Nuclear quadrupole resonance (NQR) measurements were performed on the heavy-fermion superconductor ${\mathrm{Ce}}_3{\mathrm{PtIn}}_{11}$ with $T_{\mathrm{c}}=0.32\mathrm{K}$. The temperature dependence of both spin-lattice relaxation rate $1/T_1$ and NQR spectra evidences the occurrence of two successive magnetic transitions with $T_{\mathrm{N}1}\simeq 2.2\mathrm{K}$ and $T_{\mathrm{N}2}\simeq 2.0\mathrm{K}$. In successive magnetic transitions, even though the magnetic moment at the Ce(2) site plays a major role, the magnetic moment at the Ce(1) site also contributes to some extent. While a commensurate antiferromagnetic ordered state appears for $T_{\mathrm{N}2}<T<T_{\mathrm{N}1}$, a partially incommensurate antiferromagnetic ordered state is suggested for $T<T_{\mathrm{N}2}$.

A complex orthorhombic carbon allotrope in $Pbam$ symmetry with 32 atoms in its unit cell, thus termed $Pbam$-32 carbon, was recently predicted [C. Y. He et al., Phys. Rev. Lett. 121, 175701 (2018)]. Its crystal structure comprises alternating fivefold, sixfold, and sevenfold carbon rings and exhibits reduced bonding anisotropy compared to diamond, raising the prospects of finding a superstrong material with distinct and favorable mechanical properties. Here we report findings from first-principles calculations that reveal peculiar stress-strain relations in $Pbam$-32 carbon. The obtained stress responses under various tensile and shear strains display outstanding characteristics contrasting those of traditional superhard materials like diamond and cubic boron nitride ($c$-BN). The $Pbam$-32 carbon undergoes structural deformations that produce highly isotropic stress responses under a wide variety of large tensile and shear strains, showcasing unprecedented nearly degenerate stress-strain curves along multiple deformation paths extended over ultralarge, including full-range, strains up to the bond-breaking points. These deformation modes impede or even suppress the graphitization process commonly seen in highly strained diamond and $c$-BN crystals while still sustaining large peak stresses comparable to those in diamond. Most notably, we find conspicuous bond-weakening and -breaking mechanisms stemming from bonding symmetry reduction in $Pbam$-32 carbon. At large tensile strains, a sequential bond elongation process occurs, generating a more ductile deformation past the peak stress; at large shear strains, the crystal structure goes through a similar sequential bond elongation process and, interestingly, transforms into a distinct three-dimensional network containing mixed $s{p}^2$ and $s{p}^3$ bonding states, suppressing the usual graphitization process. These more gradual bonding-state changes in the severely strained $Pbam$-32 carbon improve ductility and toughness in this superstrong carbon crystal. These insights elucidate mechanisms for toughening superstrong covalent crystals via microstructural arrangements, which shed light on rational design and development of a distinct class of superstrong materials that exhibit more isotropic mechanical responses with improved toughness under diverse loading conditions.

Due to strong electron-phonon interactions, strain engineering is a powerful tool to control quantum critical phenomena in strongly correlated oxides. Exploring this possibility requires understanding the nanoscale structure of quantum materials and the role it plays in forming the strain landscape. Here, we used a combination of x-ray nanoimaging and reciprocal space mapping to study the nanostructure of the archetypal Mott insulator ${\mathrm{VO}}_2$, featuring an insulator-to-metal and structural phase transitions. We found that ${\mathrm{VO}}_2$ thin films grown on r-cut sapphire consist of two intertwined crystal lattices, permanently inclined with respect to each other. This persistent pattern of twin domains stands out from the symmetry breaking induced by the structural phase transition and conceivably originates from the post-growth strain relaxation process. We propose a model explaining the formation of twin domains and the emergence of anisotropy in the film nanostructure. Our work suggests using miscut substrates to suppress either one or the other twin that can serve as a new tool to control strain in ${\mathrm{VO}}_2$ films.

The many-body localization transition for a Heisenberg spin chain with a speckle disorder is studied. Such a model is equivalent to a system of spinless fermions in an optical lattice with an additional speckle field. Our numerical results show that the many-body localization transition in speckle disorder falls within the same universality class as the transition in an uncorrelated random disorder, in contrast to the quasiperiodic potential typically studied in experiments. This hints at possibilities of experimental studies of the role of rare Griffiths regions and of the interplay of ergodic and localized grains at the many-body localization transition. Moreover, the speckle potential allows one to study the role of correlations in disorder on the transition. We study both spectral and dynamical properties of the system focusing on observables that are sensitive to the disorder type and its correlations. In particular, distributions of local imbalance at long times provide an experimentally available tool that reveals the presence of small ergodic grains even deep in the many-body localized phase in a correlated speckle disorder.

We demonstrate that a weak disorder in atomic positions introduces spatially localized optical modes in a dense three-dimensional ensemble of immobile two-level atoms arranged in a diamond lattice and coupled by the electromagnetic field. The frequencies of the localized modes concentrate near band edges of the unperturbed lattice. Finite-size scaling analysis of the percentiles of Thouless conductance reveals two mobility edges and yields an estimation $\nu =0.8–1.1$ for the critical exponent of the localization length. The localized modes disappear when the disorder becomes too strong and the system starts to resemble a fully disordered one where all modes are extended.

In the correlated Stokes–anti-Stokes scattering (SAS) an incident photon interacts with a Raman-active material, creating a Stokes photon and exciting a quantum vibrational mode in the medium, which is posteriorly annihilated on contact with a second incident photon, producing in turn an anti-Stokes photon. This can be accomplished by real and virtual processes. In the real process the quantum mode shared between the Stokes and anti-Stokes events is a real particle, whereas in virtual processes the pair formation is mediated by the exchange of virtual particles. Here, we introduce a Hamiltonian to describe the pair production in SAS scattering, for both types of process, when stimulated by two orthogonally polarized laser pulses in a pump-and-probe configuration. We also model the effect of the natural decay of the vibration created in the Stokes event and compute the probability of producing SAS pairs. Additionally, we follow the dynamics of the vibration by considering the Stokes and anti-Stokes fields as external reservoirs, obtaining thus a master equation for the reduced density matrix for the vibrational population. Finally, we compare our theoretical results to recently published experimental data.

We report the temperature-dependent THz to mid-infrared response of hexagonal-${\mathrm{ErMnO}}_3$ using absorption, reflectivity, and emissivity techniques from 2 to 1700 K. At low temperatures, lowest frequency vibrational modes, which extend up the lock-in ferroelectric temperature, coexist with paramagnon excitations and are associated with well-defined crystal-field rare-earth pure magnetic replicas in an intriguing phonon-magnetic convergence enhancing the multiferroic character of $h\text{−}{\mathrm{ErMnO}}_3$. Increasing the temperature, a number of vibrational bands close to the space group predicted undergo profile broadening and softening. In particular, a distinctive set of bands in the $288–329\text{−}\mathrm{c}{\mathrm{m}}^{-1}$ (300 K) range has a component whose profile is carrier screened becoming nearly fully blurred in the intermediate phase between ∼830 and ∼1500 K. Below $T_C\sim 830$ K this asymmetric band, having one component still partially screened, further splits as spin phonon interaction and the tripling of the unit cell take place revealing at $T_{\mathrm{N}}\sim 79$ K a delicate balance of long- and short-range interactions. Ambient Raman scattering brings up evidence of a Fröhlich resonance due to Coulomb interactions between carriers and the macroscopic field linked to the corresponding longitudinal optical-phonon mode. We found it is dynamically correlated to the hexagonal $c$-axis negative thermal expansion. Quantitative analyses of the mid-infrared (MIR) optical conductivity show that it also plays a role in small polarons and mediates in high-temperature bipolarones. Bipolaron profiles at high temperatures change as the sample opacity increases when at ∼900 K straight stripes turn curly toward complex vortex-antivortex domain patterns in the paraelectric phase. At still higher temperatures a low-frequency Drude contribution is triggered by electron hopping signaling an insulator-metal phase transition at ∼1600 K while the MIR response suggests coexistence between single small polarons and bipolarons. On closing, we draw a parallel with improper ferroelectrics sustaining a lattice incommensurate intermediate phase and unit-cell tripling. We argue that in the $h\text{−}R{\mathrm{MnO}}_3$ $(R=\text{rare earth}, \mathrm{Y})$ family of compounds the intermediate phase be considered incommensurate with onset at $T_{\mathrm{INC}}\sim 1500$ K and ferroelectric lock-in at $T_C\sim 830$ K delimiting this regime in $h\text{−}{\mathrm{ErMnO}}_3$.

We describe a time evolution algorithm for quantum spin chains whose Hamiltonians are composed of an infinite uniform left and right bulk part, and an arbitrary finite region in between. The left and right bulk parts are allowed to be different from each other. The algorithm is based on the time-dependent variational principle (TDVP) of matrix product states. It is inversion-free and very simple to adapt from an existing TDVP code for finite systems. The importance of working in the projective Hilbert space is highlighted. We study the quantum Ising model as a benchmark and an illustrative example. The spread of information after a local quench is studied in both the ballistic and the diffusive case. We also offer a derivation of TDVP directly from symplectic geometry.

We develop a comprehensive renormalization group (RG) approach to criticality in open Floquet systems, where dissipation enables the system to reach a well-defined Floquet steady state of finite entropy, and all observables are synchronized with the drive. We provide a detailed description of how to combine Keldysh and Floquet formalisms to account for the critical fluctuations in the weakly and rapidly driven regime. A key insight is that a reduction to the time-averaged static sector is not possible close to the critical point. This guides the design of a perturbative dynamic RG approach, which treats the time-dependent, dynamic sector associated to higher harmonics of the drive, on an equal footing with the time-averaged sector. Within this framework, we develop a weak drive expansion scheme, which enables to systematically truncate the RG flow equations in powers of the inverse drive frequency ${\mathrm{\Omega }}^{-1}$. This allows us to show how a periodic drive inhibits scale invariance and critical fluctuations of second-order phase transitions in rapidly driven open systems. Although criticality emerges in the limit ${\mathrm{\Omega }}^{-1}=0$, any finite drive frequency produces a scale that remains finite all through the phase transition. This is a universal mechanism that relies on the competition of the critical fluctuations within the static and dynamic sectors of the problem.

Controlling the preferred direction of the magnetic moments is essential for the design of spintronic devices based on ultrathin films and heterostructures. As the film thickness or the temperature is increased, the easy anisotropy axis is typically reoriented from an out-of-plane direction preferred by surface and interface energy contributions to an in-plane alignment favored by the volume anisotropy terms. We study the temperature-driven spin reorientation transition in two atomic layers of Fe on W(110) using well-tempered metadynamics simulations based on a spin model parametrized by ab initio calculations and find that the transition only takes place in the presence of the Dzyaloshinsky-Moriya interaction (DMI). This demonstrates that the chiral DMI does not only differentiate between noncollinear spin structures of different rotational senses, but it also influences the magnetic orientation of collinear magnetic configurations.

The collective excitation modes of a magnetic chiral soliton lattice (CSL) are numerically studied. We perform micromagnetic simulations of a monoaxial chiral helimagnet with an external magnetic field that forms the CSL. Due to the repulsive interaction between solitons through a ferromagnet state, collective modes described by the coupled oscillator are excited. These modes exist at a frequency of $\sim 1\mathrm{GHz}$ while the conventional ferromagnetic resonant mode exists at a frequency of $\sim 10\mathrm{GHz}$ as measured in experiments [F. Goncalves et al., Phys. Rev. B 95, 104415 (2017)]. In contrast to ferromagnetic resonance, the resonant frequency of the coupled-oscillator mode decreases by increasing the magnetic field. We clarified that the coupling energy between solitons becomes weak by increasing the distance between solitons. We also investigate the resonant condition of a precessional mode of the CSL measured in experiments.

NiO is a prototypical antiferromagnet with a characteristic resonance frequency in the THz range. From atomistic spin dynamics simulations that take into account the crystallographic structure of NiO, and in particular a magnetic anisotropy respecting its symmetry, we describe antiferromagnetic switching at THz frequency by a spin transfer torque mechanism. Subpicosecond $S$-state switching between the six allowed stable spin directions is found for reasonably achievable spin currents, like those generated by laser induced ultrafast demagnetization. A simple procedure for picosecond writing of a six-state memory is described, thus opening the possibility to speed up current logic of electronic devices by several orders of magnitude.

The ${\mathrm{V}}_{\mathrm{N}}{\mathrm{N}}_{\mathrm{B}}$ defect in hexagonal boron nitride (h-BN), comprising a nitrogen vacancy adjacent to a nitrogen-for-boron substitution, is modelled in regard to its possible usefulness in a nanophotonics device. The modelling is done on both a simple model compound and on a 2D periodic representation of the defect, considering its magnetic and spectroscopic properties. The electronic distribution in most excited states of ${\mathrm{V}}_{\mathrm{N}}{\mathrm{N}}_{\mathrm{B}}$ is found to be very open-shell in nature, and to deal with this two new computational methods are developed: one allows standard density-functional theory (DFT) calculations to be employed to evaluate state energies for doublet states with three unpaired electrons, the other introduces techniques needed to apply the vasp computational package to these and many other problems involving excited states. Also of general use, results from DFT calculations are then calibrated against those from the ab initio methods MRCI, CASPT2, CCSD, EOM-CCSD, and CCSD(T), seeking robust computational schemes. This complements previous work to reveal quite different properties for systems with odd and even numbers of electrons. These innovations allow 45 electronic states of the defect in its neutral, +1 and −1 charged forms to be considered. The charged forms of the defect are predicted to display properties of potential interest to nanophotonics.

Extensive photochemical and spectroscopic properties of the ${\mathrm{V}}_{\mathrm{B}}^{-}$ defect in hexagonal boron nitride are calculated, concluding that the observed photoemission associated with recently observed optically detected magnetic resonance is most likely of ${\left(1\right)}^3{E}^{\prime \prime }\to {\left(1\right)}^3{A}_2^{\prime }$ origin. Rapid intersystem crossing from the defect's triplet to singlet manifolds explains the observed short excited-state lifetime and very low quantum yield. New experimental results reveal smaller intrinsic spectral bandwidths than previously recognized, interpreted in terms of spectral narrowing and zero-phonon-line shifting induced by the Jahn-Teller effect. Different types of computational methods are applied to map out the complex triplet and singlet defect manifolds, including the doubly ionized formulation of the equation-of-motion coupled-cluster theory that is designed to deal with the open-shell nature of defect states, and mixed quantum-mechanics/molecular-mechanics schemes enabling 5763-atom simulations. Two other energetically feasible spectral assignments from among the singlet and triplet manifolds are considered, but ruled out based on inappropriate photochemical properties.

Spin-polarized electrons injected from a ferromagnet (FM) onto the surface of a topological insulator (TI) tend to produce a charge current transverse to the direction of the spin polarization because of the spin-momentum helical locking of the TI surface states. The charge current can be measured as an open-circuit voltage that will change in sign if the magnetization direction of the FM is reversed. Here, we model the two-dimensional transport on the TI surface coupled to a FM through a tunnel barrier (TB). The transport equations are solved analytically for two different boundary conditions on the TI surface, and the effectiveness of the TI-TB-FM junction for determining such voltage change upon FM magnetization reversal has been derived for different device dimensions. Such measurement can be used to study the spin-momentum helical locking of the TI surface states as well as for reading the FM magnetization direction in memory and logic devices based on TI-TB-FM heterostructures.

We report the direct observation of the fingerprint of the inverse Rashba-Edelstein effect (IREE) via investigations of the spin-to-charge conversion in Ta(Pt)/Cu and Cu/Ta(Pt) as a function of the heavy-metal thickness. The converted charge voltages have opposite signs for samples with reversed stacking orders in the ultrathin regime and have the same sign at higher thickness. The effect is demonstrated with two independent experimental approaches and supported by first-principles calculations. Our observations unambiguously demonstrate the existence of the IREE at heavy-metal/Cu interfaces and provide a framework for manipulating the spin-charge conversion via interface engineering.

A magnon spin-orbit coupling, induced by the dipole-dipole interaction, is derived in monoclinic-stacked bilayer honeycomb spin lattice with perpendicular magnetic anisotropy and antiferromagnetic interlayer coupling. Linear crossings are predicted in the magnon spectrum around the band minimum in $\mathrm{\Gamma }$ valley, as well as in the high-frequency range around the zone boundary. The linear crossings in $K$ and $K^{\prime }$ valleys, which connect the acoustic and optical bands, can be gapped when the intralayer dipole-dipole or Kitaev interactions exceed the interlayer dipole-dipole interaction, resulting in a phase transition from semimetal to insulator. Our results are useful for analyzing the magnon spin dynamics and transport properties in van der Waals antiferromagnets.

By using the gauge-invariant kinetic equation [Yang and Wu, Phys. Rev. B 98, 094507 (2018); Yang and Wu, Phys. Rev. B 100, 104513 (2019)], we analytically investigate the influence of the scattering on the optical properties of superconductors in the normal-skin-effect region. Both linear and second-order responses are studied under a multicycle terahertz pulse. In the linear regime, we reveal that the optical absorption ${\sigma }_{1s}\left(\omega \right)$, induced by the scattering, exhibits a crossover point at $\omega =2\left|\mathrm{\Delta }\right|$. Particularly, it is further shown that when $\omega <2\left|\mathrm{\Delta }\right|, {\sigma }_{1s}\left(\omega \right)$ from the scattering always exhibits a finite value even at low temperature, in contrast to the vanishing ${\sigma }_{1s}\left(\omega \right)$ in the anomalous-skin-effect region as the Mattis-Bardeen theory [Mattis and Bardeen, Phys. Rev. 111, 412 (1958)] revealed. In the second-order regime, responses of the Higgs mode during and after the optical pulse are studied. During the pulse, we show that the scattering causes a phase shift in the second-order response of the Higgs mode. Particularly, this phase shift exhibits a significant $\pi$ jump at $\omega =\left|\mathrm{\Delta }\right|$, which provides a very clear feature for the experimental detection. After the pulse, by studying the damping of the Higgs-mode excitation, we reveal a relaxation mechanism from the elastic scattering, which shows a monotonic enhancement with the increase of the impurity density.

Charge interference (Aharonov-Casher effect) in a long superconducting quantum phase slip wire is considered, and from this the “dual” Fraunhofer interference effect (dual to the critical current modulation of a short Josephson junction in an external magnetic field) is derived. The device that can be used to observe this effect is proposed. Furthermore, the impact of wire inhomogeneities, charge disorder, and noise on the phase slip amplitude is investigated. Although intrinsically protected against small fluctuations, the Aharonov-Casher interference resulting from jumps of random offset charges and quasiparticles can result in significant fluctuations of the measured current-voltage characteristics of the quantum phase slip wire, similar to the effects of Joule heating when averaged out over many fluctuations. Possible ways to identify and mitigate such disorder are discussed.

We report superconductivity below $T_{\mathrm{c}}=0.64$ K in the charge density wave (CDW) ordered material ${\mathrm{LaAuSb}}_2$, from measurements of the electrical resistivity, specific heat, and ac magnetic susceptibility. To investigate the interplay between superconductivity and CDW order in ${\mathrm{LaAuSb}}_2$, we measured the resistivity under pressures up to 2.0 GPa and constructed the temperature-pressure phase diagram. With the application of pressure, $T_{\mathrm{c}}$ increases gradually before exhibiting a sudden jump at around 0.64 GPa, while the CDW order is suppressed to lower temperatures before abruptly vanishing at the same pressure. We suggest that the jump of $T_{\mathrm{c}}$ may be due to the enhancement of the density of states with the closure of the CDW energy gap when CDW order is suppressed. On the other hand, the normalized upper critical field $H_{\mathrm{c}2}$ changes little with pressure, suggesting that orbital limiting is the dominant pair-breaking mechanism in ${\mathrm{LaAuSb}}_2$.