arXiv subjects

Neil Harrison

Publications and source records attributed to Neil Harrison.

32 records · Page 2Linked to original sources

arXiv2019

Hidden valence transition in URu2Si2?

The term "hidden order" refers to an as yet unidentified form of broken-symmetry order parameter that is presumed to exist in the strongly correlated electron system URu2Si2 on the basis of the reported similarity of the heat capacity at its phase transition at To~17 K to that produced by Bardeen-Cooper-Schrieffer (BCS) mean field theory. Here we show that the phase boundary in URu2Si2 has the elliptical form expected for an entropy-driven phase transition, as has been shown to accompany a change in valence. We show one characteristic feature of such a transition is that the ratio of the critical magnetic field to the critical temperature is defined solely in terms of the effective quasiparticle g-factor, which we find to be in quantitative agreement with prior g-factor measurements. We further find the anomaly in the heat capacity at To to be significantly sharper than a BCS phase transition, and, once quasiparticle excitations across the hybridization gap are taken into consideration, loses its resemblance to a second order phase transition. Our findings imply that a change in valence dominates the thermodynamics of the phase boundary in URu2Si2, and eclipses any significant contribution to the thermodynamics from a hidden order parameter.

cond-mat.str-el↗

arXiv2019

Field-induced double dome and Bose-Einstein condensation in the crossing quantum spin chain system AgVOAsO4

We present inelastic neutron scattering data on the quantum paramagnet AgVOAsO4 that establish the system is a S=1/2 alternating spin chain compound and provide a direct measurement of the spin gap. We also present experimental evidence for two different types of field-induced magnetic order between mu_0H_c1 = 8.4T and mu_0H_c2 = 48.9T, which may be related to Bose-Einstein condensation (BEC) of triplons. Thermodynamic measurements in magnetic fields up to 60T and temperatures down to 0.1K reveal a H-T phase diagram consisting of a dome encapsulating two ordered phases with maximum ordering temperatures of 3.8K and 5.3K respectively. This complex phase diagram is not expected for a single-Q BEC system and therefore establishes AgVOAsO4 as a promising multi-Q BEC candidate capable of hosting exotic vortex phases.

cond-mat.str-el↗

arXiv2018

Emergent Bound States and Impurity Pairs in Chemically Doped Shastry-Sutherland System

The search for novel unconventional superconductors is a central topic of modern condensed matter physics. Similar to other Mott insulators, Shastry-Sutherland (SSL) systems are predicted to become superconducting when chemically doped. This makes SrCu2(BO3)2, an experimental realization of SSL model, a suitable candidate and understanding of the doping effects in it very important. Here we report doping-induced emergent states in Mg-doped SrCu2(BO3)2, which remain stable up to high magnetic fields. Using four complementary magnetometry techniques and theoretical simulations, a rich impurity-induced phenomenology at high fields is discovered. The results demonstrate a rare example in which even a small doping concentration interacts strongly with both triplets and bound states of triplets, and thus plays a significant role in the magnetization process even at high magnetic fields. Moreover, our findings of the emergence of the very stable impurity pairs provide insights into the anticipated unconventional superconductivity in SrCu2(BO3)2 and related materials.

cond-mat.str-el↗

arXiv2018

Graphite in 90 T: Evidence for Strong-coupling Excitonic Pairing

Strong magnetic field induces at least two phase transitions in graphite beyond the quantum limit where many-body effects are expected. We report on a study using a state-of-the-art non-destructive magnet allowing to attain 90.5 T at 1.4 K, which reveals a new field-induced phase and evidence that the insulating state destroyed at 75 T is an excitonic condensate of electron-hole pairs. By monitoring the angle dependence of in-plane and out-of-plane magnetoresistance, we distinguish between the role of cyclotron and Zeeman energies in driving various phase transitions. We find that, with the notable exception of the transition field separating the two insulating states, the threshold magnetic field for all other transitions display an exact cosine angular dependence. Remarkably, the threshold field for the destruction of the second insulator (phase B) is temperature-independent with no detectable Landau-level crossing nearby. We conclude that the field-induced insulator starts as a weak-coupling spin-density-wave, but ends as a strong-coupling excitonic insulator of spin-polarized electron-hole pairs.

cond-mat.str-el↗

arXiv2018

Topological Excitations of Hidden Order in URu$_2$Si$_2$ Under Extreme Electric Fields

Quantum materials are epitomized by the influence of collective modes upon their macroscopic properties. Relatively few examples exist, however, whereby coherence of the ground-state wavefunction directly contributes to the conductivity. Notable examples include the quantizing effects of high magnetic fields upon the 2D electron gas, the collective sliding of charge density waves subject to high electric fields, and perhaps most notably the macroscopic phase coherence that enables superconductors to carry dissipationless currents. Here we reveal that the low temperature hidden order state of URu$_2$Si$_2$ exhibits just such a connection between the quantum and macroscopic worlds -- under large voltage bias we observe non-linear contributions to the conductivity that are directly analogous to the manifestation of phase slips in one-dimensional superconductors [1], suggesting a complex order parameter for hidden order

cond-mat.str-el↗

arXiv2018

Non-saturating Quantum Magnetization in Weyl semimetal TaAs

Detecting the spectroscopic signatures of Dirac-like quasiparticles in emergent topological materials is crucial for searching their potential applications. Magnetometry is a powerful tool for fathoming electrons in solids, yet its ability for discerning Dirac-like quasiparticles has not been recognized. Adopting the probes of magnetic torque and parallel magnetization for the archetype Weyl semimetal TaAs in strong magnetic field, we observed a quasi-linear field dependent effective transverse magnetization and a strongly enhanced parallel magnetization when the system is in the quantum limit. Distinct from the saturating magnetic responses for massive carriers, the non-saturating signals of TaAs in strong field is consistent with our newly developed magnetization calculation for a Weyl fermion system in an arbitrary angle. Our results for the first time establish a thermodynamic criterion for detecting the unique magnetic response of 3D massless Weyl fermions in the quantum limit.

cond-mat.mes-hall↗

arXiv2016

The Tricritical Point of the f-electron Antiferromagnet USb2 Driven by High Magnetic Fields

In pulsed magnetic fields up to 65T and at temperatures below the Néel transition, our magnetization and magnetostriction measurements reveal a field-induced metamagnetic-like transition that is suggestive of an antiferromagnetic to polarized paramagnetic or ferrimagnetic ordering. Our data also suggests a change in the nature of this metamagnetic-like transition from second- to first-order-like near a tricritical point at T_{tc} ~145K and H_{c}~52T. At high fields for H>H_{c} we found a decreased magnetic moment roughly half of the moment reported in low field measurements. We propose that \mathit{f-p} hybridization effects and magnetoelastic interactions drive the decreased moment, lack of saturation at high fields, and the decreased phase boundary.

cond-mat.str-el↗

arXiv2014

Thermodynamic Properties of the Quantum Spin Liquid Candidate ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ in High Magnetic Fields

We report measurements of the specific heat and magnetization of single crystal samples of the spin-1/2 kagome compound ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ (herbertsmithite), a promising quantum spin-liquid candidate, in high magnetic fields and at low temperatures. The magnetization was measured up to $μ_{0}H$ = 55 T at $T$ = 0.4 K, showing a saturation of the weakly interacting impurity moments in fields above $\sim10$ T. The specific heat was measured down to $T < 0.4$ K in magnetic fields up to 18 T, revealing $T$-linear and $T$-squared contributions. The $T$-linear contribution is surprisingly large and indicates the presence of gapless excitations in large applied fields. These results further highlight the unusual excitation spectrum of the spin liquid ground state of herbertsmithite.

cond-mat.str-el↗

arXiv2012

Towards resolution of the Fermi surface in underdoped high-Tc superconductors

We survey recent experimental results including quantum oscillations and complementary measurements probing the electronic structure of underdoped cuprates, and theoretical proposals to explain them. We discuss quantum oscillations measured at high magnetic fields in the underdoped cuprates that reveal a small Fermi surface section comprising quasiparticles that obey Fermi-Dirac statistics, unaccompanied by other states of comparable thermodynamic mass at the Fermi level. The location of the observed Fermi surface section at the nodes is indicated by a body of evidence including the collapse in Fermi velocity measured by quantum oscillations, which is found to be associated with the nodal density of states observed in angular resolved photoemission, the persistence of quantum oscillations down to low fields in the vortex state, the small value of density of states from heat capacity and the multiple frequency quantum oscillation pattern consistent with nodal magnetic breakdown of bilayer-split pockets. A nodal Fermi surface pocket is further consistent with the observation of a density of states at the Fermi level concentrated at the nodes in photoemission experiments, and the antinodal pseudogap observed by photoemission, optical conductivity, nuclear magnetic resonance Knight shift, as well as other complementary diffraction, transport and thermodynamic measurements. One of the possibilities considered is that the small Fermi surface pockets observed at high magnetic fields can be understood in terms of Fermi surface reconstruction by a form of small wavevector charge order, observed over long lengthscales in experiments such as nuclear magnetic resonance and x-ray scattering, potentially accompanied by an additional mechanism to gap the antinodal density of states.

cond-mat.supr-con↗

arXiv2012

Anisotropic $H_{c2}$ determined up to 92 T and the signature of multi-band superconductivity in Ca$_{10}$(Pt$_{4}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ superconductor

The upper critical fields, $H_{c2}$($T$), of single crystals of the superconductor Ca$_{10}$(Pt$_{4-δ}$As$_{8}$)((Fe$_{0.97}$Pt$_{0.03}$)$_{2}$As$_{2}$)$_{5}$ ($δ$ $\approx$ 0.246) are determined over a wide range of temperatures down to $T$ = 1.42 K and magnetic fields of up to $μ_{0}H$ $\simeq$ 92 T. The measurements of anisotropic $H_{c2}$($T$) curves are performed in pulsed magnetic fields using radio-frequency contactless penetration depth measurements for magnetic field applied both parallel and perpendicular to the \textbf{ab}-plane. Whereas a clear upward curvature in $H_{c2}^{\parallel\textbf{c}}$($T$) along \textbf{H}$\parallel$\textbf{c} is observed with decreasing temperature, the $H_{c2}^{\parallel\textbf{ab}}$($T$) along \textbf{H}$\parallel$\textbf{ab} shows a flattening at low temperatures. The rapid increase of the $H_{c2}^{\parallel\textbf{c}}$($T$) at low temperatures suggests that the superconductivity can be described by two dominating bands. The anisotropy parameter, $γ_{H}$ $\equiv$ $H_{c2}^{\parallel\textbf{ab}}/H_{c2}^{\parallel\textbf{c}}$, is $\sim$7 close to $T_{c}$ and decreases considerably to $\sim$1 with decreasing temperature, showing rather weak anisotropy at low temperatures.

cond-mat.supr-con↗

arXiv2008

A mechanism for Fermi-surface-topology tuned superconductivity in the cuprates

Based on recent magnetic-quantum-oscillation, ARPES, neutron-scattering and other data, we propose that superconductivity in the cuprates occurs via a convenient matching of the spatial distribution of incommensurate spin fluctuations to the amplitude and phase of the $d_{x^2-y^2}$ Cooper-pair wavefunction; this establishes a robust causal relationship between the lengthscale of the fluctuations and the superconducting coherence length. It is suggested that the spin fluctuations are driven by the Fermi surface, which is prone to nesting; they couple to the itinerant holes via the on-site Coulomb correlation energy, which inhibits double occupancy of spins or holes. The maximum energy of the fluctuations gives an appropriate energy scale for the superconducting $T_{\rm c}$. Based on this model, one can specify the design of solids that will exhibit ``high $T_{\rm c}$'' superconductivity.

cond-mat.supr-con↗

arXiv2007

Cuprate Fermi orbits and Fermi arcs: the effect of short-range antiferromagnetic order

We consider the effect of a short antiferromagnetic correlation length $ξ$ on the electronic bandstructure of the underdoped cuprates. Starting with a Fermi-surface topology similar to that detected in magnetic quantum-oscillation experiments, we show that a reduced $ξ$ gives an assymmetric broadening of the quasiparticle dispersion, resulting in simulated ARPES data very similar to those observed in experiment. Predicted features include the presence of `Fermi arcs' close to $a{\bf k}=(π/2,π/2)$, without the need to invoke a d-wave pseudogap order parameter. The statistical variation in the ${\bf k}$-space areas of the reconstructed Fermi surface pockets causes the quantum oscillations to be strongly damped, even in very strong magnetic fields, in agreement with experiment.

cond-mat.supr-con↗

arXiv2001

Superconducting properties and Fermi-surface topology of the quasi-two-dimensional organic superconductor $λ$-(BETS)$_{2}$GaCl$_{4}$

The Fermi surface topology of the organic superconductor \lbets has been determined using the Shubnikov-de Haas and magnetic breakdown effects and angle-dependent magnetoresistance oscillations. The former experiments were carried out in pulsed fields of up to 60 T, whereas the latter employed quasistatic fields of up to 30 T. All of these data show that the Fermi-surface topology of \lbets is very similar to that of the most heavily-studied organic superconductor, \cuscn, except in one important respect; the interplane transfer integral in \lbets is a factor $\sim 10$ larger than that in \cuscn . The increased three-dimensionality of \lbets is manifested in radiofrequency penetration-depth measurements, which show a clear dimensional crossover in the behaviour of $H_{c2}(T)$. The radiofrequency measurements have also been used to extract the Labusch parameter determining the fluxoid interactions as a function of temperature, and to map the flux-lattice melting curve.

cond-mat.supr-con↗

arXiv2001

On the de Haas-van Alphen effect in inhomogeneous alloys

We show that Landau level broadening in alloys occurs naturally as a consequence of random variations in the local quasiparticle density, without the need to consider a relaxation time. This approach predicts Lorentzian-broadened Landau levels similar to those derived by Dingle using the relaxation-time approximation. However, rather than being determined by a finite relaxation time $τ$, the Landau-level widths instead depend directly on the rate at which the de Haas-van Alphen frequency changes with alloy composition. The results are in good agreement with recent data from three very different alloy systems.

cond-mat.dis-nn↗