arXiv subjects

J. Singleton

Publications and source records attributed to J. Singleton.

At least 73 records · Page 4Linked to original sources

arXiv2005

Catastrophic Fermi surface reconstruction in the shape-memory alloy AuZn

AuZn undergoes a shape-memory transition at 67 K. The de Haas van Alphen effect persists to 100 K enabling the observation of a change in the quantum oscillation spectrum indicative of a catastrophic Fermi surface reconstruction at the transition. Coexistence of both Fermi surfaces at low temperatures is suggestive of an intrinsic phase separation in the bulk of the material. In addition, a Dingle analysis reveals a sharp change in the scattering mechanism at a threshold cyclotron radius, which we suggest to be related to the underlying microstructure that drives the shape-memory effect.

cond-mat.str-el↗

arXiv2005

Fermi Surface as a Driver for the Shape-Memory Effect in AuZn

Martensites are materials that undergo diffusionless, solid-state transitions. The martensitic transition yields properties that depend on the history of the material and may allow it to recover its previous shape after plastic deformation. This is known as the shape-memory effect (SME). We have succeeded in identifying the primary electronic mechanism responsible for the martensitic transition in the shape-memory alloy AuZn by using Fermi-surface measurements (de Haas-van Alphen oscillations) and band-structure calculations. This strongly suggests that electronic band structure is an important consideration in the design of future SME alloys.

cond-mat.mtrl-sci↗

arXiv2004

Landau quantization effects in the charge-density-wave system (Per)$_2M$(mnt)$_2$ (where $M=$Au and Pt)

A finite transfer integral $t_a$ orthogonal to the conducting chains of a highly one-dimensional metal gives rise to empty and filled bands that simulate an indirect-gap semiconductor upon formation of a commensurate charge-density-wave (CDW). In contrast to semiconductors such as Ge and Si with bandgaps $\sim 1$ eV, the CDW system possesses an indirect gap with a greatly reduced energy scale, enabling moderate laboratory magnetic fields to have a major effect. The consequent variation of the thermodynamic gap with magnetic field due to Zeeman splitting and Landau quantization enables the electronic bandstructure parameters (transfer integrals, Fermi velocity) to be determined accurately. These parameters reveal the orbital quantization limit to be reached at $\sim 20$ T in (Per)$_2M$(mnt)$_2$ salts, making them highly unlikely candidates for a recently-proposed cascade of field-induced charge-density wave states.

cond-mat.str-el↗

arXiv2004

Absence of field-induced charge-density wave states in (Per)$_2$Pt(mnt)$_2$

Graf {\it et al.} [Phys. Rev. Lett. {\bf 93} 076406 (2004)] recently attributed features in the magnetic-field-dependent longitudinal resistance of (Per)$_2$Pt(mnt)$_2$ to a cascade of field-induced charge-density waves (FICDWs). Here we show that a quantitative magnetotransport analysis reveals orbital quantization to be absent, disproving the presence of FICDWs. Our data show that the conduction is instead dominated by the sliding CDW collective mode at low temperatures.

cond-mat.str-el↗

arXiv2004

Suppression of the $γ-α$ structural phase transition in $Ce_{0.8} La_{0.1} Th_{0.1}$ by large magnetic fields

The $γ-α$ transition in Ce$_{0.8}$La$_{0.1}$Th$_{0.1}$ is measured as a function of applied magnetic field using both resistivity and magnetization. The $γ- α$ transition temperature decreases with increasing magnetic field, reaching zero temperature at around 56 T. The magnetic-field dependence of the transition temperature may be fitted using a model that invokes the field and temperature dependence of the entropy of the $4f$-electron moments of the $γ$ phase, suggesting that the volume collapse in Ce and its alloys is primarily driven by entropic considerations.

cond-mat.str-el↗

arXiv2004

Experimental demonstration of a new radiation mechanism: emission by an oscillating, accelerated, superluminal polarization current

We describe the experimental implementation of a superluminal ({\it i.e.} faster than light {\it in vacuo}) polarization current distribution that both oscillates and undergoes centripetal acceleration. Theoretical treatments lead one to expect that the radiation emitted from each volume element of such a polarization current will comprise a Čerenkov-like envelope with two sheets that meet along a cusp. The emission from the experimental machine is in good agreement with these expectations, the combined effect of the volume elements leading to tightly-defined beams of a well-defined geometry, determined by the source speed and trajectory. In addition, over a restricted range of angles, we detect the presence of cusps in the emitted radiation. These are due to the detection over a short time period (in the laboratory frame) of radiation emitted over a considerably longer period of source time. Consequently, the intensity of the radiation at these angles was observed to decline more slowly with increasing distance from the source than would the emission from a conventional antenna. The angular distribution of the emitted radiation and the properties associated with the cusps are in good {\it quantitative} agreement with theoretical models of superluminal sources once the effect of reflections from the earth's surface are taken into account.

physics.optics↗

arXiv2003

Angle Dependent Magnetoresistance of the Layered Organic Superconductor κ-(ET)2Cu(NCS)2: Simulation and Experiment

The angle-dependences of the magnetoresistance of two different isotopic substitutions (deuterated and undeuterated) of the layered organic superconductor κ-(ET)2Cu(NCS)2 are presented. The angle dependent magnetoresistance oscillations (AMRO) arising from the quasi-one-dimensional (Q1D) and quasi-two-dimensional (Q2D) Fermi surfaces in this material are often confused. By using the Boltzman transport equation extensive simulations of the AMRO are made that reveal the subtle differences between the different species of oscillation. No significant differences are observed in the electronic parameters derived from quantum oscillations and AMRO for the two isotopic substitutions. The interlayer transfer integrals are determined for both isotopic substitutions and a slight difference is observed which may account for the negative isotope effect previously reported [1]. The success of the semi-classical simulations suggests that non-Fermi liquid effects are not required to explain the interlayer-transport in this system.

cond-mat.str-el↗

arXiv2003

Charge-density Waves Survive the Pauli Paramagnetic Limit

Measurements of the resistance of single crystals of (Per)$_2$Au(mnt)$_2$ have been made at magnetic fields $B$ of up to 45 T, exceeding the Pauli paramagnetic limit of $B_{\rm P}\approx 37$ T. The continued presence of non-linear charge-density wave electrodynamics at $B \geq 37$ T unambiguously establishes the survival of the charge-density wave state above the Pauli paramagnetic limit, and the likely emergence of an inhomogeneous phase analogous to that anticipated to occur in superconductors.

cond-mat.str-el↗

arXiv2003

A new quantum fluid at high magnetic fields in the marginal charge-density-wave system $α$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ (where $M=$~K and Rb)

Single crystals of the organic charge-transfer salts $α$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ have been studied using Hall-potential measurements ($M=$K) and magnetization experiments ($M$ = K, Rb). The data show that two types of screening currents occur within the high-field, low-temperature CDW$_x$ phases of these salts in response to time-dependent magnetic fields. The first, which gives rise to the induced Hall potential, is a free current (${\bf j}_{\rm free}$), present at the surface of the sample. The time constant for the decay of these currents is much longer than that expected from the sample resistivity. The second component of the current appears to be magnetic (${\bf j}_{\rm mag}$), in that it is a microscopic, quasi-orbital effect; it is evenly distributed within the bulk of the sample upon saturation. To explain these data, we propose a simple model invoking a new type of quantum fluid comprising a CDW coexisting with a two-dimensional Fermi-surface pocket which describes the two types of current. The model and data are able to account for the body of previous experimental data which had generated apparently contradictory interpretations in terms of the quantum Hall effect or superconductivity.

cond-mat.str-el↗

arXiv2003

Fermi-surface topology and the effects of intrinsic disorder in a class of charge-transfer salts containing magnetic ions, β''-(BEDT-TTF)_4[(H_3O)M(C_2O_4)_3]Y

We report high-field magnetotransport measurements on β''-(BEDT-TTF)_4[(H_3O)M(C_2O_4)_3]Y, where M=Ga, Cr and Fe and Y=C_5H_5N. We observe similar Shubnikov-de Haas oscillations in all compounds, attributable to four quasi-two-dimensional Fermi-surface pockets, the largest of which corresponds to a cross-sectional area ~8.5\% of the Brillouin zone. The cross-sectional areas of the pockets are in agreement with the expectations for a compensated semimetal, and the corresponding effective masses are ~m_e, rather small compared to those of other BEDT-TTF salts. Apart from the case of the smallest Fermi-surface pocket, varying the $M$ ion seems to have little effect on the overall Fermi-surface topology or on the effective masses. Despite the fact that all samples show quantum oscillations at low temperatures, indicative of Fermi liquid behavior, the sample- and temperature-dependence of the interlayer resistivity suggest that these systems are intrinsically inhomogeneous. It is thought that intrinsic tendency to disorder in the anions and/or the ethylene groups of the BEDT-TTF molecules leads to the coexistence of insulating and metallic states at low temperatures; comparison with other charge-transfer salts suggests that this might be a rather general phenomenon. A notional phase diagram is given for the general family of β''-(BEDT-TTF)_4[(H_3O)M(C_2O_4)_3]Y salts, which exhibits several marked similarities with that of the κ-(BEDT-TTF)_2X superconductors.

cond-mat.str-el↗

arXiv2003

Experimental electronic heat capacities of $α-$ and $δ-$Plutonium; heavy-fermion physics in an element

We have measured the heat capacities of $δ-$Pu$_{0.95}$Al$_{0.05}$ and $α-$Pu over the temperature range 2-303 K. The availability of data below 10 K plus an estimate of the phonon contribution to the heat capacity based on recent neutron-scattering experiments on the same sample enable us to make a reliable deduction of the electronic contribution to the heat capacity of $δ-$Pu$_{0.95}$Al$_{0.05}$; we find $γ= 64 \pm 3$ mJK$^{-2}$mol$^{-1}$ as $T \to 0$. This is a factor $\sim 4$ larger than that of any element, and large enough for $δ-$Pu$_{0.95}$Al$_{0.05}$ to be classed as a heavy-fermion system. By contrast, $γ= 17 \pm 1$ mJK$^{-2}$mol$^{-1}$ in $α-$Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of $δ-$Pu$_{0.95}$Al$_{0.05}$, one or both of which may be associated with the formation of the $α'-$ martensitic phase. We suggest that the large $γ$-value of $δ-$Pu$_{0.95}$Al$_{0.05}$ may be caused by proximity to a quantum-critical point.

cond-mat.str-el↗

arXiv2003

Non-adiabatic small polaron hopping in the n=3 Ruddlesden-Popper compound Ca4Mn3O10

Magnetotransport properties of the compound Ca4Mn3O10 are interpreted in terms of activated hopping of small magnetic polarons in the non-adiabatic regime. Polarons are most likely formed around Mn3+ sites created by oxygen substoichiometry. The application of an external field reduces the size of the magnetic contribution to the hopping barrier and thus produces an increase in the conductivity .We argue that the change in the effective activation energy around TN is due to the crossover to VRH conduction as antiferromagnetic order sets in.

cond-mat.mtrl-sci↗

arXiv2003

The spectral and polarization characteristics of the nonspherically decaying radiation generated by polarization currents with superluminally rotating distribution patterns

We present a theoretical study of the emission from a superluminal polarization current whose distribution pattern rotates (with an angular frequency $ω$) and oscillates (with a frequency $Ω$) at the same time, and which comprises both poloidal and toroidal components. This type of polarization current is found in recent practical machines designed to investigate superluminal emission. We find that the superluminal motion of the distribution pattern of the emitting current generates localized electromagnetic waves that do not decay spherically, i.e. that do not have an intensity diminishing like ${R_P}^{-2}$ with the distance $R_P$ from their source. The nonspherical decay of the focused wave packets that are emitted by the polarization currents does not contravene conservation of energy: the constructive interference of the constituent waves of such propagating caustics takes place within different solid angles on spheres of different radii ($R_P$) centred on the source. For a polarization current whose longitudinal distribution (over an azimuthal interval of length $2π$) consists of $m$ cycles of a sinusoidal wave train, the nonspherically decaying part of the emitted radiation contains the frequencies $Ω\pm mω$; i.e. it contains {\it only} the frequencies involved in the creation and implementation of the source. This is in contrast to recent studies of the spherically decaying emission, which was shown to contain much higher frequencies. The polarization of the emitted radiation is found to be linear for most configurations of the source.

physics.optics↗

arXiv2002

The frequency spectrum of focused broadband pulses of electromagnetic radiation generated by polarization currents with superluminally rotating distribution patterns

We investigate the spectral features of the emission from a superluminal polarization current whose distribution pattern rotates with an angular frequency $ω$ and oscillates with an incommensurate frequency $Ω>ω$. This type of polarization current is found in recent practical machines designed to investigate superluminal emission. Although all of the processes involved are linear, we find that the broadband emission contains frequencies that are higher than $Ω$ by a factor of the order of $(Ω/ω)^2$. This generation of frequencies {\it not} required for the creation of the source stems from mathematically rigorous consequences of the familiar classical expression for the retarded potential. The results suggest practical applications for superluminal polarization currents as broad-band radiofrequency and infrared sources.

physics.optics↗

arXiv2002

The pressure dependence of many-body interactions in the organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$: A comparison of high pressure infrared reflectivity and Raman scattering experiments

We determine the pressure dependence of the electron-phonon coupling in $κ$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$ by comparison of high pressure Raman scattering and high pressure infrared (IR) reflectivity measurements. The Raman active molecular vibrations of the BEDT-TTF dimers stiffen by 0.1-1 %GPa$^{-1}$. In contrast, the corresponding modes in the IR spectrum are observed at lower frequency, with a pressure dependence of 0.5-5.5 %GPa$^{-1}$, due to the influence of the electron-phonon interaction. Both dimer charge-oscillation and phase-phonon models are employed to extract the pressure dependence of the electron molecular-vibration coupling for these modes. Analysis of our data suggests that the reduction of electron-phonon coupling under pressure does not account for the previously observed suppression of superconductivity under pressure and that electron-electron interactions may contribute significantly to the pairing mechanism.

cond-mat.str-el↗

arXiv2002

Comparison of the Fermi-surface topologies of kappa-(BEDT-TTF)_2 Cu(NCS)_2 and its deuterated analogue

We have measured details of the quasi one-dimensional Fermi-surface sections in the organic superconductor kappa-(BEDT-TTF)_2 Cu(NCS)_2 and its deuterated analogue using angle-dependent millimetre-wave techniques. There are significant differences in the corrugations of the Fermi surfaces in the deuterated and undeuterated salts. We suggest that this is important in understanding the inverse isotope effect, where the superconducting transition temperature rises on deuteration. The data support models for superconductivity which invoke electron-electron interactions depending on the topological properties of the Fermi surface.

cond-mat.str-el↗

arXiv2001

Magneto-optics of organic crystalline field effect transistors

Organic molecular FETs provide an experimental framework for studying the band-filling dependent properties of two-dimensional metals. Here, we propose experiments to investigate the band-filling dependent cyclotron resonance, and the effect of van Hove singularities.

cond-mat↗

arXiv2001

A statistical model for the intrinsically broad superconducting to normal transition in quasi-two-dimensional crystalline organic metals

Although quasi-two-dimensional organic superconductors such as $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$ seem to be very clean systems, with apparent quasiparticle mean-free paths of several thousand Å, the superconducting transition is intrinsically broad (e.g $\sim 1$ K wide for $T_c \approx 10$ K). We propose that this is due to the extreme anisotropy of these materials, which greatly exacerbates the statistical effects of spatial variations in the potential experienced by the quasiparticles. Using a statistical model, we are able to account for the experimental observations. A parameter $\bar{x}$, which characterises the spatial potential variations, may be derived from Shubnikov-de Haas oscillation experiments. Using this value, we are able to predict a transition width which is in good agreement with that observed in MHz penetration-depth measurements on the same sample.

cond-mat.supr-con↗