggest an analytic theory based on the effective medium approximation (EMA) which is able to describe charge-carrier transport in a disordered semiconductor with a significant degree of degeneration realized at high carrier concentrations, especially relevant in some thin-film transistors (TFTs), when the Fermi level is very close to the conduction-band edge. The EMA model is based on special averaging of the Fermi-Dirac carrier distributions using a suitably normalized cumulative density-of-state distribution that includes both delocalized states and the localized states. The principal advantage of the present model is its ability to describe universally effective drift and Hall mobility in heterogeneous materials as a function of disorder, temperature, and carrier concentration within the same theoretical formalism. It also bridges a gap between hopping and bandlike transport in an energetically heterogeneous system. The key assumption of the model is that the charge carriers move through delocalized states and that, in addition to the tail of the localized states, the disorder can give rise to spatial energy variation of the transport-band edge being described by a Gaussian distribution. It can explain a puzzling observation of activated and carrier-concentration-dependent Hall mobility in a disordered system featuring an ideal Hall effect. The present model has been successfully applied to describe experimental results on the charge transport measured in an amorphous oxide semiconductor, In-Ga-Zn-O (a-IGZO). In particular, the model reproduces well both the conventional Meyer-Neldel (MN) compensation behavior for the charge-carrier mobility and inverse-MN effect for the conductivity observed in the same a-IGZO TFT. The model was further supported by ab initio calculations revealing that the amorphization of IGZO gives rise to variation of the conduction-band edge rather than to the creation of localized states. The obtained changes agree with the one we used to describe the charge transport. We found that the band-edge variation dominates the charge transport in high-quality a-IGZO TFTs in the above-threshold voltage region, whereas the localized states need not to be invoked to account for the experimental results in this material.

Acene molecular crystals are of current interest in organic optoelectronics, both as active materials and for exploring and understanding new phenomena. Phonon scattering can be an important facilitator and dissipation mechanism in charge separation and carrier transport processes. Here, we carry out density functional theory (DFT) calculations of the structure and the full phonon dispersion of crystalline naphthalene, a well-characterized acene crystal for which detailed neutron-diffraction measurements, as well as infrared and Raman spectroscopy, are available. We evaluate the performance, relative to experiments, of DFT within the local density approximation (LDA); the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE); and a recent van der Waals–corrected nonlocal correlation (vdW-DF-cx) functional. We find that the vdW-DF-cx functional accurately predicts lattice parameters of naphthalene within 1%. Intermolecular and intramolecular phonon frequencies across the Brillouin zone are reproduced within 7.8% and 1%, respectively. As expected, LDA (PBE) underestimates (overestimates) the lattice parameters and overestimates (underestimates) phonon frequencies, demonstrating their shortcomings for predictive calculations of weakly bound materials. If the unit cell is fixed to the experimental lattice parameters, PBE is shown to lead to improved phonon frequencies. Our study provides a detailed understanding of the phonon spectrum of naphthalene, and highlights the importance of including van der Waals dispersion interactions in predictive calculations of lattice parameters and phonon frequencies of molecular crystals and related organic materials.

We performed calculations of momentum distributions of annihilating electron-positron pairs in various fully relaxed vacancy defects in SiC. We used self-consistent two-component density functional theory schemes to find the electronic and positronic densities and wave functions in the considered systems. Using the one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) we calculated the line-shape parameters $S$ and $W$. We emphasize the effect of the experimental resolution and the choice of the integration ranges for the $S$ and $W$ parameters on the distributions of the points corresponding to different defects in the $S\left(W\right)$ plot. We performed calculation for two polytypes of SiC, $3C$, and $6H$ and showed that for silicon vacancies and clusters containing this defect there were no significant differences between the Doppler spectra. The results of the Doppler spectra calculations were compared with experimental data obtained for $n$-type $6H$-SiC samples irradiated with 4-MeV Au ions. We observed a good general agreement between the measured and calculated points.

We study the physical origin of extreme optical chirality of subwavelength arrays of chiral holes in metal. We reconstruct the nanoscale relief of the hole arrays by the atomic-force microscopy and post-process the data to acquire an average unit-cell shape clear of noise and defects. For this shape, we perform the electromagnetic finite difference time domain simulations that reproduce all important features observed by the light-transmission experiments, including the notably strong circular dichroism and optical activity covering the whole range of possible values. To interpret the simulation results, we develop a chiral coupled-mode model which yields analytical expressions that fit accurately the numerical data in a broad wavelength range. Our conclusions undoubtedly link the extreme optical chirality to the plasmon resonances of chiral holes and the associated chiral Fano-type transmission resonance.

We demonstrate an area law bound on the ground state entanglement entropy of a wide class of gapless quantum states of matter using a strategy called local entanglement thermodynamics. The bound depends only on thermodynamic data, actually a single exponent, the hyperscaling violation exponent $\theta$. All systems in $d$ spatial dimensions obeying our scaling assumptions and with $\theta <d-1$ obey the area law, while systems with $\theta =d-1$ can violate the area law at most logarithmically. We also discuss the case of frustration-free Hamiltonians and show that to violate the area law more than logarithmically these systems must have an unusually large number of low energy states. Finally, we make contact with the recently proposed $s$-source framework and argue that $\theta$ and $s$ are related by $s=2^{\theta }$.

We study the energy transport in a system of two half-infinite XXZ chains initially kept separated at different temperatures, and later connected and let free to evolve unitarily. By changing independently the parameters of the two halves, we highlight, through bosonization and time-dependent matrix-product-state simulations, the different contributions of low-lying bosonic modes and of fermionic quasiparticles to the energy transport. In the simulations we also observe that the energy current reaches a finite value which only slowly decays to zero. The general picture that emerges is the following. Since integrability is only locally broken in this model, a preequilibration behavior may appear. In particular, when the sound velocities of the bosonic modes of the two halves match, the low-temperature energy current is almost stationary and described by a formula with a nonuniversal prefactor interpreted as a transmission coefficient. Thermalization, characterized by the absence of any energy flow, occurs only on longer time scales which are not accessible with our numerics.

We report an optical spectroscopic study of $\mathrm{SrMnS}{\mathrm{b}}_2$, a low-carrier-density metal. As temperature is decreased, our measurements reveal a large increase in the quasiparticle plasma frequency, which is highly unusual for a metal. This seemingly anomalous behavior can be accounted for using a “three-band” model of the multiband electronic structure of $\mathrm{SrMnS}{\mathrm{b}}_2$ that includes two conduction bands and one valence band. The second conduction band is assumed to be heavy and its minimum is taken to be close to, but not intersecting, the Fermi level. At finite temperature, quasiparticles are thermally redistributed between the two conduction bands, leading to an increase in the optical effective mass and a decrease in the plasma frequency. The temperature dependence of the low-lying interband optical transitions and the Hall number can also be understood using our model. The phenomenology of such a three-band scenario has not been widely considered to date in optical spectroscopic studies. Our results provide an explanation for the puzzling optical properties that have previously been reported in a number of topical low-carrier-density metals and semimetals and lay a foundation for future optical studies of these materials.

The density matrix renormalization group method is introduced in energy space to study Anderson impurity models. The method allows for calculations in the thermodynamic limit and is advantageous for studying not only the dynamical properties, but also the quantum entanglement of the ground state at the vicinity of an impurity quantum phase transition. This method is applied to obtain numerically exactly the ground-state phase diagram of the single-impurity Anderson model on the honeycomb lattice at half-filling. The calculation of local static quantities shows that the phase diagram contains two distinct phases, the local moment (LM) phase and the asymmetric strong coupling (ASC) phase, but no Kondo screening phase. These results are supported by the local spin and charge excitation spectra, which exhibit qualitatively different behavior in these two phases and also reveal the existence of the valence fluctuating point at the phase boundary. For comparison, we also study the low-energy effective pseudogap Anderson model using the method introduced here. Although the high-energy excitations are obviously different, we find that the ground-state phase diagram and the asymptotically low-energy excitations are in good quantitative agreement with those for the single-impurity Anderson model on the honeycomb lattice, thus providing a quantitative justification for the previous studies based on low-energy approximate approaches. Furthermore, we find that the lowest entanglement level is doubly degenerate for the LM phase, whereas it is singlet for the ASC phase and is accidentally threefold degenerate at the valence fluctuating point. This should be contrasted with the degeneracy of the energy spectrum because the ground state is found to be always singlet. Our results therefore clearly demonstrate that the low-lying entanglement spectrum can be used to determine with high accuracy the phase boundary of the impurity quantum phase transition.

To achieve a fundamental understanding of the multiferroic behavior and electronic properties of intrinsic vacancies in $\mathrm{BiFe}{\mathrm{O}}_3$, here we performed first-principles calculations based on hybrid Hartree-Fock density functional theories, which can accurately describe defect electronic structures. Oxygen vacancies, which behave as deep donors with high concentrations under oxygen-poor conditions, reduce the magnetic moments at neighboring Fe ions in the neutral state, while charged oxygen vacancies induce additional ferroelectric polarizations. Cation vacancies, on the other hand, are likely to form under oxygen-rich conditions and result in multiferroic properties distinct from those induced by oxygen vacancies. Bi vacancies act as triple-shallow acceptors and strongly suppress spontaneous polarization regardless of charge states, while Fe vacancies locally interfere with both electric and spin polarization and are thus regarded as multiferroic singular points in $\mathrm{BiFe}{\mathrm{O}}_3$. A rich variety of the multiferroic behavior of vacancies can be systematically understood from the localized/delocalized features of defect states, and the different formation conditions for vacancies provide a strategy to tailor the multiferroic properties of $\mathrm{BiFe}{\mathrm{O}}_3$ through control of the concentration and charge states of vacancies.

We study periodically driven bosonic scalar field theories in the infinite $N$ limit. It is well known that the free theory can undergo parametric resonance under monochromatic modulation of the mass term and thereby absorb energy indefinitely. Interactions in the infinite $N$ limit terminate this increase for any choice of the UV cutoff and driving frequency. The steady state has nontrivial correlations and is synchronized with the drive. The $O\left(N\right)$ model at infinite $N$ provides the first example of a clean interacting quantum system that does not heat to infinite temperature at any drive frequency.

We investigate experimentally the retrapping of the phase in a $\phi$ Josephson junction upon return of the junction to the zero-voltage state. Since the Josephson energy profile $U_0\left(\psi \right)$ in $\phi$ JJ is a $2\pi$ periodic double-well potential with minima at $\psi =\pm \phi mod2\pi$, the question is at which of the two minima $-\phi$ or $+\phi$ the phase will be trapped upon return from a finite voltage state during quasistatic decrease of the bias current (tilt of the potential). By measuring the relative population of two peaks in escape histograms, we determine the probability of phase trapping in the $\pm \phi$ wells for different temperatures. Our experimental results agree qualitatively with theoretical predictions. In particular, we observe an onset of the butterfly effect with an oscillating probability of trapping. Unexpectedly, this probability saturates at a value different from 50% at low temperatures.

The resistivity $\rho$ and Hall resistivity ${\rho }_H$ are measured on FeSe at pressures up to $P=28.3$ kbar in magnetic fields up to $B=14.5$ T. The $\rho \left(B\right)$ and ${\rho }_H\left(B\right)$ curves are analyzed with multicarrier models to estimate the carrier density and mobility as a function of $P$ and temperature ($T\le 110$ K). It is shown that the pressure-induced antiferromagnetic transition is accompanied by an abrupt reduction of the carrier density and scattering. This indicates that the electronic structure is reconstructed significantly by the antiferromagnetic order.

In this Rapid Communication we derive the full gauge-invariant electromagnetic response beyond the BCS level using the fermionic superfluid path integral. In the process we identify and redress a failure to satisfy the compressibility sum rule; this shortcoming is associated with the conventional path-integral formulation of BCS-level electrodynamics. The approach in this paper builds on an alternative saddle point scheme. At the mean field level, this leads to the well known gauge-invariant electrodynamics of BCS theory and to the satisfaction of the compressibility sum rule. Moreover, this scheme can be readily extended to address arbitrary higher order fluctuation theories (for example, at the Gaussian level.) At any level this approach will lead to a gauge invariant and compressibility sum rule consistent treatment of electrodynamics and thermodynamics.