arXiv subjects

P. Walmsley

Publications and source records attributed to P. Walmsley.

At least 19 recordsLinked to original sources

arXiv2023

Ultrafast spatiotemporal dynamics of a charge-density wave using femtosecond dark-field momentum microscopy

Understanding phase competition and phase separation in quantum materials requires access to the spatiotemporal dynamics of electronic ordering phenomena on a micro- to nanometer length- and femtosecond timescale. While time- and angle-resolved photoemission (trARPES) experiments provide sensitivity to the femtosecond dynamics of electronic ordering, they typically lack the required spatial resolution. Here, we demonstrate ultrafast dark-field photoemission microscopy (PEEM) using a momentum microscope, providing access to ultrafast electronic order on the microscale. We investigate the prototypical charge-density wave (CDW) compound TbTe3 in the vicinity of a buried crystal defect, demonstrating real- and reciprocal-space configurations combined with a pump-probe approach. We find CDW order to be suppressed in the region covered by the crystal defect, most likely due to locally imposed strain. Comparing the ultrafast dynamics in different areas of the sample reveals a substantially smaller response to optical excitation and faster relaxation of excited carriers in the defect area, which we attribute to enhanced particle-hole scattering and defect-induced relaxation channels.

cond-mat.str-el↗

arXiv2021

Coherent Modulation of Quasiparticle Scattering Rates in a Photoexcited Charge-Density-Wave System

We present a complementary experimental and theoretical investigation of relaxation dynamics in the charge-density-wave (CDW) system TbTe$_3$ after ultrafast optical excitation. Using time- and angle-resolved photoemission spectroscopy, we observe an unusual transient modulation of the relaxation rates of excited photocarriers. A detailed analysis of the electron self-energy based on a nonequilibrium Green's function formalism reveals that the phase space of electron-electron scattering is critically modulated by the photoinduced collective CDW excitation, providing an intuitive microscopic understanding of the observed dynamics and revealing the impact of the electronic band structure on the self-energy.

cond-mat.mtrl-sci↗

arXiv2020

Nonequilibrium Charge-Density-Wave Order Beyond the Thermal Limit

The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium. Recent discoveries, such as the optical enhancement of the critical temperature in certain superconductors and the photo-stabilization of hidden phases, have turned this field into an important research frontier. Here, we demonstrate nonthermal charge-density-wave (CDW) order at electronic temperatures far greater than the thermodynamic transition temperature. Using time- and angle-resolved photoemission spectroscopy and time-resolved X-ray diffraction, we investigate the electronic and structural order parameters of an ultrafast photoinduced CDW-to-metal transition. Tracking the dynamical CDW recovery as a function of electronic temperature reveals a behaviour markedly different from equilibrium, which we attribute to the suppression of lattice fluctuations in the transient nonthermal phonon distribution. A complete description of the system's coherent and incoherent order-parameter dynamics is given by a time-dependent Ginzburg-Landau framework, providing access to the transient potential energy surfaces.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic breakdown and charge density wave formation: a quantum oscillation study of the rare-earth tritellurides

The rare-earth tritellurides ($R$Te$_3$, where $R$ = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y) form a charge density wave state consisting of a single unidirectional charge density wave for lighter $R$, with a second unidirectional charge density wave, perpendicular and in addition to the first, also present at low temperatures for heavier $R$. We present a quantum oscillation study in magnetic fields up to 65T that compares the single charge density wave state with the double charge density wave state both above and below the magnetic breakdown field of the second charge density wave. In the double charge density wave state it is observed that there remain several small, light pockets with the largest occupying around 0.5% of the Brillouin zone. By applying magnetic fields above the independently determined magnetic breakown field, the quantum oscillation frequencies of the single charge density wave state are recovered, as expected in a magnetic breakdown scenario. Measurements of the electronic effective mass do not show any divergence or significant increase on the pockets of Fermi surface observed here as the putative quantum phase transition between the single and double charge density wave states is approached.

cond-mat.str-el↗

arXiv2019

Comparison of temperature and doping dependence of nematic susceptibility near a putative nematic quantum critical point

Strong electronic nematic fluctuations have been discovered near optimal doping for several families of Fe-based superconductors, motivating the search for a possible link between these fluctuations, nematic quantum criticality, and high temperature superconductivity. Here we probe a key prediction of quantum criticality, namely power law dependence of the associated nematic susceptibility as a function of composition and temperature approaching the compositionally-tuned putative quantum critical point. To probe the 'bare' quantum critical point requires suppression of the superconducting state, which we achieve by using large magnetic fields, up to 45 T, while performing elastoresistivity measurements to follow the nematic susceptibility. We performed these measurements for the prototypical electron-doped pnictide, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, over a dense comb of dopings. We find that close to the putative quantum critical point, the nematic susceptibility appears to obey power law behavior over almost a decade of variation in composition, consistent with basic notions of nematic quantum criticality. Paradoxically, however, we also find that the temperature dependence for compositions close to the critical value cannot be described by a single power law. This is surprising as power law scaling in both doping and temperature is expected close to a quantum critical point.

cond-mat.str-el↗

arXiv2019

Growth of nematic susceptibility in the field-induced normal state of an iron-based superconductor revealed by elastoresistivity measurements in a 65 T pulsed magnet

In the iron-based superconductors, both nematic and magnetic fluctuations are expected to enhance superconductivity and may originate from a quantum critical point hidden beneath the superconducting dome. The behavior of the non-superconducting state can be an important piece of the puzzle, motivating in this paper the use of high magnetic fields to suppress superconductivity and measure the nematic susceptibility of the normal state at low temperatures. We describe experimental advances which make it possible to measure a resistive gauge factor (which is a proxy for the nematic susceptibility) in the field-induced normal state in a 65 T pulsed magnet, and report measurements of the gauge factor of a micromachined single crystal of Ba(Fe$_{0.926}$Co$_{0.074}$)$_2$As$_2$ at temperatures down to 1.2 K. The nematic susceptibility increases monotonically in the field-induced normal state as the temperature decreases, consistent with the presence of a quantum critical point nearby in composition.

cond-mat.str-el↗

arXiv2018

A sharp increase in the density of states in PbTe approaching a saddle point in the band structure

PbTe is a leading mid-range thermoelectric material with a $zT$ that has been enhanced by, amongst other methods, band engineering. Here we present an experimental study of the Hall effect, quantum oscillations, specific heat, and electron microprobe analysis that explores the evolution of the electronic structure of PbTe heavily doped with the `ideal' acceptor Na up to the solubility limit. We identify two phenomenological changes that onset as the electronic structure deviates from a Kane-type dispersion at around 180meV; a qualitative change in the field dependence of the Hall effect indicative of an increase in the high-field limit and a change in the Fermiology, and a sharp increase in the density of states as a function of energy. Following consideration of three possible origins for the observed phenomenology we conclude that the most likely source is non-ellipsoidicity of the $L$-pocket upon approach to a saddle point in the band structure, which is evidenced directly by our quantum oscillation measurements. Comparison to density functional theory calculations imply that this evolution of the electronic structure may be a key contributor to the large thermopower in PbTe.

cond-mat.mtrl-sci↗

arXiv2018

Mass enhancement in multiple bands approaching optimal doping in a high-temperature superconductor

Pnictides provide an opportunity to study the effects of quantum criticality in a multi-band high temperature superconductor. Quasiparticle mass divergence near optimal doping, observed in two major classes of high-temperature superconductors, pnictides and cuprates, is a direct experimental indicator of enhanced electronic interactions that accompany quantum criticality. Whether quasiparticles on all Fermi surface pockets in BaFe2(As1-xPx)2 are affected by quantum criticality is an open question, which specific heat measurements at high magnetic fields can directly address. Here we report specific heat measurements up to 35T in BaFe2(As1-xPx)2 over a broad doping range, 0.44 <= x <= 0.6. We observe saturation of C/T in the normal state at all dopings where superconductivity is fully suppressed. Our measurements demonstrate that quasiparticle mass increases towards optimal doping in multiple pockets, some of which exhibit even stronger mass enhancement than previously reported from quantum oscillations of a single pocket.

cond-mat.supr-con↗

arXiv2018

Direct spectroscopic evidence for mixed-valence Tl in the low carrier-density superconductor Pb$_{1-x}$Tl$_x$Te

Upon doping with Tl the narrow band-gap semiconductor PbTe exhibits anomalously high temperature superconductivity despite a very low carrier density as well as signatures of the Kondo effect despite an absence of magnetic moments. These phenomena have been explained by invoking 2$e$ fluctuations of the valence of the Tl dopants but a direct measurement of the mixed-valency implied by such a mechanism has not been reported to date. In this work we present the unambiguous observation of multiple valences of Tl in Tl-doped PbTe via photo emission spectroscopy measurements. It is shown via our quantitative analysis that the suppression of the carrier density at compositions exhibiting superconductivity and Kondo-like behaviour is fully accounted for by mixed valency, thus arguing strongly against a self-compensation scenario proposed elsewhere for this material and strengthening the case for valence fluctuation models. In addition to the identification of Tl$^+$ and Tl$^{3+}$ a possible third intermediate local charge-density is tentatively suggested by full fits to the data, the implications of which are discussed in the context of the charge-Kondo effect.

cond-mat.supr-con↗

arXiv2018

Competing soft phonon modes at the charge-density-wave transitions in DyTe$_3$

The family of rare-earth tritellurides RTe$_3$ features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [1]. What sets the heavier members of the RTe3 series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wavevector $q_{CDW,2}\parallel [100]$ of almost the same magnitude but orthogonal to the ordering wavevector $q_{CDW,1}\parallel [001]$ observed at higher temperatures [2]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of DyTe$_3$. In particular, we show that there are several phonon modes along both in-plane directions, which respond to the onset of the CDW transition at $T_{CDW,1}=308\,\rm{K}$. Surprisingly, these soft modes close to $q_{CDW,2}=(0.68,0,0)$ show strong softening near $T_{CDW,1}$ but do not exhibit any response to the lower-temperature transition at $T_{CDW,2}=68\,\rm{K}$. Our results indicate that the low-temperature CDW order is not just the 90° rotated analogue of the one appearing at high temperatures.

cond-mat.mtrl-sci↗

arXiv2017

Evidence of incoherent carriers associated with resonant impurity levels and their influence on superconductivity in the anomalous superconductor Pb$_{1-x}$Tl$_x$Te

We present a combined experimental and theoretical study of the evolution of the Fermi surface of the anomalous superconductor Pb$_{1-x}$Tl$_x$Te as a function of thallium concentration, drawing on a combination of magnetotransport measurements (Shubnikov de Haas oscillations and Hall coefficient), Angle Resolved Photoemission Spectroscopy (ARPES), and density functional theory (DFT) calculations of the electronic structure. Our results indicate that for Tl concentrations beyond a critical value the Fermi energy coincides with resonant impurity states in Pb$_{1-x}$Tl$_x$Te, and we rule out the presence of an additional valence band maximum at the Fermi energy. Through comparison to non-superconducting Pb$_{1-x}$Na$_x$Te we argue that the presence of these states at the Fermi energy provides the pairing interaction and thus also the anomalously high temperature superconductivity in this material.

cond-mat.supr-con↗

arXiv2016

Determination of the resistivity anisotropy of orthorhombic materials via transverse resistivity measurements

Measurements of the resistivity anisotropy can provide crucial information about the electronic structure and scattering processes in anisotropic and low-dimensional materials, but quantitative measurements by conventional means often suffer very significant systematic errors. Here we describe a novel approach to measuring the resistivity anisotropy of orthorhombic materials, using a single crystal and a single measurement, that is derived from a $\fracπ{4}$ rotation of the measurement frame relative to the crystallographic axes. In this new basis the transverse resistivity gives a direct measurement of the resistivity anisotropy, which combined with the longitudinal resistivity also gives the in-plane elements of the conventional resistivity tensor via a 5-point contact geometry. This is demonstrated through application to the charge-density wave compound ErTe$_3$, and it is concluded that this method presents a significant improvement on existing techniques in many scenarios.

cond-mat.str-el↗

arXiv2016

Inverse correlation between quasiparticle mass and Tc in a cuprate high-Tc superconductor

Close to a zero temperature transition between ordered and disordered electronic phases, quantum fluctuations can lead to a strong enhancement of the electron mass and to the emergence of competing phases such as superconductivity. A correlation between the existence of such a quantum phase transition and superconductivity is quite well established in some heavy fermion and iron-based superconductors and there have been suggestions that high temperature superconductivity in the copper oxide materials (cuprates) may also be driven by the same mechanism. Close to optimal doping, where the superconducting transition temperature $T_c$ is maximum in the cuprates, two different phases are known to compete with superconductivity: a poorly understood pseudogap phase and a charge ordered phase. Recent experiments have shown a strong increase in quasiparticle mass $m^*$ in the cuprate YBa$_2$Cu$_3$O$_{7-δ}$ as optimal doping is approached suggesting that quantum fluctuations of the charge ordered phase may be responsible for the high-$T_c$ superconductivity. We have tested the robustness of this correlation between $m^*$ and $T_c$ by performing quantum oscillation studies on the stoichiometric compound YBa$_2$Cu$_4$O$_8$ under hydrostatic pressure. In contrast to the results for YBa$_2$Cu$_3$O$_{7-δ}$, we find that in YBa$_2$Cu$_4$O$_8$ the mass decreases as $T_c$ increases under pressure. This inverse correlation between $m^*$ and $T_c$ suggests that quantum fluctuations of the charge order enhance $m^*$ but do not enhance $T_c$.

cond-mat.supr-con↗

arXiv2016

Fermi surface evolution of Na-doped PbTe studied through density functional theory calculations and Shubnikov-de Haas measurements

We present a combined experimental and theoretical study of the evolution of the low-temperature Fermi surface of lead telluride, PbTe, when holes are introduced through sodium substitution on the lead site. Our Shubnikov-de-Haas measurements for samples with carrier concentrations up to $9.4\times10^{19}$cm$^{-3}$ (0.62 Na atomic $\%$) show the qualitative features of the Fermi surface evolution (topology and effective mass) predicted by our density functional (DFT) calculations within the generalized gradient approximation (GGA): we obtain perfect ellipsoidal L-pockets at low and intermediate carrier concentrations, evolution away from ideal ellipsoidicity for the highest doping studied, and cyclotron effective masses increasing monotonically with doping level, implying deviations from perfect parabolicity throughout the whole band. Our measurements show, however, that standard DFT calculations underestimate the energy difference between the L-point and $Σ$-line valence band maxima, since our data are consistent with occupation of a single Fermi surface pocket over the entire doping range studied, whereas the calculations predict an occupation of the $Σ$-pockets at higher doping. Our results for low and intermediate compositions are consistent with a non-parabolic Kane-model dispersion, in which the L-pockets are ellipsoids of fixed anisotropy throughout the band, but the effective masses depend strongly on Fermi energy.

cond-mat.mtrl-sci↗

arXiv2014

Anomalous critical fields in quantum critical superconductors

Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity. However, the exact mechanism by which this occurs remains poorly understood. The iron-pnictide superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ is perhaps the clearest example to date of a high temperature quantum critical superconductor, and so it is a particularly suitable system in which to study how the quantum critical fluctuations affect the superconducting state. Here we show that the proximity of the QCP yields unexpected anomalies in the superconducting critical fields. We find that both the lower and upper critical fields strongly violate the expectations from the conventional theory taking into account the observed mass enhancement near the QCP. These anomalous behaviours of the critical fields imply that the energy of superconducting vortices is enhanced, possibly due to a microscopic mixing of antiferromagnetism and superconductivity, suggesting that a highly unusual vortex state is realised in quantum critical superconductors.

cond-mat.supr-con↗

arXiv2014

Superconducting gap structure of BaFe_2(As_{1-x}P_{x})_2

We present a study of the superconducting gap structure in the iron-pnictide series BaFe2(As1-xPx)2. By measuring the variation of the specific heat as a function of temperature and magnetic field we are able to determine the number and Fermi surface location of the nodes in the superconducting gap. In particular, from measurements of the variation of the specific heat as the magnetic field is rotated in the ab plane of the sample we conclude that the nodes are in the [110] directions. Then from a quantitative analysis of the temperature and field dependence of the specific heat we further conclude that nodes exists on all Fermi surface sheets.

cond-mat.supr-con↗

arXiv2013

Quasiparticle mass enhancement close to the quantum critical point in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

We report a combined study of the specific heat and de Haas-van Alphen effect in the iron-pnictide superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Our data when combined with results for the magnetic penetration depth give compelling evidence for the existence of a quantum critical point (QCP) close to $x=0.30$ which affects the majority of the Fermi surface by enhancing the quasiparticle mass. The results show that the sharp peak in the inverse superfluid density seen in this system results from a strong increase in the quasiparticle mass at the QCP.

cond-mat.supr-con↗

arXiv2012

A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1-xPx)2

In a superconductor, the ratio of the carrier density, $n$, to their effective mass, $m^*$, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, $λ_L$. In two dimensional systems, this ratio $n/m^*$ ($\sim 1/λ_L^2$) determines the effective Fermi temperature, $T_F$. We report a sharp peak in the $x$-dependence of $λ_L$ at zero temperature in clean samples of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ at the optimum composition $x = 0.30$, where the superconducting transition temperature $T_c$ reaches a maximum of 30\,K. This structure may arise from quantum fluctuations associated with a quantum critical point (QCP). The ratio of $T_c/T_F$ at $x = 0.30$ is enhanced, implying a possible crossover towards the Bose-Einstein condensate limit driven by quantum criticality.

cond-mat.supr-con↗