# Physics - Other Publications (50)

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## Physics - Other Publications

Metal atoms and small clusters introduced into superfluid helium (He II) concentrate there in quantized vortices to form (by further coagulation) the thin nanowires. The nanowires' thickness and structure are well predicted by a double-staged mechanism. On the first stage the coagulation of cold particles in the vortex cores leads to melting of their fusion product, which acquires a spherical shape due to surface tension. Read More

Of late, there has been intense interest in the realization of topological phases in very experimentally accessible classical systems like mechanical metamaterials and photonic crystals. Subjecting them to a time-dependent driving protocol further expands the diversity of possible topological behavior. We introduce a very realistic experimental proposal for a mechanical Floquet Chern insulator using a lattice of masses equipped with time-varying electromagnets. Read More

The synchronized magnetization dynamics in ferromagnets on a nonmagnetic heavy metal caused by the spin Hall effect is investigated theoretically. The direct and inverse spin Hall effects near the ferromagnetic/nonmagnetic interface generates longitudinal and transverse electric currents. The phenomenon is known as the spin Hall magnetoresistance effect, whose magnitude depends on the magnetization direction in the ferromagnet due to the spin transfer effect. Read More

We formulate part I of a rigorous theory of ground states for classical, finite, Heisenberg spin systems. The main result is that all ground states can be constructed from the eigenvectors of a real, symmetric matrix with entries comprising the coupling constants of the spin system as well as certain Lagrange parameters. The eigenvectors correspond to the unique maximum of the minimal eigenvalue considered as a function of the Lagrange parameters. Read More

The focus of this paper is on the use of transfer functions to comprehend the formation of band gaps in locally resonant acoustic metamaterials. Identifying a recursive approach for any number of serially arranged locally resonant mass in mass cells, a closed form expression for the transfer function is derived. Analysis of the end-to-end transfer function helps identify the fundamental mechanism for the band gap formation in a finite metamaterial. Read More

We discuss the type of the general macroscopic parity-violating effects, when there is the current along the vortex, which is concentrated in the vortex core. We consider vortices in superfluids, which contain the Weyl points. In the vortex core the positions of the Weyl points form the skyrmion structure. Read More

We demonstrate vector magnetometry with an ensemble of nitrogen-vacancy (NV) centers in diamond without the need for an external bias field. The anisotropy of the electric dipole moments of the NV center reduces the ambiguity of the optically detected magnetic resonances upon polarized visible excitation. Further lifting of the remaining ambiguities is achieved via application of an appropriately linearly polarized microwave field, which enables suppression of spin-state transitions of a certain crystallographic NV orientation. Read More

We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk SmB6 , which produce local magnetic fields of about ~1. Read More

Controllable and yet robust confinement of light is vital for many applications. Flat bands induced by symmetries are proposed for robust localization of light. Here we show that localization arises at the exceptional point, where by breaking the parity-time symmetry and without entering to the broken phase a flat band is generated. Read More

We analyze many body localization (MBL) in an interacting quasi-periodic system in one-dimension. We explore effects of nearest-neighbour repulsion on a system of spin-less fermions in which below a threshold value of quasi-periodic potential $h < h_c$, the system has single particle mobility edge at $\pm E_c$ while for $ h > h_c$ all the single particle states are localized. We demonstrate based on our numerical calculation of participation ratio in the Fock space and Shannon entropy, that both for $h < h_c$ and $h > h_c$, the interacting system can have many-body mobility edge. Read More

We report a {51}^V nuclear magnetic resonance investigation of the frustrated spin-1/2 chain compound LiCuVO_{4}, performed in pulsed magnetic fields and focused on high-field phases up to 55 T. For the crystal orientations H // c and H // b we find a narrow field region just below the magnetic saturation where the local magnetization remains uniform and homogeneous, while its value is field dependent. This behavior is the first microscopic signature of the spin-nematic state, breaking spin-rotation symmetry without generating any transverse dipolar order, and is consistent with theoretical predictions for the LiCuVO_{4} compound Read More

The symmetry properties of the dynamical matrix are well described in multiple classic textbooks. This short paper revisits the issue to demonstrate alternative form of dynamical matrix which explicitly shows its symmetry and reality in common cases. Read More

The theory of "Adiabatic Fountain Resonance" with superfluid $^4$He is clarified. In this geometry a film region between two silicon wafers bonded at their outer edge opens up to a central region with a free surface. We find that the resonance in this system is not a Helmholtz resonance as claimed by Gasparini and co-workers, but in fact is a fourth sound resonance. Read More

We demonstrate that a circularly polarized ultrashort laser pulse can induce a significant transient chirality in an achiral transparent dielectric. For moderate laser intensities, this is a $\chi^{(3)}$ effect that vanishes under the assumption of an instantaneous nonlinear polarization response. For near-infrared laser fields as strong as 1 V/{\AA}, nonperturbative electron dynamics become important. Read More

Displacements of atoms and molecules away from lattice sites in helium and parahydrogen solids at low temperature have been studied by means of Quantum Monte Carlo simulations. In the bcc phases of He-3 and He-4, atomic displacements are largely quantum-mechanical in character, even at melting. The computed Lindemann ratio at melting is found to be in good agreement with experimental results for He-4. Read More

Using Monte Carlo simulations, we study thermal and critical properties of two systems, in which domain walls and so-called $Z_2$-vortices as topological defects are presented. The main model is a lattice version of the $O(3)$ principal chiral model. We find a first order transition and give qualitative arguments that the first order is induced by topological defects. Read More

We here elaborate on a quantitative argument to support the validity of the Collatz conjecture, also known as the (3x + 1) or Syracuse conjecture. The analysis is structured as follows. First, three distinct fixed points are found for the third iterate of the Collatz map, which hence organise in a period 3 orbit of the original map. Read More

**Authors:**V. Lauer, M. Schneider, T. Meyer, C. Dubs, P. Pirro, T. Brächer, F. Heussner, B. Lägel, V. I. Vasyuchka, A. A. Serga, B. Hillebrands, A. V. Chumak

We demonstrate experimentally that a thermal gradient in a nanostructured YIG(66 nm)/Pt(7 nm) bilayer can trigger coherent spin dynamics in the YIG layer due to a spin Seebeck effect (SSE) induced spin-transfer torque (STT). The experiment is performed on a 4 {\mu}m long and 475 nm wide YIG/Pt nanowire at room temperature. Magnetization precession is excited by applying 50 ns long dc pulses to the structure, and detected using Brillouin light scattering (BLS) spectroscopy. Read More

The role played by zero-point contribution, also called quantum noise or vacuum fluctuations, in the quantum expression of the fluctuation-dissipation theorem (FDT) is a long-standing open problem widely discussed by the physicist community since its announcement by Callen and Welton pioneer paper of 1951 [1]. From one hand, it has the drawbacks of: (i) the expectation value of its energy is infinite, (ii) it produces an ultraviolet catastrophe of the noise power spectral density and, (iii) it lacks of an experimental validation under thermal equilibrium conditions. From another hand, by imposing appropriate boundary conditions and eliminating divergences by regulation techniques, vacuum fluctuations are the source of an attractive force between opposite conducting plates, firstly predicted by Casimir in 1948 [2] and later validated experimentally with increasing accuracy. Read More

The possibility is considered that, after nuclear recoil caused by elastic scattering with Light Dark Matter, atoms in superfluid He4 may create quantized vortex rings, which can thereafter be detected. The concept for sort of telescope for LDM is presented, which would be sensitive to its motion through the Galactic LDM wind. A demonstrator is proposed, which makes use of thermal neutrons. Read More

A large Anomalous Hall Effect was recently observed in Mn$_{3}$X (X= Sn, Ge), noncollinear antiferrmagnets with a triangular network of spins. Here, we present a study of thermal and thermoelectric response in Mn$_{3}$Sn. In absence of magnetic field, Berry curvature generates off-diagonal thermal(Righi-Leduc) and thermoelectric(Nernst) signals, which are easily detectable at room temperature and can be inverted with a small magnetic field. Read More

We show how one can easily construct simple, non-empirical and parameter-free exchange functionals by extending the well-know local-density approximation (LDA) to finite uniform electron gases. These new generalized local-density approximation (GLDA) functionals use only two quantities: the electron density $\rho$ and the curvature of the Fermi hole $\alpha$. These alternative "rung 2" functionals can be easily coupled with generalized-gradient approximation (GGA) functionals to form a new family of "rung 3" meta-GGA (MGGA) functionals that we have named factorizable MGGAs (FMGGAs). Read More

We combine quasiparticle interference simulation (theory) and atomic resolution scanning tunneling spectro-microscopy (experiment) to visualize the interference patterns on a type-II Weyl semimetal Mo$_{x}$W$_{1-x}$Te$_2$ for the first time. Our simulation based on first-principles band topology theoretically reveals the surface electron scattering behavior. We identify the topological Fermi arc states and reveal the scattering properties of the surface states in Mo$_{0. Read More

Numerical simulations in a tight-binding model have shown that an intersection of topologically protected one-dimensional chiral channels can function as a beam splitter for non-interacting fermions on a two-dimensional lattice \cite{Qiao2011,Qiao2014}. Here we confirm this result analytically in the corresponding continuum $\mathbf{k}\cdot\mathbf{p}$ model, by solving the associated two-dimensional Dirac equation, in the presence of a `checkerboard' potential that provides a right-angled intersection between two zero-line modes. The method by which we obtain our analytical solutions is systematic and potentially generalizable to similar problems involving intersections of one-dimensional systems. Read More

Calculating the degree of non-Markovianity of a dissipative process is a difficult task, even for the dynamics of a single qubit, given the complex maximization problem. In this work, focusing on the entanglement-based quantifier of non-Markovianity, we present an analytical solution for such an optimization problem. We then propose a computable non-Markovianity measure based on generalized robustness of entanglement, an entanglement measure that can be readily calculated by a semidefinite programming method. Read More

Closed expression for the Green's function of the stationary two-dimensional Schrodinger equation for an electron in group-VI dichalcogenides in the presence of a magnetic field is obtained in terms of the Whittaker functions. The resulting Green's function operator is a~$8 \times 8$ matrix consisting of four block-diagonal~$2\times 2$ matrices, each of them characterized by different values of valley index and electron spin. The obtained results are used to calculate local density of states induced by a neutral delta-like impurity in the presence of a magnetic field within the lowest Born approximation. Read More

We study the lateral Casimir force experienced by a particle that rotates near a planar surface. The origin of this force lies in the symmetry breaking induced by the particle rotation in the vacuum and thermal fluctuations of its dipole moment, and, therefore, in contrast to lateral Casimir forces previously described in the literature for corrugated surfaces, it exists despite the translational invariance of the planar surface. Working within the framework of fluctuational electrodynamics, we derive analytical expressions for the lateral force and analyze its dependence on the geometrical and material properties of the system. Read More

Asymptotic state of an open quantum system can undergo qualitative changes upon small variation of system parameters. We demonstrate it that such 'quantum bifurcations' can be appropriately defined and made visible as changes in the structure of the asymptotic density matrix. By using an $N$-boson open quantum dimer, we present quantum diagrams for the pitchfork and saddle-node bifurcations in the stationary case and visualize a period-doubling transition to chaos for the periodically modulated dimer. Read More

Locally resonant metamaterials are characterized by bandgaps at wavelengths that are much larger than the lattice size, enabling low-frequency vibration attenuation. Typically, bandgap analyses and predictions rely on the assumption of traveling waves in an infinite medium, and do not take advantage of modal representations typically used for the analysis of the dynamic behavior of finite structures. Recently, we developed a method for understanding the locally resonant bandgap in uniform finite metamaterial beams using modal analysis. Read More

In circuit quantum electrodynamics, an artificial "circuit atom" can couple to a quantized microwave radiation much stronger than its real atomic counterpart. The celebrated quantum Rabi model describes the simplest interaction of a two-level system with a single-mode boson field. When the coupling is arbitrary large, the bare multilevel structure of a realistic circuit atom cannot be ignored even if the circuit is strongly anharmonic. Read More

We give an introduction to the theory of multi-partite entanglement. We begin by describing the "coordinate system" of the field: Are we dealing with pure or mixed states, with single or multiple copies, what notion of "locality" is being used, do we aim to classify states according to their "type of entanglement" or to quantify it? Building on the general theory of multi-partite entanglement - to the extent that it has been achieved - we turn to explaining important classes of multi-partite entangled states, including matrix product states, stabilizer and graph states, bosonic and fermionic Gaussian states, addressing applications in condensed matter theory. We end with a brief discussion of various applications that rely on multi-partite entangled states: quantum networks, measurement-based quantum computing, non-locality, and quantum metrology. Read More

The symmetry of single-molecule magnets (SMMs) dictates their spin quantum dynamics, influencing how such systems relax via quantum tunneling of magnetization (QTM). By reducing a system's symmetry, through the application of a magnetic field or uniaxial pressure, these dynamics can be modified. We report measurements of the magnetization dynamics of a crystalline sample of the high-symmetry [Mn12O12(O2CCH3)16(CH3OH)4]CH3OH SMM as a function of uniaxial pressure applied either parallel or perpendicular to the sample's "easy" magnetization axis. Read More

We characterize the influence that external noise, with both spatial and temporal correlations, has on the scale dependence of the reaction parameters of a cubic autocatalytic reaction diffusion (CARD) system. Interpreting the CARD model as a primitive reaction scheme for a living system, the results indicate that power-law correlations in environmental fluctuations can either decrease or increase the rates of nutrient decay and the rate of autocatalysis (replication) on small spatial and temporal scales. Read More

Computer simulations show that liquids of molecules with harmonic intramolecular bonds may have "pseudoisomorphic" lines of approximately invariant dynamics in the thermodynamic phase diagram. We demonstrate that these lines can be identified by requiring scale invariance of the inherent-structure reduced-unit low-frequency vibrational spectrum evaluated for a single equilibrium configuration. This rationalizes why excess-entropy scaling, density scaling, and isochronal superposition apply for many liquids with internal degrees of freedom. Read More

Two schemes are presented that mitigate the effect of errors and decoherence in short depth quantum circuits. The size of the circuits for which these techniques can be applied is limited by the rate at which the errors in the computation are introduced. Near term applications of early quantum devices, such as quantum simulations, rely on accurate estimates of expectation values to become relevant. Read More

Synthesis and extensive structural, pyroelectric, magnetic, dielectric and magneto-electric characterizations are reported for polycrystalline Co4Nb2O9 towards unraveling the multiferroic state especially in reference to the magnetic spin flop transition. Magnetic measurements confirm the Co4Nb2O9 becomes antiferromagnetic (AFM) at around 28 K but no clear evidence for spin-flop effect was found. Associated with the magnetic phase transition, a sharp peak in pyroelectric current indicates the appearance of the strong magneto-electric coupling below Neel temperature (TN) with a large coupling constant upto 17. Read More

Nuclear spin relaxation is studied in n-GaAs thick layers and microcavity samples with different electron densities. We reveal that both in metallic samples where electrons are free and mobile, and in insulating samples, where electrons are localized, nuclear spin relaxation is strongly enhanced at low magnetic field. The origin of this effect could reside in the quadrupole interaction between nuclei and fluctuating electron charges, that has been proposed to drive nuclear spin dynamics at low magnetic fields in the insulating samples. Read More

We propose a scheme for engineering compressed spatial states in a two-dimensional parabolic potential with a spin-orbit coupling by selective spin measurements. This sequence of measurements results in a coordinate-dependent density matrix with probability maxima achieved at a set of lines or at a two dimensional lattice. The resultant probability density depends on the spin-orbit coupling and the potential parameters and allows one to obtain a broad class of localized pure states on demand. Read More

We present a general method for constructing effective field theories for non-relativistic superfluids, generalizing the previous approaches of Greiter, Witten, and Wilczek, and Son and Wingate to the case of several superfluids in solution. We investigate transport in mixtures with broken parity and find a parity odd "Hall drag" in the presence of independent motion as well as a pinning of mass, charge, and energy to sites of nonzero relative velocity. Both effects have a simple geometric interpretation in terms of the signed volumes and directed areas of various sub-complexes of a "velocity polyhedron": the convex hull formed by the endpoints of the velocity vectors of a superfluid mixture. Read More

This review is based on lectures given by M. J. Duff summarising the far reaching contributions of Ettore Majorana to fundamental physics, with special focus on Majorana fermions in all their guises. Read More

We consider the Jastrow pair-product wavefunction for the strongly correlated Bose systems, in our case liquid helium-4. An ansatz is proposed for the pair factors which consists of a numeric solution to a modified and parametrized pair scattering equation. We consider a number of such simple one-variable parametrizations. Read More

In order to simplify the theoretical description of spasers, a gain medium is commonly represented by a two-level system. A realistic model, however, should have four levels. By using the Lindblad equations we develop a description of such a system and show that depending on ratios of the Rabi frequency and the rate of relaxation of the polarization, a four-level system may be reduced to one of two effective two-level systems that reproduce the key properties of a four-level system. Read More

The correlation function of radiation from a high-quality semiconductor microcavity at the resonant laser excitation demonstrates oscillations with surprisingly long-period and damping times of a nanosecond range. It was shown that the oscillations are not attributed to weak Rabi interaction between long-lived exciton states and intracavity electromagnetic field. The study of a response with high spectral resolution had revealed that the oscillations arise if a spectral position as well as a period of longitudinal laser modes is similar to modulation components of a microcavity transmission spectrum. Read More

Two-dimensional Weyl superconductor is the most elusive member of a group of materials with Weyl fermions as low-energy excitations. Here, we propose to realize this state in a heterostructure consisting of thin films of half-metal and spin-singlet superconductor. In particular, for the $d$-wave case, a very robust two-dimensional Weyl superconductor (dWSC) is realized independent of the orientation of the spontaneous magnetization of the half-metal. Read More

Many realizations of solid-state qubits involve couplings to leakage states lying outside the computational subspace, posing a threat to high-fidelity quantum gate operations. Mitigating leakage errors is especially challenging when the coupling strength is unknown, e.g. Read More

A continuum limit treatment of planar spin chains with arbitrary S is presented. The difference between integer and half-integer spins is emphasised. While isotropic half-integer spin chains are gapless,and have power-law decay of correlations at T = 0 with exponent eta = 1, integer spin systems have a singlet ground state with a gap for S=1 excitations and exponential decay of correlations. Read More

Non-locality is one of the most striking signatures of the topological nature of Weyl semimetals. We propose to probe the non-locality in these materials via a measurement of a magnetic field dependent Coulomb drag between two sheets of graphene which are separated by a three-dimensional slab of Weyl semimetal. We predict a new mechanism of Coulomb drag, based on cyclotron orbits that are split between opposite surfaces of the semi-metal. Read More

The interest in the properties of edge states in Chern insulators and in $\mathbb{Z}_2$ topological insulator has increased rapidly in recent years. We present calculations on how to influence the transport properties of chiral and helical edge states by modifications of the edges in the Haldane and in the Kane-Mele model. The Fermi velocity of the chiral edge states becomes direction-dependent as does the spin-dependent Fermi velocity of the helical edge states. Read More

We report a Raman study of the effect of temperature on the self-energies of optical phonons in a number of transition metals with hexagonal-close-packed structure. Anisotropic softening of phonon energies and narrowing of phonon linewidths with increasing temperature are observed. These effects are reproduced in the calculations of phonon spectral functions based on \textit{ab initio} electronic structures and with carrier scattering by phonons taken into account. Read More

We study the role played by noise on the bounded state of a two-particle QW with interaction, as introduced in [1]. The bounded ("molecular") state can be effectively described as a one-particle QW in 1D, with a coin operator which depends on the extra phase that is acquired by the interaction at each time step. The noise is introduced by a random change in the value of the phase during the evolution, from a constant probability distribution within a given interval. Read More