The interaction between phonons and high-energy excitations of electronic origin in cuprates and their role in the superconducting mechanisms is still controversial. Here we use coherent vibrational time-domain spectroscopy together with density functional and dynamical mean-field theory calculations to establish a direct link between the $c$-axis phonon modes and the in-plane electronic charge excitations in optimally doped $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_7$. The nonequilibrium Raman tensor is measured by means of the broadband “coherent-phonon” response in pump-probe experiments and is qualitatively described by our model using density functional theory in the frozen-phonon approximation plus single-band dynamical mean-field theory to account for the electronic correlations. The major outcome of our experimental and theoretical study is to establish the link between out-of-plane copper ion displacements and the in-plane electronic correlations, and to estimate at a few unit cells the correlation length of the associated phonon mode. The approach introduced here could help in revealing the complex interplay between fluctuations of different nature and spatial correlation in several strongly correlated materials.

Density functional theory calculations using the quasiharmonic approximation have been used to calculate the solid Hugoniot of two polytypes of boron carbide up to 100 GPa. Under the assumption that segregation into the elemental phases occurs around the pressure that the ${\mathrm{B}}_{11}{\mathrm{C}}_{\text{p}}$(CBC) polytype becomes thermodynamically unstable with respect to boron and carbon, two discontinuities in the Hugoniot, one at 50 GPa and one at 90–100 GPa, are predicted. The former is a result of phase segregation, and the latter a phase transition within boron. First-principles molecular dynamics (FPMD) simulations were employed to calculate the liquid Hugoniot of ${\mathrm{B}}_4\mathrm{C}$ up to 1.5 TPa, and the results are compared to recent experiments carried out at the Omega Laser Facility up to 700 GPa [D. E. Fratanduono, Phys. Rev. B 94, 184107 (2016)]. A generally good agreement between theory and experiment was observed. Analysis of the FPMD simulations provides evidence for an amorphous, but covalently bound, fluid below 438 GPa, and an atomistic fluid at higher pressures and temperatures.

The magnetoelectric coupling, a phenomenon inducing magnetic (electric) polarization by application of an external electric (magnetic) field and first conjectured by Curie in 1894, is observed in most of the multiferroics and used for many applications in various fields such as data storage or sensing. However, its microscopic origin is a long-standing controversy in the scientific community. An intense revival of interest developed in the beginning of the 21st century due to the emergence of multiferroic frustrated magnets in which the ferroelectricity is magnetically induced and which present an inherent strong magnetoelectric coupling. The Dzyaloshinskii-Moriya interaction (DMI) well accounts for such ferroelectricity in systems with a noncollinear magnetic order such as the ${\mathrm{RMnO}}_3$ manganites. The DMI effect is, however, inadequate for systems presenting ferroelectricity induced by quasicollinear spin arrangements such as the prominent ${\mathrm{RMn}}_2{\mathrm{O}}_5$ manganites. Among different microscopic mechanisms proposed to resolve this incompatibility, the exchange-striction model stands as the most invoked candidate. In this scenario, the polar atomic displacements originate from the release of a frustration caused by the magnetic order. Despite its theoretical description 15 years ago, this mechanism had yet to be unambiguously validated experimentally. The breakthrough finally comes from ${\mathrm{SmMn}}_2{\mathrm{O}}_5$ presenting a unique magnetic order revealed by powder neutron diffraction. The unique orientation of its magnetic moment establishes the missing element that definitely validates the exchange striction as the effective mechanism for the spin-induced ferroelectricity in this series. More generally, this is a proof of concept that validates this model on actual systems, facilitating the development of a new generation of multiferroics with unrivaled magnetoelectric properties.

We model changes of magnetic ordering in Mn-based antiperovskite nitrides driven by biaxial lattice strain at zero and at finite temperature. We employ a noncollinear spin-polarized density functional theory to compare the response of the geometrically frustrated exchange interactions to a tetragonal symmetry breaking (the so called piezomagnetic effect) across a range of ${\mathrm{Mn}}_3\mathrm{AN}$ (A = Rh, Pd, Ag, Co, Ni, Zn, Ga, In, Sn) at zero temperature. Building on the robustness of the effect we focus on ${\mathrm{Mn}}_3\mathrm{GaN}$ and extend our study to finite temperature using the disordered local moment (DLM) first-principles electronic structure theory to model the interplay between the ordering of Mn magnetic moments and itinerant electron states. We discover a rich temperature-strain magnetic phase diagram with two previously unreported phases stabilized by strains larger than 0.75% and with transition temperatures strongly dependent on strain. We propose an elastocaloric cooling cycle crossing two of the available phase transitions to achieve simultaneously a large isothermal entropy change (due to the first-order transition) and a large adiabatic temperature change (due to the second-order transition).

We analyze general zero mode properties of the parent Hamiltonian of the unprojected Jain-2/5 state. We characterize the zero mode condition associated to this Hamiltonian via projection onto a four-dimensional two-particle subspace for given pair angular momentum, for the disk and similarly for the spherical geometry. Earlier numerical claims in the literature about ground-state uniqueness on the sphere are substantiated on analytic grounds, and related results are derived. Preference is given to second-quantized methods, where zero mode properties are derived not from given analytic wave functions, but from a “lattice” Hamiltonian and associated zero mode conditions. This method reveals new insights into the guiding-center structure of the unprojected Jain-2/5 state, in particular, a system of dominance patterns following a “generalized Pauli principle,” which establishes a complete one-to-one correspondence with the edge mode counting. We also identify one-body operators that function as generators of zero modes.

In numerical simulations of classical and quantum lattice systems, 2D corner transfer matrices (CTMs) and 3D corner tensors (CTs) are a useful tool to compute approximate contractions of infinite-size tensor networks. In this paper we show how the numerical CTMs and CTs can be used, additionally, to extract universal information from their spectra. We provide examples of this for classical and quantum systems, in 1D, 2D, and 3D. Our results provide, in particular, practical evidence for a wide variety of models of the correspondence between $d$-dimensional quantum and $(d+1)$-dimensional classical spin systems. We show also how corner properties can be used to pinpoint quantum phase transitions, topological or not, without the need for observables. Moreover, for a chiral topological PEPS we show by examples that corner tensors can be used to extract the entanglement spectrum of half a system, with the expected symmetries of the $SU{\left(2\right)}_k$ Wess-Zumino-Witten model describing its gapless edge for $k=1,2$. We also review the theory behind the quantum-classical correspondence for spin systems and provide a numerical scheme for quantum state renormalization in 2D using CTs. Our results show that bulk information of a lattice system is encoded holographically in efficiently-computable properties of its corners.

We study both noncentrosymmetric and time-reversal breaking Weyl semimetal systems under a strong magnetic field with the Coulomb interaction. The three-dimensional bulk system is reduced to many mutually interacting quasi-one-dimensional wires. Each strongly correlated wire can be approached within the Tomonaga-Luttinger liquid formalism. Including impurity scatterings, we inspect the localization effect and the temperature dependence of the electrical resistivity. The effect of a large number of Weyl points in real materials is also discussed.

Supersymmetry (SUSY) is a symmetry transforming bosons to fermions and vice versa. Indications of its existence have been extensively sought after in high-energy experiments. However, signatures of SUSY have yet to be detected. In this paper we study a condensed matter realization of SUSY on the edge of a Read-Rezayi quantum Hall state, given by filling factors of the form $\nu =\frac{k}{k+2}$, where $k$ is an integer. As we show explicitly, this strongly interacting state exhibits an $\mathcal{N}=2$ SUSY. This allows us to use a topological invariant—the Witten index—defined specifically for supersymmetric theories, to count the difference between the number of bosonic and fermionic zero modes in a circular edge. In this system, we argue that the edge hosts $k+1$ protected zero modes. We further discuss the stability of SUSY with respect to generic perturbations and find that much of the above results remain unchanged. In particular, these results directly apply to the well-established $\nu =1/3$ Laughlin state, in which case SUSY is a robust property of the edge theory. These results unveil a hidden topological structure on the long-studied Read-Rezayi states.

Chiral multiferroic langasites have attracted attention due to their doubly chiral magnetic ground state within an enantiomorphic crystal. We report on a detailed resonant soft x-ray diffraction study of the multiferroic ${\mathrm{Ba}}_3{\mathrm{TaFe}}_3{\mathrm{Si}}_2{\mathrm{O}}_{14}$ at the Fe $L_{2,3}$ and oxygen $K$ edges. Below $T_N$ ($\approx 27K$) we observe the satellite reflections $(0,0,\tau ), (0,0,2\tau ), (0,0,3\tau )$, and $(0,0,1-3\tau )$ where $\tau \approx 0.140\pm 0.001$. The dependence of the scattering intensity on x-ray polarization and azimuthal angle indicate that the odd harmonics are dominated by the out-of-plane ($\widehat{\mathbf{c}}$ axis) magnetic dipole while the $(0,0,2\tau )$ originates from the electron density distortions accompanying magnetic order. We observe dissimilar energy dependencies of the diffraction intensity of the purely magnetic odd-harmonic satellites at the Fe $L_3$ edge. Utilizing first-principles calculations, we show that this is a consequence of the loss of threefold crystal symmetry in the multiferroic phase.

Composed of earth-abundant elements, ${\mathrm{ZnSnN}}_2$ is a promising semiconductor for photovoltaic and photoelectrochemical applications. However, basic properties such as the precise value of the band gap and the band alignment to other semiconductors are still unresolved. For instance, reported values for the band gap vary from 1.4 to 2.0 eV. In addition, doping in ${\mathrm{ZnSnN}}_2$ remains largely unexplored. Using density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional, we investigate the electronic structure of ${\mathrm{ZnSnN}}_2$, its band alignment to GaN and ZnO, and the possibility of $p$-type doping. We find that the position of the valence-band maximum of ${\mathrm{ZnSnN}}_2$ is 0.39 eV higher than that in GaN, yet the conduction-band minimum is close to that in ZnO, which suggests that achieving $p$-type conductivity is likely as in GaN, yet it may be difficult to control unintentional $n$-type conductivity as in ZnO. Among possible $p$-type dopants, we explore Li, Na, and K substituting on the Zn site. We show that while ${\mathrm{Li}}_{\mathrm{Zn}}$ is a shallow acceptor, ${\mathrm{Na}}_{\mathrm{Zn}}$ and ${\mathrm{K}}_{\mathrm{Zn}}$ are deep acceptors, which we trace back to large local relaxations around the Na and K impurities due to the atomic size mismatches.

A comprehensive theoretical analysis of photoinduced forces in an illuminated nanojunction, formed between an atomic force microscopy tip and a sample, is presented. The formalism is valid within the dipolar approximation and includes multiple scattering effects between the tip, sample, and a planar substrate through a dyadic Green's function approach. This physically intuitive description allows a detailed look at the quantitative contribution of multiple scattering effects to the measured photoinduced force, effects that are typically unaccounted for in simpler analytical models. Our findings show that the presence of the planar substrate and anisotropy of the tip have a substantial effect on the magnitude and the spectral response of the photoinduced force exerted on the tip. Unlike previous models, our calculations predict photoinduced forces that are within range of experimentally measured values in photoinduced force microscopy (PiFM) experiments.

We have investigated magnetic and ferroelectric (dielectric) properties of multiferroic ${\mathrm{CuFe}}_{0.982}{\mathrm{Ga}}_{0.018}{\mathrm{O}}_2, {\mathrm{CuFe}}_{0.965}{\mathrm{Ga}}_{0.035}{\mathrm{O}}_2$, and ${\mathrm{CuFe}}_{0.95}{\mathrm{Al}}_{0.05}{\mathrm{O}}_2$ under applied uniaxial pressure $p$ up to 600 MPa. Unlike the results of the almost same experiments on ${\mathrm{CuFeO}}_2$ [Tamatsukuri et al., Phys. Rev. B 94, 174402 (2016)], we have found that the application of $p$ induces a new ferroelectric phase, which is different from the well-studied spin-driven ferroelectric phase with helical magnetic ordering, in all the doped samples investigated here. We have also constructed the temperature versus $p$ magnetoelectric phase diagrams of the three samples. The ferroelectric polarization in the $p$-induced ferroelectric phase lies along the [110] direction as in the helical magnetoferroelectric phase, and its value is comparable with or larger than that in the helical magnetoferroelectric phase. The magnetic structure in the $p$-induced ferroelectric phase seems to be of a collinear sinusoidal type. Although this magnetic structure itself does not break the inversion symmetry, it is considered to play an important role in the origin of ferroelectricity in the $p$-induced ferroelectric phase through the spin-lattice coupling in this system.

Neutron single-crystal diffraction and rotational anisotropy optical second harmonic generation data are presented resolving the nature of the structural distortion realized in electron-doped (${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x$)${}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ with $x=0.035$ and $x=0.071$. Once electrons are introduced into the bilayer spin-orbit assisted Mott insulator ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, previous studies have identified the appearance of a low-temperature structural distortion and have suggested the presence of a competing electronic instability in the phase diagram of this material. Our measurements resolve a lowering of the structural symmetry from monoclinic $C2/c$ to monoclinic $P{2}_1/c$ and the creation of two unique Ir sites within the chemical unit cell as the lattice distorts below a critical temperature $T_S$. Details regarding the modifications to oxygen octahedral rotations and tilting through the transition are discussed as well as the evolution of the low-temperature distorted lattice as a function of carrier substitution.

The structural properties, energetics, and polarizations of perovskite-based thin-film oxide systems are computed as a function of biaxial strain state and epitaxial orientation, employing an automated computational workflow based on density functional theory. A total of 14 compositions are considered, of the form $\mathit{AB}{\mathrm{O}}_3$, with $A$ = Ba, K, Na, Pb, and Sr and $B$ = Hf, Sn, Ti, Zr, Nb, Ta, and V site cations chosen to yield tolerance factors with values ranging between 0.95 and 1.1. Three biaxial strain states corresponding to epitaxial growth of (100)-, (110)-, and (111)-oriented films are considered, with misfit strains ranging between $-4\%$ to 4%. Results are presented for the series of perovskite-derived phases, and their corresponding symmetries, which are energetically favorable as a function of misfit strain, along with their corresponding equilibrium atomic positions, lattice parameters, and electric polarizations. The results demonstrate robust trends of in-plane polarization enhancement under tensile strain for all epitaxial orientations, and out-of-plane polarization enhancement with compression for the (100)- and (110)-oriented films. Strains corresponding to the (111)-growth orientation lead to a wider variety of out-of-plane polarization behavior, with ${\mathrm{BaTiO}}_3$ showing anomalous diminishing polarization with compression. Epitaxial orientation is shown to have a strong effect on the nature of strain-induced phase transitions, with (100)-oriented systems tending to have smooth, second-order transitions and (110)- and (111)-oriented systems more commonly exhibiting first-order transitions. The significance of this effect for device applications is discussed, and a number of systems are identified as potentially interesting for ferroelectric thin-film applications based on energetic stability and polarization behavior. Analysis of polarization behavior across different orientations reveals distinct groups into which compositions can be organized, some of which have polarization dependencies on misfit strain that have not been reported previously.

The compression behavior of deuterated ice VII and VIII was investigated by high pressure neutron scattering in the pressure range 2–13.7 GPa between 93 and 300 K. We establish equations of state which contain accurate values for the bulk moduli $B_0$, their pressure derivatives $B_0^{\prime }$, as well as the ambient pressure volume $V_0$. These equations of state hold over a large part of the stability domain of ice VII, by comparison with available x-ray data, and to at least $\approx 13$ GPa for ice VIII. They are indistinguishable at low pressures, but beyond $\approx 7$ GPa and at low temperatures ice VIII appears to become stiffer than expected. This might be related to an anomalous phonon hardening observed previously in ice VIII in this $P/T$ range [D. D. Klug et al., Phys. Rev. B 70, 144113 (2004)].

We propose a computational method of the Loschmidt amplitude in a generic spin system on the basis of the complex semiclassical analysis on the spin-coherent state path integral. We demonstrate how the dynamical transitions emerge in the time evolution of the Loschmidt amplitude for the infinite-range transverse Ising model with a longitudinal field, exposed by a quantum quench of the transverse field $\mathrm{\Gamma }$ from $\infty$ or zero. For both initial conditions, we obtain the dynamical phase diagrams that show the presence or absence of the dynamical transition in the plane of transverse field after a quantum quench and the longitudinal field. The results of semiclassical analysis are verified by numerical experiments. Experimental observation of our findings on the dynamical transition is also discussed.