Low-temperature Excitations Research Articles

Radiative thermalization in semiclassical simulations of light-matter interaction

Prediction of the equilibrium populations in quantum dynamics simulations of molecules exposed to black-body radiation has proved challenging for semiclassical treatments, with the usual Ehrenfest and Maxwell-Bloch methods exhibiting serious failures. In this context, we explore the behavior of a recently introduced semiclassical model of light-matter interaction derived from a dissipative Lagrangian [C. M. Bustamante, E. D. Gadea, A. Horsfield, T. N. Todorov, M. C. Gonz\'alez Lebrero, and D. A. Scherlis, Phys. Rev. Lett. 126, 087401 (2021)]. It is shown that this model reproduces the Boltzmann populations for two-level systems, predicting the black-body spectra in approximate agreement with Planck's distribution. In multilevel systems, small deviations from the expected occupations are seen beyond the first excited level. By averaging over fast oscillations, a rate equation is derived from the dissipative equation of motion that makes it possible to rationalize these deviations. Importantly, it enables us to conclude that this model will produce the correct equilibrium populations provided the occupations of the lowest levels remain close to unity, a condition satisfied at low temperature or small excitations.

Observation of three magnetic states in spinel MnFe2O4 single crystals

Due to their unique structure that hosts geometric frustration, spinel oxides with magnetic ions in both the tetrahedral and octahedral environments undergo multiple spin rearrangement transitions. From the magnetization and thermal properties measurements of MnFe2O4, we have found three magnetic states occurring at TFI-1 = 575 K, TFI-2 = 50 K, and Tx = 15 K. Detailed analyses of the specific heat and thermal conductivity reveal a significant contribution from the low-temperature magnetic excitations. Especially, there is a large negative magneto-thermal conductivity with a maximum magnitude near TFI-2, which becomes positive below Tx. The possible origin of each magnetic state is discussed.

Lifetimes of magnons in two-dimensional diluted ferromagnetic systems

Spin dynamics in low dimensional magnetic systems has been of fundamental importance for a long time and has currently received an impetus owing to the emerging field of nanoelectronics. Knowledge of the spin wave lifetimes, in particular, can be favorable for future potential applications. We investigate the low-temperature spin wave excitations in two-dimensional disordered ferromagnetic systems, with a particular focus on the long wavelength magnon lifetimes. A semi-analytical Green's functions based approach is used to determine the dynamical spectral functions, for different magnetic impurity concentrations, from which the intrinsic linewidth is extracted. We obtain an unambiguous $q^4$ scaling of the magnon linewidth which is ascribed to the disorder induced damping of the spin waves, thereby settling a longstanding unresolved issue on the wave-vector dependence. Our findings are also in good agreement with previous theoretical studies on Heisenberg ferromagnets. Additionally, we demonstrate the futility of using the low moments associated with the spectral densities to evaluate the magnon dispersions and lifetimes.

Long wavelength spin dynamics in diluted magnetic systems: Scaling of magnon lifetime

Abstract Spin wave excitations in disordered magnetic systems have been one of the most widely studied fields in condensed matter physics for several decades. However, a careful and extensive search reveals a longstanding controversy on one important aspect, which is the wave-vector dependence of the spin wave intrinsic linewidth. We theoretically investigate the low-temperature spin wave excitations in disordered (diluted) ferromagnetic systems with a particular focus on the linewidth behavior in the long wavelength limit ( q → 0 ). The linewidth is extracted from a proper finite size analysis of the dynamical spectral functions, taking into account the effects of disorder and spin fluctuations treated within self-consistent local RPA. We obtain an unambiguous q5 scaling of the intrinsic linewidth, which is attributed to the disorder induced damping of the spin waves. This is in agreement with some previous theoretical studies on the Heisenberg ferromagnets, although the exchange interactions were mostly restricted to nearest neighbors unlike in our case. We also demonstrate the difficulties in extracting the correct scaling of the linewidth as it is sensitive to the q values considered, and one can obtain an incorrect q-dependence if the q's are not sufficiently small. Finally, our findings are discussed in the light of prospective spintronics applications.

Heat Capacity Study of Relaxors

The thermodynamic properties of various relaxors of PMN, PMT, PZN, PST, PMN–PT, PZN–PT, and BTZ have been reviewed and the excess thermodynamic properties are discussed. In lead-containing complex perovskite, excess heat capacity owing to the formation of PNR and glass-like low-temperature excitations are clearly observed. Studies of solid solutions indicate the difficulty of forming PNR with rhombohedral symmetry in the ferroelectric phase and the possible growth of a polar region with tetragonal symmetry in the compounds. Thus, the lead-free Ba-based relaxor BTZ35 can be considered as the frozen state of a displacive-type phase transition without disorder. Keywords: BTZ, heat capacity, PMN, PMN–PT, PMT, polar nanoregion, PST, PZN, PZN–PT, relaxor.

Low-temperature spin excitations in frustrated ZnCr2O4probed by high-field thermal conductivity

The magnetoelastic excitations of spin frustrated ZnCr${}_{2}$O${}_{4}$ are studied by the magnetic field dependence of the thermal conductivity $k$ down to 50 mK. Above the first-order magnetostructural transition at ${T}_{N,S}$ \ensuremath{\approx} 12.5 K, spin fluctuations are strongly coupled to acoustic phonons, leading to a glasslike dependence of $k$($T$), up to ${\ensuremath{\Theta}}_{\mathrm{CW}}$. In the symmetry broken phase below ${T}_{N,S}$, $k$ shows a dominant magnetic contribution even at the lowest temperatures probed in this work. Application of a magnetic field above 2.5 T destabilizes the spin-bond structure, leading to a sudden increase and a nonconventional temperature dependence of the thermal conductivity. The possibility of the coexistence of gapped and gapless excitations in this magnetic phase is discussed.

Exact solution of the 1D Hubbard model in the atomic limit with inter-site magnetic coupling

In this paper we present for the first time the exact solution in the narrow-band limit of the 1D extended Hubbard model with nearest-neighbour spin-spin interactions described by an exchange constant J. An external magnetic field h is also taken into account. This result has been obtained in the framework of the Green's functions formalism, using the Composite Operator Method. By means of this theoretical background, we have studied some relevant features such as double occupancy, magnetization, spin-spin and charge-charge correlation functions and derived a phase diagram for both ferro (J>0) and anti-ferro (J<0) coupling in the limit of zero temperature. We also report a study on density of states, specific heat, charge and spin susceptibilities. In the limit of zero temperature, we show that the model exhibits a very rich phase diagram characterized by different magnetic orders and by the coexistence of charge and spin orderings at commensurate filling. Moreover, our analysis at finite temperature of density of states and response functions shows the presence of low-temperature charge and spin excitations near the phase boundaries.

Low-temperature excitations within the Bethe approximation

We propose the variational quantum cavity method to construct a minimal energy subspace of wavevectors that are used to obtain some upper bounds for the energy cost of the low-temperature excitations. Given a trial wavefunction we use the cavity method of statistical physics to estimate the Hamiltonian expectation and to find the optimal variational parameters in the subspace of wavevectors orthogonal to the lower-energy wavefunctions. To this end, we write the overlap between two wavefunctions within the Bethe approximation, which allows us to replace the global orthogonality constraint with some local constraints on the variational parameters. The method is applied to the transverse Ising model and different levels of approximations are compared with the exact numerical solutions for small systems.

Discrete antiferromagnetic spin-wave excitations in the giant ferric wheel Fe18

The low-temperature elementary spin excitations in the AFM molecular wheel Fe18 were studied experimentally by inelastic neutron scattering and theoretically by modern numerical methods, such as dynamical density matrix renormalization group or quantum Monte Carlo techniques, and analytical spin-wave theory calculations. Fe18 involves eighteen spin-5/2 Fe(III) ions with a Hilbert space dimension of 10^14, constituting a physical system that is situated in a region between microscopic and macroscopic. The combined experimental and theoretical approach allowed us to characterize and discuss the magnetic properties of Fe18 in great detail. It is demonstrated that physical concepts such as the rotational-band or L&E-band concepts developed for smaller rings are still applicable. In particular, the higher-lying low-temperature elementary spin excitations in Fe18 or AFM wheels in general are of discrete antiferromagnetic spin-wave character.

Neutron scattering experiments and simulations near the magnetic percolation threshold ofFexZn1−xF2

The low temperature excitations in the anisotropic antiferromagnetic Fe_{1-x} Zn_x F_2 for x=0.25 and 0.31, at and just above the magnetic percolation threshold concentration x_p=0.25, were measured using inelastic neutron scattering. The excitations were simulated for x=0.31 using a localized, classical excitation model, which accounts well for the energies and relative intensities of the excitations observed in the scattering experiments.

Spin Ice: Magnetic Excitations without Monopole Signatures Using Muon Spin Rotation

Theory predicts the low temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (μSR) experiment [S. T. Bramwell et al., Nature (London) 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation-the so-called second Wien effect. We demonstrate via a new series of μSR experiments in Dy(2)Ti(2)O(7) that such an effect is not observable in a TF μSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.

Calorimetric evidence for nodes in the overdoped Ba(Fe0.9Co0.1)2As2

We present low-temperature specific heat of the electron-doped Ba(Fe0.9Co0.1)2As2, which does not show any indication of an upturn down to 400 mK, the lowest measuring temperature. The lack of a Schottky-like feature at low temperatures or in magnetic fields up to 9 T enables us to identify enhanced low-temperature quasiparticle excitations and to study anisotropy in the linear term of the specific heat. Our results cannot be explained by a single or multiple isotropic superconducting gap, but are consistent with multi-gap superconductivity with nodes on at least one Fermi surface sheet.

Equations of two-fluid hydrodynamics in the Hubbard model

A set of equations is obtained using stationary and nonstationary superconductivity theories in terms of the Hubbard model. The equations near Tc and for a weak magnetic field have the general form of superconductivity equations with a strongly anisotropic Fermi surface. Calculations are performed using a kinetic equation for quasiparticles. The low-temperature collective excitations in a superconductor are studied. An explicit relationship for the temperature dependence of second sound is obtained.

Intrinsic peculiarities of real material realizations of a spin-1/2 kagomé lattice

Spin-1/2 magnets with kagomé geometry, being for years a generic object of theoretical investigations, have few real material realizations. Recently, a DFT-based microscopic model for two such materials, kapellasite Cu3Zn(OH)6Cl2 and haydeeite Cu3Mg(OH)6Cl2, was presented [Janson O, Richter J and Rosner H arxiv:0806.1592]. Here, we focus on the intrinsic properties of real spin-1/2 kagomé materials having influence on the magnetic ground state and the low-temperature excitations. We find that the values of exchange integrals are strongly dependent on O—H distance inside the hydroxyl groups, present in most spin-1/2 kagomé compounds up to date. Besides the original kagomé model, considering only the nearest neighbour exchange, we emphasize the crucial role of the exchange along the diagonals of the kagomé lattice.

Quantum critical fluctuations in cobaltate superconductor Nax(H3O)zCoO2 · yH2O

In cobaltate superconductor Nax(H3O)zCoO2·yH2O with bilayered hydrate (BLH) structure, Co nuclear magnetic resonance (NMR) measurement was performed to investigate the low-temperature magnetic excitations in magnetic field. The nuclear spin-lattice relaxation rate (1/T1) of Co was measured in the field range of 1.5–14T parallel to the CoO2 plane. In the normal state, 1/T1T follows the temperature dependence expressed as 1/T1T∝1/T-θ in all the field range. At the field lower than 10T, 1/T1T continues to increase down to Tc and decrease due to superconductivity below Tc. T1T=Const. behavior is not observed at all above Tc. When the field is higher than 10T, 1/T1T does not show clear decrease, but stop increasing below T∗. Using Tc and T∗ we determine, a H–T phase diagram is developed. The field dependence of Tc at low fields is as small as dTc/dH∼−0.06K/T. A novel phase characterized by a small kink of 1/T1T seems to be present at high-field region above 10T.

Unconventional spin-density wave in Bechgaard salt(TMTSF)2NO3

Among many Bechgaard salts, $(\mathrm{TMTSF}{)}_{2}{\mathrm{NO}}_{3}$, where TMTSF denotes tetramethyltetraselenafulvalene, exhibits very anomalous low-temperature properties. Unlike the case of the conventional spin-density wave (SDW), $(\mathrm{TMTSF}{)}_{2}{\mathrm{NO}}_{3}$ undergoes the SDW transition at ${T}_{C}\ensuremath{\approx}9.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and the low-temperature quasiparticle excitations are gapless. Also, it is known that $(\mathrm{TMTSF}{)}_{2}{\mathrm{NO}}_{3}$ does not exhibit superconductivity even under pressure, while a field-induced SDW is found in $(\mathrm{TMTSF}{)}_{2}{\mathrm{NO}}_{3}$ only for $P=8.5\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$ and $B&gt;20\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Here we show that both the angle-dependent magnetoresistance data and the nonlinear Hall resistance of $(\mathrm{TMTSF}{)}_{2}{\mathrm{NO}}_{3}$ at ambient pressure are interpreted satisfactorily in terms of an unconventional spin-density wave. Based on these facts, we propose a new phase diagram for Bechgaard salts.

E-band excitations in the magnetic Keplerate moleculeFe30

Low-temperature excitations in the magnetic Keplerate molecule ${\mathrm{Fe}}_{30}$ as calculated by linear spin-wave theory (SWT), modified linear SWT, and spin-level mean-field theory (SLMFT) are compared to recent inelastic neutron scattering results by Garlea et al. [Phys. Rev. B 73, 024414 (2006)]. SLMFT reproduces a part of the experimental spectrum rather well, but not all of it. SWTs yield a small fraction of the $E$-band excitations and hence are not capable of a complete description of the excitation spectrum.

High-Energy Spin Excitations in the Electron-Doped SuperconductorPr0.88LaCe0.12CuO4−δwithTc=21 K

We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of electron-doped superconducting Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$ ($T_c=21\pm 1$ K) over a wide energy range (4 meV$\le\hbar\omega\le 330$ meV). The effect of electron-doping is to cause a wave vector ($Q$) broadening in the low-energy ($\hbar\omega\le 80$ meV) commensurate spin fluctuations at ($\pi$,$\pi$) and to suppress the intensity of spin-wave-like excitations at high energies ($\hbar\omega\ge 100$ meV). This leads to a substantial redistribution in the spectrum of the local dynamical spin susceptibility $\chi^{\prime\prime}(\omega)$, and reveals a new energy scale similar to that of the lightly hole-doped YB$_2$Cu$_3$O$_{6.353}$ ($T_c=18$ K).

Reduction of two-dimensional dilute Ising spin glasses

The recently proposed reduction method is applied to the Edwards-Anderson model on bond-diluted square lattices. This allows, in combination with a graph-theoretical matching algorithm, to calculate numerically exact ground states of large systems. Low-temperature domain-wall excitations are studied to determine the stiffness exponent y_2. A value of y_2=-0.281(3) is found, consistent with previous results obtained on undiluted lattices. This comparison demonstrates the validity of the reduction method for bond-diluted spin systems and provides strong support for similar studies proclaiming accurate results for stiffness exponents in dimensions d=3,...,7.

Two-magnon excitations observed by neutron scattering in the two-dimensional spin-52Heisenberg antiferromagnetRb2MnF4

The low-temperature magnetic excitations of the two-dimensional spin-5/2 square-lattice Heisenberg antiferromagnet Rb2MnF4 have been probed using pulsed inelastic neutron scattering. In addition to dominant sharp peaks identified with one-magnon excitations, a relatively weak continuum scattering is also observed at higher energies. This is attributed to neutron scattering by pairs of magnons and the observed intensities are consistent with predictions of spin wave theory.