We use first-principles theory to predict that the application of uniaxial compressive strain leads to a transition from an antiferromagnetic insulator to a ferromagnetic half-metal phase in ${\mathrm{LaMnO}}_3$. We identify the Q2 Jahn-Teller mode as the primary mechanism that drives the transition, indicating that this mode can be used to tune the lattice, charge, and spin coupling. Applying $\simeq 6$ GPa of uniaxial pressure along the [010] direction activates the transition to a half-metallic $\mathit{pseudocubic}$ state. The half-metallicity opens the possibility of producing colossal magnetoresistance in the stoichiometric ${\mathrm{LaMnO}}_3$ compound at significantly lower pressure compared to recently observed investigations using hydrostatic pressure.

We study collective amplitude modes of the superconducting order parameter in strongly coupled electron-phonon systems described by the Holstein model using the nonequilibrium dynamical mean-field theory with the self-consistent Migdal approximation as an impurity solver. The frequency of the Higgs amplitude mode is found to coincide with the superconducting gap even in the strongly coupled (beyond BCS) regime. Besides the Higgs mode, we find another collective mode involving the dynamics of both the phonons and the superconducting order parameter. The frequency of this mode, higher than twice the renormalized phonon frequency in the superconducting phase, is shown to reflect a strong electron-mediated phonon-phonon interaction. Both of collective modes are predicted to contribute to time-resolved photoemission spectra after a strong laser pump as vertex corrections to produce resonance peaks, which allows one to distinguish them from quasiparticle excitations.

Recently, $R{\text{Mn}}_2{\text{O}}_5$ ($R=\text{rare}$ earth, $\text{Bi}, \text{Y}$) type multiferroics have drawn considerable attention, because of magnetically induced ferroelectricity along with an extremely large magnetoelectric coupling. Here, we present a detailed study on ${\text{NdMn}}_2{\text{O}}_5$ which is a crucial composition between the nonferroelectric ${\text{PrMn}}_2{\text{O}}_5$ and ferroelectric ${\text{SmMn}}_2{\text{O}}_5$. We report the results of heat capacity, magnetization, dielectric permittivity, and electric polarization measurements along with an accurate description of the structural and microscopic magnetic properties obtained from high resolution x-ray and neutron diffraction studies. We show that ${\text{NdMn}}_2{\text{O}}_5$ is ferroelectric, although the magnitude of polarization is much weaker than that of the other multiferroic members. The direction of the polarization is along the crystallographic $b$ axis and its magnitude can be tuned with the application of a magnetic field. Moreover, unlike the other multiferroic members of this series, ferroelectricity in ${\text{NdMn}}_2{\text{O}}_5$ emerges in an incommensurate magnetic state. The present study also provides evidence in support of the influence of the rare-earth size on the magnetoelectric phase diagram.

The superconducting transition temperature $T_{\mathrm{c}}$ of ammoniated metal-doped FeSe ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_y{M}_x\mathrm{FeSe}$ $(M$: metal atom) has been scaled with the FeSe plane spacing, and it has been suggested that the FeSe plane spacing depends on the location of metal atoms in ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_y{M}_x\mathrm{FeSe}$ crystals. Although the crystal structure of ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_y\mathrm{L}{\mathrm{i}}_x\mathrm{FeSe}$ exhibiting a high $T_{\mathrm{c}}$ (∼44 K) was determined from neutron diffraction, the structure of ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_y{M}_x\mathrm{FeSe}$ exhibiting a low $T_{\mathrm{c}}$ (∼32 K) has not been determined thus far. Here, we determined the crystal structure of ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_y\mathrm{C}{\mathrm{s}}_{0.4}\mathrm{FeSe}(T_{\mathrm{c}}=33\mathrm{K})$ through the Rietveld refinement of the x-ray diffraction (XRD) pattern measured with synchrotron radiation at 30 K. The XRD pattern was analyzed based on two different models, on-center and off-center, under a space group of $I4/mmm$. In the on-center structure, the Cs occupies the $2a$ site and the N of $\mathrm{N}{\mathrm{H}}_3$ may occupy either the $4c$ or $2b$ site, or both. In the off-center structure, the Cs may occupy either the $4c$ or $2b$ site, or both, while the N occupies the $2a$ site. Only an on-center structure model in which the Cs occupies the $2a$ and the N of $\mathrm{N}{\mathrm{H}}_3$ occupies the $4c$ site provided reasonable results in the Rietveld analysis. Consequently, we concluded that ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_y\mathrm{C}{\mathrm{s}}_{0.4}\mathrm{FeSe}$ can be assigned to the on-center structure, which produces a smaller FeSe plane spacing leading to the lower $T_{\mathrm{c}}$.

Guided by a simple strategy in search of new superconducting materials, we predict that high-temperature superconductivity can be realized in classes of high-density materials having strong ${\mathit{sp}}^3$ chemical bonding and high lattice symmetry. We examine in detail sodalite carbon frameworks doped with simple metals such as Li, Na, and Al. Though such materials share some common features with doped diamond, their doping level is not limited, and the density of states at the Fermi level in them can be as high as that in the renowned $\mathrm{Mg}{\mathrm{B}}_2$. Together with other factors, this boosts the superconducting temperature $\left(T_c\right)$ in the materials investigated to higher levels compared to doped diamond. For example, the $T_c$ of sodalitelike $\mathrm{Na}{\mathrm{C}}_6$ is predicted to be above 100 K. This phase and a series of other sodalite-based superconductors are predicted to be metastable phases but are dynamically stable. Owing to the rigid carbon framework of these and related dense carbon materials, these doped sodalite-based structures could be recoverable as potentially useful superconductors.

We have carried out a microscopic study of the dynamic structure factor of liquid ${}^4\mathrm{He}$ across the normal-superfluid transition temperature using the path integral Monte Carlo method. The ill-posed problem of the inverse Laplace transform, from the imaginary-time intermediate scattering function to the dynamic response, is tackled by stochastic optimization. Our results show a quasiparticle peak and a small and broad multiphonon contribution. In spite of the lack of strength in the collective peaks, we clearly identify the rapid dropping of the roton peak amplitude when crossing the transition temperature $T_{\lambda }$. Other properties such as the static structure factor, static response, and one-phonon contribution to the response are also calculated at different temperatures. The changes of the phonon-roton spectrum with the temperature are also studied. An overall agreement with available experimental data is achieved.

Topological materials are characterized by an electronic band structure with nontrivial topological properties. In this paper we introduce a basis of operators for the linear space of operators spanned by charge-neutral fermion bilinears. These band-projected density operators are constructed using directly the eigenfunctions of the electronic energy band structure and there is no need to assume a flat Berry curvature. As a result, our set of operators has a wider range of validity and is sensitive to physical phenomena which are not detectable in the flat-curvature limit. In particular, we show that the Berry monopole configuration of a Weyl semimetal give rises to a nonvanishing Jacobiator for these band-projected density operators, implying the emergence of nonassociativity at the location of the Weyl nodes. The resulting nonassociativity observes the fundamental identity, the defining property of the Nambu bracket, and so one may call this a nonassociative Nambu geometry. We also derive the corresponding uncertainty principle.

Nitrogen-vacancy (NV) centers in diamond have emerged as valuable tools for sensing and polarizing spins. Motivated by potential applications in chemistry, biology, and medicine, we show that NV-based sensors are capable of detecting single spin targets even if they undergo diffusive motion in an ambient thermal environment. Focusing on experimentally relevant diffusion regimes, we derive an effective model for the NV-target interaction, where parameters entering the model are obtained from numerical simulations of the target motion. The practicality of our approach is demonstrated by analyzing two realistic experimental scenarios: (i) time-resolved sensing of a fluorine nuclear spin bound to an N-heterocyclic carbene-ruthenium (NHC-Ru) catalyst that is immobilized on the diamond surface and (ii) detection of an electron spin label by an NV center in a nanodiamond, both attached to a vibrating chemokine receptor in thermal motion. We find in particular that the detachment of a fluorine target from the NHC-Ru carrier molecule can be monitored with a time resolution of a few seconds.

We report the time-resolved photoluminescence spectroscopy of nanoseconds-scale hot-carrier (HC) cooling dynamics in InAs/GaAs quantum dot superlattices (QDSLs). We demonstrate supra 1000-K time-averaged carrier temperature in the InAs/GaAs QDSLs from one-dimensional density of states restricting the phase space and energy-momentum conservation in the carrier scattering processes. The InAs/GaAs QDSLs HC energy dissipation rate was much smaller than that for InAs/GaAs multiple quantum wells and nearly excitation-photon-density independent, implying reduced efficiency of carrier-carrier scattering.

The quantum transport properties of graphene and monolayer ${\text{MoS}}_2$ superconductor heterostructures have been of considerable importance in the last few years. Layered nature of molybdenum disulfide permits the superconducting correlation induction. Moreover, peculiar dynamical features of monolayer ${\mathrm{MoS}}_2$, such as valence band spin splitting in the nondegenerate $K$ and $K^{\prime }$ valleys originated from strong spin-orbit coupling, and considerable direct band gap can make it potentially a useful material for electronics applications. Using the Dirac-like Hamiltonian of ${\mathrm{MoS}}_2$, with taking into account the related mass asymmetry and topological contributions, we investigate the effect of spin-triplet $p$-wave pairing symmetry on the superconducting excitations, resulting in the Andreev reflection process and Andreev bound state in the corresponding normal-superconductor (NS) and superconductor-normal-superconductor (SNS) structures, respectively. We study how the resulting subgap conductance and Josephson current are affected by the particular symmetry of order parameter. The signature of $p_x$-wave symmetry is found to decline the subgap superconducting energy excitations and, consequently, slightly suppress the Andreev reflection in the case of the $p$-doped S region. The essential dynamical parameters $\lambda$ and $\beta$ of ${\mathrm{MoS}}_2$ have significant effect on the both tunneling conductance and Josephson current. Particularly, the considered $p$-wave symmetry in the superconducting bound energies may feature the zero energy states at the interfaces. The critical current oscillations as a function of length of junction are obtained in the $p$-doped S region.

In this work, we find an analytical flat-membrane solution to the saddle point equations, derived by F. Guinea et al. [Phys. Rev. B 89, 125428 (2014)], for the case of a suspended graphene membrane of circular shape. We also find how different buckled membrane solutions bifurcate from the flat membrane at critical temperatures and membrane radii. The saddle point equations take into account electron-phonon coupling and this coupling provides a residual stress even for a flat graphene layer. Below a critical temperature (which is exceedingly high for an infinite layer) or above a critical size that depend on boundary conditions, different buckling modes that may be the germ of rippling appear. Our results provide the opportunity to develop new feasible experiments dealing with buckling in small suspended graphene membranes that could verify them. These experiments may also be used to fit the phonon-electron coupling constant or the bending energy.

We present an analytical description of the electron energy loss near plasmonic nanostructures with cylindrical symmetry as a three-step process: (i) electron-induced excitation of surface plasmon modes, (ii) their propagation and reflection, and (iii) the backaction on the electrons. The model incorporates relativistic effects, retardation, and light emission, and can treat arbitrary tilted electron trajectories. Convincing agreement with recent experimental data is demonstrated.

We systematically study the high-resolution and polarized Raman spectra of multilayer (ML) ${\mathrm{MoTe}}_2$. The layer-breathing (LB) and shear (C) modes are observed in the ultralow-frequency region, which are used to quantitatively evaluate the interlayer coupling in ML ${\mathrm{MoTe}}_2$ based on the linear chain model, in which only the nearest interlayer coupling is considered. The Raman spectra on three different substrates verify the negligible substrate effect on the phonon frequencies of ML ${\mathrm{MoTe}}_2$. Ten excitation energies are used to measure the high-frequency modes of $N$-layer ${\mathrm{MoTe}}_2$ ($N\mathrm{L} {\mathrm{MoTe}}_2$; $N$ is an integer). Under the resonant excitation condition, we observe $N$–dependent Davydov components in ML ${\mathrm{MoTe}}_2$, originating from the Raman-active $A_1^{\prime }\left(A_{1g}^2\right)$ modes at ∼$172\mathrm{c}{\mathrm{m}}^{-1}$. More than two Davydov components are observed in $N\mathrm{L} {\mathrm{MoTe}}_2$ for $N>4$ by Raman spectroscopy. The $N$-dependent Davydov components are further investigated based on the symmetry analysis. A van der Waals model only considering the nearest interlayer coupling has been proposed to well understand the Davydov splitting of high-frequency $A_1^{\prime }\left(A_{1g}^2\right)$ modes. The different resonant profiles for the two Davydov components in 3L ${\mathrm{MoTe}}_2$ indicate that proper excitation energy of $\sim 1.8-2.2$ eV must be chosen to observe the Davydov splitting in ML ${\mathrm{MoTe}}_2$. Our work presents a simple way to identify layer number of ultrathin ${\mathrm{MoTe}}_2$ flakes by the corresponding number and peak position of Davydov components. Our work also provides a direct evidence from Raman spectroscopy of how the nearest van der Waals interactions significantly affect the frequency of the high-frequency intralayer phonon modes in multilayer ${\mathrm{MoTe}}_2$ and expands the understanding on the lattice vibrations and interlayer coupling of transition metal dichalcogenides and other two-dimensional materials.

Geometrically imposed force cancellations lead to ultralow friction between rigid incommensurate crystalline asperities. Elastic deformations may avert this cancellation but are difficult to treat analytically in finite and three-dimensional systems. We use atomic-scale simulations to show that elasticity affects the friction only after the contact radius $a$ exceeds a characteristic length set by the core width of interfacial dislocations $b_{\text{core}}$. As $a$ increases past $b_{\text{core}}$, the frictional stress for both incommensurate and commensurate surfaces decreases to a constant value. This plateau corresponds to a Peierls stress that drops exponentially with increasing $b_{\text{core}}$ but remains finite.

We study analytically and with the numerical time-evolving block decimation method the dynamics of an impurity in a bath of spinless fermions with nearest-neighbor interactions in a one-dimensional lattice. The bath is in a Mott insulator state with alternating sites occupied and the impurity interacts with the bath by repulsive on-site interactions. We find that when the magnitudes of the on-site and nearest-neighbor interactions are close to each other, the system shows excitations of two qualitatively distinct types. For the first type, a domain wall and an antidomain wall of density propagate into opposite directions, while the impurity stays at the initial position. For the second one, the impurity is bound to the antidomain wall while the domain wall propagates, an excitation where the impurity and bath are closely coupled.

We evaluate the entanglement entropy of exactly solvable Hamiltonians corresponding to general families of three-dimensional topological models. We show that the modification to the entropic area law due to three-dimensional topological properties is richer than the two-dimensional case. In addition to the reduction of the entropy caused by a nonzero vacuum expectation value of contractible loop operators, a topological invariant emerges that increases the entropy if the model consists of nontrivially braiding anyons. As a result the three-dimensional topological entanglement entropy provides only partial information about the two entropic topological invariants.

We address the problem of Dirac fermions interacting with transversal optical phonons. A gap in the spectrum of fermions leads to the emergence of the Chern-Simons excitations in the spectrum of phonons. We study the effect of those excitations on observable quantities: the phonon dispersion, the phonon spectral density, and the Hall conductivity.