SEARCH · Searcharxiv

Results for “cond-mat.str-el”

Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 307 records · Page 17Linked to original sources

arXiv2010

Dielectric properties and spin-phonon coupling in multiferroic double perovskite Bi$_2$CoMnO$_6$

First-principles electronic structure calculations have been performed for the double perovskite Bi$_2$CoMnO$_6$ in its non-centrosymmetric polar state using generalized gradient approximation plus Hubbard U approach. We find that while Co is in a high spin state, Mn is in an intermediate spin state. The calculated dynamical charge tensors are anisotropic reflecting a low-symmetry structure of the compound. Magnetic structure dependent phonon frequencies indicate the presence of spin-phonon coupling. Using Berry phase method, we obtain a spontaneous electronic polarization of 5.88 $μC/cm^2$ which is close to the experimental value observed for a similar compound, Bi$_2$NiMnO$_6$.

cond-mat.mtrl-sci↗

arXiv2010

Ordered States and Nonlinear Large-Scale Excitations in a Plane Magnet with Spin s=1

We study ordered states and topological excitations in a quasi-two-dimensional magnet, modeled by a square lattice with spins $s {=} 1$ at all sites, and the Hamiltonian with biquadratic exchange interaction between nearest neighbor sites. We propose two effective Hamiltonians for description of large-scale excitations in the two-dimensional case. They describe excitations of the mean field in a nematic phase and a mixed ferromagnetic-nematic phase. It is shown that the effective Hamiltonians are minimized on configurations with fixed topological charge. These topological excitations can arise at low temperatures and cause a destruction of a long-range order in the two-dimensional system.

cond-mat.str-el↗

arXiv2010

Electrically stabilized magnetic vortex and antivortex states in magnetic dielectrics

The micromagnetic distribution in a dielectric nanoparticle is theoretically considered. It is shown that the existence of inhomogeneous magnetoelectric interaction in magnetic dielectrics provides the possibility to stabilize the vortex and antivortex state. The estimation of the critical voltage necessary for vortex/antivortex nucleation in bismuth ferrite and iron garnet nanoparticles yields a value of +/-150 V. This system can be considered as electrically switchable two state-logic magnetic element.

cond-mat.mtrl-sci↗

arXiv2010

Specific heat of Ba0.68K0.32Fe2As2: evidence for multiband strong-coupling superconductivity

The specific heat of high-purity $\rm Ba_{0.68}K_{0.32}Fe_2As_2$ single crystals with the highest reported superconducting $T_c$ = 38.5 K was studied. The electronic specific heat, $C_p$, below $T_c$ shows two gap features, with $Δ_1 \approx 11$ meV and $Δ_2 \approx 3.5$ meV obtained from an $α$-model analysis. The reduced gap value, $2Δ^{\rm max} / k_B T_c \approx 6.6$, the magnitude of the specific heat jump, $ΔC_p(T_c)/T_c$, and its slope below $T_c$ exhibit strong-coupling character. We also show that an Eliashberg model with two hole and two electron bands gives the correct values of $T_c$, the superconducting gaps, and the temperature dependence of the free-energy difference.

cond-mat.supr-con↗

arXiv2010

An Impurity Solver Using the Time-Dependent Variational Matrix Product State Approach

We use the time dependent variational matrix product state (tVMPS) approach to investigate the dynamical properties of the single impurity Anderson model (SIAM). Under the Jordan-Wigner transformation, the SIAM is reformulated into two spin-1/2 XY chains with local magnetic fields along the z-axis. The chains are connected by the longitudinal Ising coupling at the end points. The ground state of the model is searched variationally within the space spanned by the matrix product state (MPS). The temporal Green's functions are calculated both by the imaginary and real time evolutions, from which the spectral information can be extracted. The possibility of using the tVMPS approach as an impurity solver for the dynamical mean field theory is also addressed. Finite temperature density operator is obtained by the ancilla approach. The results are compared to those from the Lanczos and the Hirsch-Fye quantum Monte-Carlo methods.

cond-mat.str-el↗

arXiv2010

Superconductivity at 33 K in "111" single crystals at ambient pressure

We have successfully grown single crystalline NaFeAs with cobalt or phosphor doping. Bulk superconductivity occurs in NaFe$_{0.95}$Co$_{0.05}$As at 19 K while much higher transition temperature is observed in NaFeAs$_{0.8}$P$_{0.2}$, in which the Tc of 33 K is even higher than the highest value realized by applying pressure in NaFeAs and all other isoelectronic element substituted samples. We discuss these behaviours by comparison with 122 and 1111 systems. We hope that our findings will help improve our understanding of iron-based superconductivity.

cond-mat.supr-con↗

arXiv2012

Photo-induced high-temperature order-disorder phase transition in CaSnO3 perovskite revealed by Raman spectroscopy

Calcium stannate perovskite (CaSnO3) has been studied by Raman spectroscopy at two excitation wavelengths (514.5 nm and 632.8 nm). A new first-order order-disorder phase transition induces Raman frequency shifts and line width doubling at 121C on heating (94C on cooling), seen in experiments using the 514.5 nm line of an Ar+-ion laser. The transition is also seen when using a 623.8 nm He-Ne laser and by differential scanning calorimetry (DSC), but without strong order-disorder character, indicating that the phase transition is dependent on photo-excitation. High-temperature powder X-ray diffraction measurements provide thermal expansion coefficients of ax = 13.9 x 10-6 K-1, ay = 2.7 x 10-6 K-1, az = 14.3 x 10-6 K-1. The phase transition is postulated to be associated with photo-excited charged and conductive nanoscale ferroelectric order-disorder. As such, CaSnO3 could represent the first in a new class of optoelectronic materials with additional potential photocatalytic properties.

cond-mat.mtrl-sci↗

arXiv2010

Intermediate coupling model of cuprates: adding fluctuations to a weak coupling model of pseudogap and superconductuctivity competition

We demonstrate that many features ascribed to strong correlation effects in various spectroscopies of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations. The self energy is calculated over the full doping range from half filling to the overdoped system. In the normal state, the spectral function reveals four subbands: two widely split incoherent bands representing the remnant of the two Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses in the overdoped regime. The resulting coherent subbands closely resemble our earlier mean-field results. Here we present an overview of the combined results of our mean-field calculations and the newer extensions into the intermediate coupling regime.

cond-mat.supr-con↗

arXiv2013

Neutron scattering and muSR investigations of the spin liquid state with quenched disorder in LuCuGaO4

LuCuGaO4 has magnetic Cu2+ and diamagnetic Ga3+ ions distributed on a triangular bilayer and is suggested to undergo a spin-glass transition with T_g approximately 0.4 K. Using muSR and neutron scattering measurements, we show that at low temperature the spins form a short-range correlated state with spin fluctuations detectable over a wide range of timescales: at 0.05 K magnetic fluctuations can be detected in both the muSR time window and also extending beyond 7 meV in the inelastic neutron scattering response, indicating magnetic fluctuations spanning timescales between 1E-5 and 1E-10 seconds. The dynamical susceptibility scales according to the form χ''(ω)T^α, with α=1, throughout the measured temperature range (0.05 - 50 K). These effects are associated with quantum fluctuations and some degree of structural disorder in ostensibly quite different materials, including certain heavy fermion alloys, kagome spin liquids, quantum spin glasses, and valence bond glasses. We therefore suggest LuCuGaO4 is an interesting model compound for the further examination of disorder and quantum magnetism.

cond-mat.mtrl-sci↗

arXiv2013

Theoretical Reproduction of the Lineshapes of Nonlinear Conductance Observed in a Quantum Point Contact

The nonlinear conductance observed in a quantum point contact is theoretically reproduced for the entire range of applied bias. The single-impurity Anderson model with two reservoirs at different chemical potentials is studied for a sequential change of the gate voltage. The imbalance in the left and right Kondo coupling strength is introduced by the displacement of the Kondo impurity by the electric field produced in the constriction of quantum point contact. We reveal the origin of the side peaks and study the behavior of the height and width of the zero-bias anomaly.

cond-mat.mes-hall↗

arXiv2011

Non-local effects of edge excitations in the quantum Hall regime

We use a novel sample geometry to study non-local effects of edge excitations in the integer quantum Hall effect regime. We find that the condition of local equilibrium at the quantum Hall edge is affected by the diffusion of dynamically polarized nuclei. Our analysis indicates, that the nuclear diffusion is effectively one-dimensional in the present experiment.

cond-mat.mes-hall↗

arXiv2011

Competing s-wave and d-wave order in A15 superconductors

Competing pairing channels have been proposed in superfluid $^3$He, in the high temperature cuprate superconductors, and pnictide superconductors. While strong experimental support exists for this in superfluid $^3$He, the case for superconductors is weaker. Here, we show that $A15$ superconductors provide a compelling example for competing superconducting order. We present microscopic arguments that $A15$ superconductors have a competing d-wave order in addition to usual s-wave superconductivity and show that this d-wave order provides a unifying explanation for a series of diverse experimental results. These include: the emergence of a sharp mode in the superconducting state seen by Raman spectroscopy; the observed shrinking of the vortex core at anomalously low fields; and the large anisotropy measured in the upper critical field. We further show that multiple superconducting gaps can appear when these $A15$ superconductors are strained.

cond-mat.supr-con↗

arXiv2010

Classical Phase Transitions of Geometrically Constrained O($N$) Spin Systems

We study the phase transition between the high temperature algebraic liquid phase and the low temperature ordered phase in several different types of locally constrained O(N) spin systems, using a unified constrained Ginzburg-Landau formalism. The models we will study include: 1, O(N) spin-ice model with cubic symmetry; 2, O(N) spin-ice model with easy-plane and easy-axis anisotropy; 3, a novel O(N) "spin-plaquette" model, with a very different local constraint from the spin-ice. We calculate the renormalization group equations and critical exponents using a systematic ε= 4 - d expansion with constant N, stable fixed points are found for large enough N. In the end we will also study the situation with softened constraints, the defects of the constraints will destroy the algebraic phase and play an important role at all the transitions.

cond-mat.stat-mech↗

arXiv2010

Magnetic properties of a strongly correlated system on the Bethe lattice

We study the influence of an external magnetic field h on the phase diagram of a system of Fermi particles living on the sites of a Bethe lattice with coordination number z and interacting through on-site U and nearest-neighbor V interactions. This is a physical realization of the extended Hubbard model in the narrow-band limit. Our results establish that the magnetic field may dramatically affect the critical temperature below which a long-range charge ordered phase is observed, as well as the behavior of physical quantities, inducing, for instance, magnetization plateaus in the magnetization curves. Relevant thermodynamic quantities - such as the specific heat and the susceptibility - are also investigated at finite temperature by varying the on-site potential, the particle density and the magnetic field.

cond-mat.str-el↗

arXiv2010

Approximate diagonalization method for many-fermion Hamiltonians

The limits of direct unitary transformation of many-fermion Hamiltonians are explored. Practical application of such transformations requires that effective many-body interactions be discarded over the course of a calculation. The truncation of the Hamiltonian leads to finite errors and in some cases divergences. A new formalism is proposed to manage errors and avoid divergences. Removing all interactions from a many-fermion Hamiltonian reduces it to fermion number operators allowing for direct calculation of eigenvalues. If the same transformations are applied to the bare fermions, eigenfermions are produced whose Slater determinants form eigenstates. This enables a hierarchy of diagonalization methods of increasing accuracy as fewer interactions are discarded from the Hamiltonian.

cond-mat.str-el↗

arXiv2010

Comment on "Measurement of x-ray absorption spectra of overdoped high-temperature cuprate superconductors: Inapplicability of the single-band Hubbard model"

In a recent Letter, Peets, et al. measured the x-ray intensity at the oxygen K-edge in overdoped La$_{2-x}$Sr$_x$CuO$_{4\pmδ}$ (LSCO) and Tl$_2$Ba$_2$CuO$_{6+δ}$. They claimed that, unlike the underdoped samples of LSCO and YBa$_2$Cu$_3$O$_x$ in which the integrated intensity increases at least linearly with doping, it saturates abruptly for a hole count exceeding $x\approx 0.23$. They interpreted the saturation as a breakdown of the 1-band Hubbard model in the cuprates. However, we show that their results are in quantitative agreement with the 1-band Hubbard model.

cond-mat.str-el↗

arXiv2010

Three-magnon bound states in exactly rung-dimerized spin ladders

Three magnon bound states in all total spin sectors of general nonintegrable exactly rung-dimerized spin ladder are obtained by Bethe Ansatz. Basing on this result a dispersion law for $m$-magnon ($m>3$) bound states is conjectured. It is suggested that (depending on correlations between coupling constants) a behavior of the gas parameter (density of rung-triplets) on the boundary of the rung-dimerized phase may either be smooth or have a leap.

cond-mat.str-el↗

arXiv2010

Disorder from disorder and confinement in the quantum Ising model in the pyrochlore lattice

At zero temperature, the classical antiferromagnetic Ising model on the pyrochlore lattice is a spin disorder phase of the critical spin correlation. It is a deconfined phase in that the binding energy of the monopole-anti-monopole pair is independent of their distance of separation. We show that turning on a transverse magnetic field turns it into the cooperative paramagnet, and the spin correlation becomes exponential decay. Furthermore, it introduces the quantum confinement (of magnetic monopoles), where the binding energy of the pair is proportional to their distance of separation. This disorder state undergoes adiabatic transition to the paramagnetic state in the large field limit. The effective Hamiltonian (without magnetic monopoles) in small field is the Ising Hamiltonian plus ring exchange interaction.

cond-mat.str-el↗