Deceptive signals of phase transitions in small magnetic clusters

We present an analysis of the thermodynamic properties of small transition-metal clusters and show how the commonly used indicators of phase transitions such as peaks in the specific heat or magnetic susceptibility can lead to deceptive interpretations of the underlying physics. The analysis of the distribution of zeros of the canonical partition function in the whole complex temperature plane reveals the nature of the transition. We show that signals in the magnetic susceptibility at positive temperatures have their origin at zeros lying at negative temperatures.     

具有非磁性缺陷的二维自旋玻璃模型

本文提出具有非磁性缺陷(n_m^1/2l<<1)的二维RKKY自旋玻璃模型,分别给出不同温区比热和磁化率的表达式.同时应用渗流理论分析转变温区自旋集团的行为,说明比热膝盖伏峰值和磁化率尖拐状峰值的实验事实. 关键词：     

Coulomb Crystals with Isotopic Impurities

Vibration modes and thermodynamic properties of a body‐centered cubic (bcc) Coulomb crystal with a small admixture of substitutional isotopic impurities are studied analytically applying the perturbation theory of disordered crystal spectra and the Lifshitz‐Krein trace formula. We calculate the density of phonon states of the perfect bcc Coulomb crystal and use it to compute the heat capacity of the crystal with impurities in a wide range of temperatures. It is shown that the ratio of an impurity contribution to the crystal heat capacity over the perfect crystal specific heat tends to a constant in the low‐temperature quantum regime and decays as T^–2 in the classic regime of high temperatures. It is also shown that even a small concentration of heavy impurities amplifies significantly the total crystal heat capacity. The results are compared with those obtained using the more conventional linear mixing theory. It is demonstrated that both methods give similar results at all tempera‐tures when the impurity mass is not too different from that of the base ions but a strong discrepancy is observed at low and intermediate temperatures when impurities are noticeably lighter or heavier. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nearly magnetic antiparallel pairs in dilute alloys

When a dilute alloy exhibits exchange enhancement of the isolated impurity susceptibility, it is shown that pairs of impurities may exhibit a low lying anti-parallel fluctuation mode such that the pair susceptibility in a uniform field is less enhanced than that of the isolated impurity, while the specific heat may be strongly enhanced. N.M.R. properties of such a pair should exhibit a small shift in frequency and a fast nuclear spin lattice relaxation time, leading to possibly large deviations from the Korringa relation.     

Ground states and thermal states of the random field Ising model

The random field Ising model is studied numerically at both zero and positive temperature. Ground states are mapped out in a region of random field and external field strength. Thermal states and thermodynamic properties are obtained for all temperatures using the Wang-Landau algorithm. The specific heat and susceptibility typically display sharp peaks in the critical region for large systems and strong disorder. These sharp peaks result from large domains flipping. For a given realization of disorder, ground states and thermal states near the critical line are found to be strongly correlated--a concrete manifestation of the zero temperature fixed point scenario.     

Properties of magnetically dilute alloys

Recently it has been shown by analysing Mössbauer spectra of dilute alloys of iron in gold that the anisotropic part of the exchange interaction between iron near neighbours determines the direction of magnetisation of such iron moments at low temperatures. Estimates of the important interactions between moments, which are here identified as the isotropic and anisotropic parts of the near neighbour exchange, and the isotropic exchange between further neighbours via the conduction electrons, have been obtained from available experimental data. The near neighbour exchange causes spins within random clusters linked as near neighbours to align parallel (the isotropic part) and to prefer certain directions in the cluster (the anisotropic part). The further neighbour exchange is responsible for the cooperative magnetic transition shown in Mössbauer and small field susceptibility results. The final model leads to qualitative interpretations of the temperature and field dependence of the susceptibility, and is able to reproduce the hysteresis loops observed at low temperatures for samples cooled in different fields. The effects of the interactions on the entropy and specific heats are discussed, and it is shown that the linear specific heat contributions at low temperatures occur because of the anisotropic interactions.     

Investigation of the phase transitions in a model of metal magnetic Fe/V superlattice

A model of iron/vanadium superlattice has been proposed to study the phase transition in multilayers. The temperature dependences of the magnetization, susceptibility, and specific heat have been obtained. Two specific heat peaks are revealed at small ratios between the interlayer and intralayer exchanges.     

Observation of a Kondo effect for a dilute alloy containing Sm impurities

Measurements of the depressions of the superconducting transition temperature T_c with Sm impurity concentration and the specific heat jump at T_c as a function of T_c, and the temperature dependences of the normal state specific heat and magnetic susceptibility are reported for the matrix impurity system (LaSm)Sn₃. The results constitute the first definitive evidence of a Kondo effect for a dilute alloy containing Sm impurities.     

Tricritical phenomena at the gamma-->alpha transition in Ce0.9-xLaxTh0.1 alloys

The gamma-->alpha isostructural transition in the Ce0.9-xLaxTh0.1 system is measured as a function of La alloying using specific heat, magnetic susceptibility, resistivity, thermal expansivity or striction measurements. A line of discontinuous transitions, as indicated by the change in volume, decreases exponentially from 118 K to close to 0 K with increasing La doping, and the transition changes from being first-order to continuous at a critical concentration, x(c) approximately 0.14. At the tricritical point, the coefficient of the linear T term in the specific heat gamma and the magnetic susceptibility increase rapidly near x(c) and approach large values at x=0.35 signifying that a heavy Fermi-liquid state evolves at large doping. The Wilson ratio reaches a value above 2 for a narrow range of concentrations near x(c), where the specific heat and susceptibility vary most rapidly with the doping concentration.     

Dynamics of Random One-Dimensional ±J Systems

Finite chains of a two-state random Potts spin model with periodic boundary conditions are studied within Glauber dynamics. The spin exchange is assumed random with frustration between ferro and antiferromagnetic values (±J). Time-dependent fluctuations are induced by periodic temperature oscillations. Master type differential equations for spin correlation functions are solved within linear response theory. The spectrum of relaxation times are calculated at different temperatures. The ±J Potts glass chains undergo a zero temperature phase transition. The barriers against inversion of the spin chain take only two values; 0 and 2|J|. The temperature behaviour of specific heat is characterized by rounded peaks. The frequency dependence displays two plateaus for the real part of specific heat and two corresponding peaks for the imaginary part. The dynamic specific heat is not affected by the longest relaxing mode like susceptibility. The time separation of the modes is demonstrated by the Cole-Cole plots.     

High field magnetic susceptibility of NiCr and NiV alloys

The variation of the density of states at the Fermi level with impurity concentration is investigated by means of magnetic measurements. This variation is found to be small for NiCr and NiV dilute alloys, in contrast with specific heat results.     

Delocalization of the Ce 4f shell in amorphous Ce75.5Co24.5

The amorphous alloy Ce_75.5Co_24.5 prepared by melt spinning has been studied through measurements of the magnetic susceptibility, magnetization, electrical resistivity, thermoelectric power and specific heat. The results are interpreted in terms of a homogeneous intermediate valence state of the Ce ions. This is inferred from a temperature-independent magnetic susceptibility at low temperature and the absence of magnetic ordering, a large linear term in the specific heat, and aT ² dependence of the electrical resistivity at low temperature followed by a steep increase with temperature up to 50 K. At this temperature, the thermoelectric power displays a maximum. The intrinsic properties are partially obscured at low temperatures by a contribution from roughly a few percent of magnetic impurities, presumably Ce³⁺ ions. They manifest themselves by an increase of the susceptibility towards low temperatures and by a broad Schottky-like contribution to the specific heat resulting from the excitation of magnetic clusters.Dedicated to Prof. Dr. S. Methfessel on the occasion of his 60th birthday     

Physical properties of the β-Ti6Sn5 system

The current work reports the specific heat, the resistivity and the magnetic susceptibility of both single-crystal and polycrystalline β-Ti₆Sn₅ at cryogenic temperatures. The effects of small additions of magnetic and non-magnetic impurities were examined. Resistivity and magnetic susceptibility measurements reveal that the undoped material is a Pauli paramagnet displaying Fermi-liquid behaviour, while ferromagnetic ordering was observed at T150K with small additions of Ce, La or Co. Analysis of the electronic specific heat γ and magnetic susceptibility gives an unexpectedly large Wilson ratio R _w of 1.76, a value indicative of correlated electron behaviour. We present the general physical properties and based on the sensitivity of the magnetic properties to doping, show evidence that β-Ti₆Sn₅ exhibits a ground state in close proximity to a non-magnetic and magnetic phase boundary.     

Understanding the heavy fermion phenomenology from a microscopic model

We solve the 3D periodic Anderson model using a two impurity cluster dynamical mean field theory. We obtain the temperature versus hybridization phase diagram. Approaching the quantum critical point (QCP) both the Néel and lattice Kondo temperatures decrease and they do not cross at the lowest temperature we reached. While strong ferromagnetic spin fluctuation on the Kondo side is observed, our result suggests the critical static spin susceptibility is local in space at the QCP. We observe in the crossover region a logarithmic temperature dependence in the specific heat coefficient and spin susceptibility.     

Specific heat and spin susceptibility of dilute alloys*

The change in the specific heat and spin susceptibility of a metal of valence Z due to the introduction of a small number of impurity atoms with valence (Z + q) is calculated by the Thomas-Fermi method. It is shown that in the case of copper an exact Thomas-Fermi calculation accounts approximately for the seemingly linear behaviour of the change in C_v and X from q = 1 to q = 3 observed in recent experiments by Rayne. It does not, however, account for absolute values. In an appendix a check on the validity of the Thomas-Fermi theory is made for q < 1 by recalculating the specific heat and spin susceptibility by means of the techniques of many-body perturbation theory. The different methods give essentially the same results.     

Compensation effect in an exchange enhanced superconductor: [(La.993Lu.007)1−xGdx]Ru2

We have measured the upper critical field, H_c2, of alloys of the form [(La_.993Lu_.007)_1?xGd_x]Ru₂. For large $\text{(x?0.02)}$ concentrations of Gd, the critical fields are re-entrant. For small (x?0.01) concentrations of Gd, the critical fields cross the pure sample critical fields and rise above them due to compensation of the externally applied magnetic field by the negative effective internal field of the magnetic impurity. Compensation occurs at relatively low fields (25 KG) in this system as a result of substantial exchange enhancement. From the concentration and critical field of the most compensated sample, we find a value for the exchange integral, J(0) = ?(15+?2) meV, and a susceptibility enhancement factor in excellent agreement with EPR, specific heat and static susceptibility measurements.     

Electrical and thermal transport properties of dilute Pr in Pd

We have measured susceptibility, electrical resistivity thermopower, thermal conductivity, specific heat, thermal lattice expansion and Hall effect of dilute alloys of Pr (x = 0.014, 0.03) and of Lu (x = 0.014) in Pd. We find a rather strong valence instability at room temperature from L_III X-ray absorption (v = 3.1), which manifests itself by a concentration-independent characteristic temperature, by a resistivity minimum and by a strong anomally of the thermopower. The susceptibility also indicates strong magnetic polarisation of the surrounding Pd-atoms through the excited crystal field levels but not through the ground state of the impurity. The neutron scattering shows several sharp crystal field excitations between 0 and 1 meV and a very broad quasi-elastic scattering at higher temperatures with the 4f-form factor. We take this also as evidence for a weak coupling of valence instabilities to the low lying crystal field levels of the impurity and a strong coupling to the higher ones.     

Collective Diffusion and Strange-Metal Transport

We study a particular combination of charge and heat currents, which is decoupled with the heat current. The "heat-decoupled" (HD) current can be transported by diffusion at long distances, when the thermoelectric effect is small, large, or balanced. Using holographic models with momentum relaxation, we illustrate that the different thermoelectric effects correspond to the high temperatures and strong disorder, low temperatures, or special critical index. Meanwhile, the Einstein-like relation and the diffusion/chaos relation may be emergent. Assuming that the existence and features of HD modes appear in strange metals, we can predict that when the thermoelectric effect is not very large, the scaling of resistivity is predominantly controlled by the HD susceptibility and chaos; otherwise more physics is required.     

Correlated giant dielectric peaks and antiferromagnetic transitions near room temperature in pure and alkali-doped BaMnO(3-δ)

We report structural, magnetic, dielectric and thermal properties of single-crystal BaMnO(2.99) and its derivatives BaMn(0.97)Li(0.03)O(3) and Ba(0.97)K(0.03)MnO(3). The hexagonal 15R-BaMnO(2.99) perovskite phase is a known antiferromagnetic insulator that orders at a Néel temperature T(N) = 220 K. We find dilute Li and K doping change the ratio of cubic to hexagonal layers and cause drastic changes in the dielectric and magnetic properties. Unusually large high-temperature magnetoelectric shifts (up to 85%) are observed near temperatures at which pronounced peaks in the dielectric constant are observed for applied electric fields along either the c or a axis, respectively. The temperatures of the dielectric peaks are strongly correlated with anomalies in the c- or a-axis magnetic susceptibility and the specific heat for all compositions studied. All our data suggest that the strongly anisotropic magnetic and dielectric anomalies (which occur near, or above room temperature) originate from the same Mn ion sites, which implies these materials form an exceptional class of magnetoelectrics.     

Nonstoichiometry and low-temperature magnetic properties of FeSi crystals

The results of experimental and theoretical studies of the low-temperature properties of FeSi crystals are presented. The specific features of the magnetic susceptibility are shown to be related to the superparamagnetic behavior of impurity clusters. The thermomagnetic hysteresis phenomena observed are explained using the model of exchange-coupled clusters.