arXiv subjects

R. D. McDonald

Publications and source records attributed to R. D. McDonald.

63 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↗

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

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↗

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↗

arXiv2001

Cyclotron resonance harmonics in the organic superconductor $β^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$; observation of a new kind of effective mass renormalization

Cyclotron resonance, along with its second and third harmonics, has been observed in the quasi-two-dimensional organic superconductor $β^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$. The harmonic content is richly angle-dependent, and is interpreted in terms of the interlayer warping of the Fermi surface giving rise to the cyclotron resonance. In a departure from Kohn's theorem, but in agreement with recent theoretical predictions, the effective mass deduced from the cyclotron resonance measurements is greater than that determined from magnetic quantum oscillations.

cond-mat.str-el↗

arXiv2001

The pressure dependence of electron-phonon coupling 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 constant in $κ$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$ by comparison of high pressure Raman scattering and high pressure infrared reflectivity (IR) measurements. We use comparison of the IR reflection spectrum from deuterated and protonated samples to aid the deconvolution of several overlapping phonon modes. These coupled modes are modelled with a Green's function to extract the linear pressure dependence of the individual modes. 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. The stronger pressure dependence of the central C=C mode of the BEDT-TTF molecule in the IR, is discussed. Our analysis suggests that reduction of electron-phonon coupling under pressure does not account for observed suppression of superconductivity under pressure.

cond-mat.str-el↗