We report evidence for a 1.0-THz self-starting mirrorless acoustic phonon parametric oscillator (MAPPO) produced from acousto-optic phase-conjugate degenerate four-wave (D4WM) mixing in a THz laser-pumped silicon doping superlattice (DSL). The DSL was grown by molecular beam epitaxy on a (100) boron-doped silicon substrate. A superconducting NbTiN subwavelength grating was used to couple the THz laser radiation into the DSL. Superconducting granular aluminum bolometric detection, coupled with Si:B piezophonon spectroscopy, revealed excitation of THz coherent compressional and shear waves, along the $\langle 111\rangle$ direction only. The Bragg scattering condition for distributed feedback, and the energy conservation requirement for the D4WM process, were both verified. A THz MAPPO could provide a test bed for studies of nonclassical acoustic phonon fields.

Understanding the temperature-dependent behavior of phonons that affects the thermal transport properties is crucial for developing efficient thermoelectric materials. Previously, SnSe was reported to exhibit a record high thermoelectric performance above 900 K, which was debated in subsequent reports. Nonetheless, there is a growing consensus that anharmonicity is one of the contributing factors which yields ultralow thermal conductivity in SnSe. While there is experimental and theoretical evidence for anharmonicity in SnSe, its effect on the entire phonon band structure, which is responsible for the thermal transport, is investigated here. We performed a combined temperature-dependent polarized Raman spectroscopic and heat-capacity study on fully dense single-crystalline SnSe, which revealed that the anharmonicity is driven by soft optical modes in the $b\text{−}c$ plane. These modes exhibited strong high-temperature broadening, indicating ultrashort phonon lifetimes and high scattering rates. Analysis of the Raman peak frequencies and linewidths revealed phonon decay to be dominated by a three-phonon scattering process. Consistent with this observation, the analysis of our temperature-dependent heat-capacity data also revealed the presence of strong anharmonicity in SnSe. The anharmonic coefficients ${\alpha }_R$ and ${\alpha }_C$ calculated from the Raman and heat-capacity measurements, respectively, are in excellent agreement with each other. This study provides a deeper understanding of the role of phonon-phonon scattering and anharmonicity leading to outstanding thermal transport properties of SnSe.

The localized magnon modes of isolated $k\pi$ skyrmions on a field-polarized background are analyzed based on the Landau-Lifshitz-Gilbert equation within the terms of an atomistic classical spin model, with system parameters based on the Pd/Fe biatomic layer on Ir(111). For increasing skyrmion order $k$ a higher number of excitation modes are found, including modes with nodes in the radial eigenfunctions. It is shown that at low fields $2\pi$ and $3\pi$ skyrmions are destroyed via a burst instability connected to a breathing mode, while $1\pi$ skyrmions undergo an elliptic instability. At high fields all $k\pi$ skyrmions collapse due to the instability of a breathing mode. The effective damping parameters of the spin waves are calculated in the low Gilbert damping limit, and they are found to diverge in the case of the lowest-lying modes at the burst and collapse instabilities but not at the elliptic instability. It is shown that the breathing modes of $k\pi$ skyrmions may become overdamped at higher Gilbert damping values.

Recent advances in momentum-resolved electron energy-loss spectroscopy (MEELS) and resonant inelastic x-ray scattering (RIXS) now allow one to access the charge response function with unprecedented versatility and accuracy. This allows for the study of excitations, which were inaccessible recently, such as low-energy and finite momentum collective modes. The SU(2) theory of the cuprates is based on a composite order parameter with SU(2) symmetry fluctuating between superconductivity and charge order. The phase where it fluctuates is a candidate for the pseudogap phase of the cuprates. This theory has a signature, enabling its strict experimental test, which is the fluctuation between these two orders, corresponding to a charge 2 spin 0 mode at the charge ordering wave vector. Here, we derive the influence of this SU(2) collective mode on the charge susceptibility in both strong and weak coupling limits, and discuss its relation to MEELS, RIXS, and Raman experiments. We find two peaks in the charge susceptibility at finite energy, whose middle is the charge ordering wave vector, and discuss their evolution in the phase diagram.

We study a three-dimensional single-band repulsive Hubbard model at weak coupling. We establish the superconducting phase diagram in the parameter space of the chemical potential and the out-of-plane hopping strength. The model continuously connects the Hubbard model in two and three dimensions. We confirm previously established results in these limits, and identify a rich structure of competing order parameters in between. Specifically, we find five types of $p$- and $d$-wave orders. In several regions of the phase diagram, even when the Fermi surface is a corrugated cylinder, the ground state is a time-reversal-symmetry-breaking superconductor with nodes, i.e., a Weyl superconductor.

We analyze the crossover from Kondo to weak-link regime by means of a model of tunable bond impurities in the middle of a spin-1/2 XXZ Heisenberg chain. We study the Kondo screening cloud and estimate the Kondo length by combining perturbative renormalization group approach with the exact numerical calculation of the integrated real-space spin-spin correlation functions. We show that, when the spin impurity is symmetrically coupled to the two parts of the chain with realistic values of the Kondo coupling strengths and spin-parity symmetry is preserved, the Kondo length takes values within the reach of nowadays experimental technology in ultracold-atom setups. In the case of nonsymmetric Kondo couplings and/or spin parity broken by a nonzero magnetic field applied to the impurity, we discuss how Kondo screening redistributes among the chain as a function of the asymmetry in the couplings and map out the shrinking of the Kondo length when the magnetic field induces a crossover from Kondo impurity to weak-link physics.

Using migration enhanced molecular beam epitaxy, we synthesize single- and bilayer ${\mathrm{WSe}}_2$ on epitaxial graphene/SiC(0001) and investigate the electronic properties of their edges with different terminations using scanning tunneling microscopy/spectroscopy (STM/S). Density functional theory calculations predict edge states at both Se and W edges, whose position and spatial distribution depend on how the edges are terminated. STS reveals that whether edge states at the nominal W edge are detectable depends on the number and spatial distribution of additional Se, whereas edge states on the Se edge are always detectable. Our findings indicate that edge termination plays an important role on edge states, and modification of the edge is a viable means to tailor the electronic properties of ${\mathrm{WSe}}_2$ nanostructures.

We introduce the notion, and develop the theory of, local-noise spectroscopy (LNS)—a tool to study the properties of systems far from equilibrium by means of flux density correlations. As a test bed, we apply it to biased molecular junctions. This tool naturally extends those based on local fluxes, while providing complementary information on the system. As examples of the rich phenomenology that one can study with this approach, we show that LNS can be used to yield information on microscopic properties of bias-induced light emission in junctions, provide local resolution of intrasystem interactions, and employed as a nanothermometry tool. Although LNS may, at the moment, be difficult to realize experimentally, it can nonetheless be used as a powerful theoretical tool to infer a wide range of physical properties on a variety of systems of present interest.

Scanning tunneling microscopy (STM) has been utilized for imaging and manipulation of self-trapped holes on the surface of the wide band-gap semiconductor $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. A positively charged surface layer comprised of localized holes with ${10}^{13}\mathrm{c}{\mathrm{m}}^{-2}$ density has been observed for $n$-doped samples. We show that the surface layer can be populated by hole pumping from the STM tip. A transition between the glassy phase and ordered striped phase of self-trapped holes has also been observed. Our analysis indicates that the saturated two-dimensional density of self-trapped holes may be determined by balance of self-trapping and Coulomb repulsion energies.

In this paper, we predict the occurrence of topological nontrivial phase in hexagonal antiperovskite systems. By carefully investigating the evolution of band structure, we have studied the pressure induced topological phase transition in ${\mathrm{Ba}}_3\mathrm{BiP}$, ${\mathrm{Ba}}_3\mathrm{BiN}$, and ${\mathrm{Sr}}_3\mathrm{BiN}$ compounds using the hybrid functional calculations. The nontrivial topology has been verified by computing Dirac like surface dispersion and topological invariant ${\mathrm{Z}}_2$ index via parity analysis. The unconventional spin texture has been analyzed which guaranteed the absence of impurity induced backscattering on the boundary of the sample while respecting the time reversal symmetry. Our simulation confirms the chemical and mechanical stability of all three compounds. The present study introduces an important class of hexagonal antiperovskite compounds in the topological regime and are believed to capture ample attention from both the theoretical and experimental front.

The influence of Fermi level position and annealing ambient on the zinc vacancy $V_{\mathrm{Zn}}$ generation and Al diffusion is studied in monocrystalline zinc oxide (ZnO). From secondary-ion mass spectrometry and positron annihilation spectroscopy results, a quadratic dependence between the concentrations of $V_{\mathrm{Zn}}$ and Al is established, demonstrating the Fermi level dependence of the formation of the electrically compensating $-2$ charge state of $V_{\mathrm{Zn}}$ in conductive $n$-type ZnO crystals. In contrast, thermal treatment in the zinc-rich ambient is shown to efficiently reduce the $V_{\mathrm{Zn}}$ concentration and related complexes. Using a reaction-diffusion model, the diffusion characteristics of Al at different donor background concentrations are fully accounted for by mobile (${\mathrm{Al}}_{\mathrm{Zn}}{V}_{\mathrm{Zn}}{)}^{-}$ pairs. These pairs form via the migration and reaction of isolated $V_{\mathrm{Zn}}^{2-}$ with the essentially immobile ${\mathrm{Al}}_{\mathrm{Zn}}^{+}$. We obtain a migration barrier for the ${\left({\mathrm{Al}}_{\mathrm{Zn}}{V}_{\mathrm{Zn}}\right)}^{-}$ pair of $2.4\pm 0.2$ eV, in good agreement with theoretical predictions. In addition to strongly alter the shape of the Al diffusion profiles, increasing the donor background concentration also results in an enhanced effective Al diffusivity, attributed to a reduction in the ${\mathrm{V}}_{\mathrm{Zn}}^{2-}$ formation energy as the Fermi level position increases.

Linear and nonlinear magnetophotonic properties of periodic arrays of nickel nanodimers are governed by the interplay of the (local) optical response of individual nanoparticles and (nonlocal) diffraction phenomena. The redistribution of light intensity between diffracted beams when a diffraction order onsets or disappears is known as Wood's anomaly. Here, angular and magnetic-field-dependent near-infrared spectroscopic measurements, performed for different optical wavelengths and grating constants, discriminate between the linear and nonlinear excitation mechanisms of Wood's anomalies. In the nonlinear regime, evidenced by the magnetic second-harmonic generation, Wood's anomaly is characterized by an order-of-magnitude larger effect in intensity redistribution between the diffracted beams as compared to the linear case. The nonlinear Wood's anomaly manifests itself also in the nonlinear magnetic contrast highlighting the prospects of nonlinear magnetophotonics.

All-optical switching (AOS) of magnetization in ferri- and ferromagnetic thin films has in recent years attracted a strong interest since it allows magnetization reversal in the absence of applied magnetic field. Here we investigate AOS in ${[\mathrm{Co}/\mathrm{Pt}]}_N$ multilayers. The coercivity ($H_C$) of the multilayers was tuned either by varying the bilayer repetition number ($N$) or the sample temperature ($T$). During the AOS experiments, we first illuminated the multilayers by a sequence of femtosecond laser pulses with varying fluence, light polarization, and repetition rate. The optically affected area was then imaged with magneto-optical Kerr microscopy. Our results indicate that the optical pulses can trigger either AOS or initiate an all-optical domain formation (AODF). The laser fluence required for AOS scales linearly with $H_C$ and depends on a precise tuning of laser pulse fluence, repetition rate, and light polarization. Furthermore, the magnetic response of the samples at a varying ambient temperature (down to 50 K) and for different time intervals between subsequent laser pulses point to the crucial role of domain wall dynamics in optical control of magnetization in ferromagnetic multilayers.

We consider a superconductor in which the density of states at the Fermi level or the pairing interaction is driven periodically with a frequency larger than the superconducting gap in the collisionless regime. We show by numerical and analytical computations that a subset of quasiparticle excitations enter into resonance and perform synchronous Rabi oscillations leading to cyclic population inversion with a frequency that depends on the amplitude of the drive. As a consequence a different “Rabi-Higgs” mode emerges. Turning off the drive at different times and modulating the strength allows access to all known dynamical phases of the order parameter: persistent oscillations, oscillations with damping and overdamped dynamics. We discuss physical realizations of the drive and methods to detect the dynamics.

It is well known that conventional harmonic lattice dynamics cannot be applied to dynamically unstable crystals at 0 K, such as the high-temperature body centered cubic (bcc) phase of crystalline Zr. Predicting phonon spectra at finite temperature requires the calculation of force constants to the third, fourth, and even higher orders; however, it remains challenging to determine at which order the Taylor expansion of the potential-energy surface for different materials should be cut off. Molecular dynamics, on the other hand, intrinsically includes arbitrary orders of phonon anharmonicity; however, its accuracy is severely limited by the empirical potential field used. Recently, machine learning algorithms have emerged as promising tools to build accurate potentials for molecular dynamics simulation. In this paper, we approach the problem of predicting phonon dispersion at finite temperature by performing molecular dynamics simulations with machine learning driven potential fields. We developed Gaussian approximation potential models for both the hexagonal close packed (hcp) phase and the bcc phase of zirconium crystals. The developed potential field is first validated with static properties including energy-volume relationship, elastic constants, and phonon dispersions at 0 K. Molecular dynamics simulations are then performed to stochastically sample the potential-energy surface and to calculate the phonon dispersion at elevated temperatures. The phonon renormalization in bcc Zr is successfully captured by the molecular dynamics simulation at 1188 K. The instability of the bcc structure is found to originate from the double-well shape of the potential-energy surface where the local maximum is located in an unstable equilibrium position. The stabilization of the bcc phase at high temperature is due to the dynamical average of the low-symmetry minima of the double well due to atomic vibrations.

We study analytically an organic hydrogen-bonded material, ${\mathrm{H}}_2\mathrm{SQ}$, whose dynamics is governed by (i) a four-spin plaquette interaction $J_0$, (ii) Ising-like intramolecular interaction $J_1$, (iii) a dipole-dipole interaction $J_2$, and (iv) an external transverse Zeeman field $K$; the symbols denote the strength of each interaction such that $J_0>J_1>J_2$, while $K$ can be taken arbitrary. We chart out the classical phase diagram going to an ordered state from a disordered, degenerate ground state. Our spin-wave analysis above the classical ground states shows that there is no order-disorder transition associated with this. We argue that this happens due to the existence of a macroscopic number of symmetry operators. For finite $J_2$, the system shows a paraelectric to ferroelectric transition giving four global degenerate ferroelectric orders for which the high-field spin-wave spectrum is found to be gapless and linear near the phase transition point. In low-field, the spin-wave spectrum is gapped and quadratic. We perform a perturbative continuous unitary transformation (PCUT) to improve the results of spin-wave analysis and obtain the ground-state energy, the one-particle dispersion, and the gap and phase boundary in various parameter regimes. In the end, we briefly discuss the mapping of this model to a toric code model (TCM) for large $J_0$ and small field values $K$.