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N. Harrison

Publications and source records attributed to N. Harrison.

At least 91 records · Page 5Linked to original sources
arXiv2011

Direct observation of multiple spin zeroes in the underdoped high temperature superconductor YBa2Cu3O6+x

We report the direct observation of multiple `spin zeroes' in angle-dependent magnetic quantum oscillations measured up to 85T in YBa2Cu3O6+x, at which the amplitude falls to a deep minimum accompanied by a phase inversion of the measured quantum oscillations, enabling the product of the effective mass and effective g-factor m*g* to be tightly constrained. We find an evolution of the location of the spin zeros with applied magnetic field, and suggest that this effect and the absence of a spin zero at low angles can be produced by more than one Fermi surface component, and an effective g-factor with a subtle anisotropy between in-plane and out-of-plane crystalline directions.

cond-mat.supr-con↗
arXiv2011

Chemical potential oscillations from a single nodal pocket in the underdoped high-Tc superconductor YBa2Cu3O6+x

The mystery of the normal state in the underdoped cuprates has deepened with the use of newer and complementary experimental probes. While photoemission studies have revealed solely `Fermi arcs' centered on nodal points in the Brillouin zone at which holes aggregate upon doping, more recent quantum oscillation experiments have been interpreted in terms of an ambipolar Fermi surface, that includes sections containing electron carriers located at the antinodal region. To address the question of whether an ambipolar Fermi surface truly exists, here we utilize measurements of the second harmonic quantum oscillations, which reveal that the amplitude of these oscillations arises mainly from oscillations in the chemical potential, providing crucial information on the nature of the Fermi surface in underdoped YBa2Cu3O6+x. In particular, the detailed relationship between the second harmonic amplitude and the fundamental amplitude of the quantum oscillations leads us to the conclusion that there exists only a single underlying quasi-two dimensional Fermi surface pocket giving rise to the multiple frequency components observed via the effects of warping, bilayer splitting and magnetic breakdown. A range of studies suggest that the pocket is most likely associated with states near the nodal region of the Brillouin zone of underdoped YBa2Cu3O6+x at high magnetic fields.

cond-mat.supr-con↗
arXiv2011

Protected nodal electron pocket from multiple-Q ordering in underdoped high temperature superconductors

A multiple wavevector (Q) reconstruction of the Fermi surface is shown to yield a profoundly different electronic structure to that characteristic of single wavevector reconstruction, despite their proximity in energy. We consider the specific case in which ordering is generated by Qx = [2πa, 0] and Qy = [0,2πb] (in which a = b = 1/4) - similar to those identified in neutron diffraction and scanning tunneling microscopy experiments, and more generally show that an isolated pocket adjacent to the nodal point knodal = [\pm π/2, \pm π/2] is a protected feature of such a multiple-Q model, potentially corresponding to the nodal 'Fermi arcs' observed in photoemission and the small size of the electronic heat capacity found in high magnetic fields - importantly, containing electron carriers which can yield negative Hall and Seebeck coefficients observed in high magnetic fields.

cond-mat.str-el↗
arXiv2010

Fermi Surface of Cr$_{1-x}$V$_x$ across the Quantum Critical Point

We have measured de Haas-van Alphen oscillations of Cr$_{1-x}$V$_x$, $0 \le x \le 0.05$, at high fields for samples on both sides of the quantum critical point at $x_c=0.035$. For all samples we observe only those oscillations associated with a single small hole band with magnetic breakdown orbits of the reconstructed Fermi surface evident for $x x_c$ is further evidence for strong fluctuation scattering of these charge carriers well into the paramagnetic regime. We find no significant mass enhancement of the carriers in the single observed band at any $x$. An anomalous field dependence of the dHvA signal for our $x=0.035$ crystal at particular orientations of the magnetic field is identified as due to magnetic breakdown that we speculate results from a field induced SDW transition at high fields.

cond-mat.str-el↗
arXiv2010

Compensated electron and hole pockets in an underdoped high Tc superconductor

We report quantum oscillations in the underdoped high Tc YBCO over a wide range in magnetic field 28<B<85 T corresponding to ~12 oscillations, enabling the Fermi surface topology to be mapped to high resolution. As earlier reported by Sebastian et al., we find a Fermi surface comprising multiple pockets, as revealed by the additional distinct quantum oscillation frequencies and harmonics reported in this work. We find the originally reported broad low frequency Fourier peak at 535 T to be clearly resolved into three separate peaks at 460 T, 532 T and 602 T. Our increased resolution and angle-resolved measurements identify these frequencies to originate from two similarly sized pockets with greatly contrasting degrees of interlayer corrugation. The spectrally dominant frequency originates from a pocket (α) that is almost ideally two-dimensional in form. In contrast, the newly resolved weaker adjacent spectral features originate from a deeply corrugated pocket (γ). On comparison with band structure, the d-wave symmetry of the interlayer dispersion locates the minimally corrugated αpocket at the 'nodal' point where holes are located in a translational symmetry-broken scenario, and the significantly corrugated γpocket at the 'antinodal' point in the Brillouin zone, where electrons are located in a translational symmetry-broken scenario. Translational symmetry breaking by an SDW is suggested from the strong suppression of Zeeman splitting for the spectrally dominant pocket, additional evidence for which is provided from the harmonics we resolve in the present experiments. Given the similarity in αand γpocket sizes, their opposite carrier type and the previous report of a diverging effective mass, we discuss the possibility of a secondary Fermi surface instability at low dopings of the excitonic insulator type, associated with the metal-insulator QCP.

cond-mat.supr-con↗
arXiv2009

Fermi liquid behaviour in an underdoped high Tc superconductor

We use magnetic quantum oscillations in the underdoped high Tc superconductor YBa_2Cu_3O_(6+x) (x=0.56) measured over a broad range of temperatures 100 mK<T<18 K to extract the form of the distribution function describing the low-lying quasiparticle excitations in high magnetic fields. Despite the proximity of YBa_2Cu_3O_(6+x) (x=0.56) to a Mott insulating state, various broken symmetry ground states and/or states with different quasiparticle statistics, we find that our experimental results can be understood in terms of quasiparticle excitations obeying Fermi-Dirac statistics as in the Landau Fermi liquid theory.

cond-mat.supr-con↗
arXiv2009

Dirac nodal pockets in the antiferromagnetic parent phase of FeAs superconductors

We show that previously measured small Fermi surface pockets within the antiferromagnetic phase of SrFe2As2 and BaFe2As2 are consistent with a Dirac dispersion modulated by interlayer hopping, giving rise to a Dirac point in k-space and a cusp in the magnetic field angle-dependent magnetic quantum oscillation frequencies. These findings support the existence of a nodal spin-density wave in these materials, which could play an important role in protecting the metallic state against localization effects. The speed of the Dirac fermions in SrFe2As2 and BaFe2As2 is found to be 14-20 times slower than in graphene, suggesting that the pnictides provide a laboratory for exploring the effects of strongly interacting Dirac fermions.

cond-mat.str-el↗
arXiv2009

Metal-insulator quantum critical point beneath the high Tc superconducting dome

An enduring question in correlated systems concerns whether superconductivity is favoured at a quantum critical point (QCP) characterised by a divergent quasiparticle effective mass. Despite such a scenario being widely postulated in high Tc cuprates and invoked to explain non-Fermi liquid transport signatures, experimental evidence is lacking for a critical divergence under the superconducting dome. We use ultra-strong magnetic fields to measure quantum oscillations in underdoped YBa2Cu3O6+x, revealing a dramatic doping-dependent upturn in quasiparticle effective mass at a critical metal-insulator transition beneath the superconducting dome. Given the location of this QCP under a plateau in Tc in addition to a postulated QCP at optimal doping, we discuss the intriguing possibility of two intersecting superconducting subdomes, each centred at a critical Fermi surface instability.

cond-mat.str-el↗
arXiv2009

Spin-order driven Fermi surface revealed by quantum oscillations in an underdoped high Tc superconductor

We use quantum oscillation measurements to distinguish between spin and orbital components of the lowest energy quasiparticle excitations in YBa2Cu3O6.54, each of which couple differently to a magnetic field. Our measurements reveal the phase of the observed quantum oscillations to remain uninverted over a wide angular range, indicating that the twofold spin degeneracy of the Landau levels is virtually unaltered by the magnetic field. The inferred suppression of the spin degrees of freedom indicates a spin density-wave is responsible for creation of the small Fermi surface pockets in underdoped YBa2Cu3O6+x - further suggesting that excitations of this phase are important contributors to the unconventional superconducting pairing mechanism.

cond-mat.supr-con↗
arXiv2009

Determining the in-plane Fermi surface topology in underdoped high Tc superconductors using angle-dependent magnetic quantum oscillations

We propose a quantum oscillation experiment by which the rotation of an underdoped YBa2Cu3O6+x sample about two different axes with respect to the orientation of the magnetic field can be used to infer the shape of the in-plane cross-section of corrugated Fermi surface cylinder(s). Deep corrugations in the Fermi surface are expected to give rise to nodes in the quantum oscillation amplitude that depend on the magnitude and orientation of the magnetic induction B. Because the symmetry of electron and hole cyclinders within the Brillouin zone are expected to be very different, the topology can provide essential clues as to the broken symmetry responsible for the observed oscillations.

cond-mat.supr-con↗
arXiv2009

2D XY Behavior observed in quasi-2D quantum Heisenberg antiferromagnets

The magnetic properties of a new family of molecular-based quasi-two dimension $S=1/2$ Heisenberg antiferromagnets are reported. Three compounds, ($Cu(pz)_2(ClO_4)_2$, $Cu(pz)_2(BF_4)_2$, and $[Cu(pz)_2(NO_3)](PF_6)$) contain similar planes of Cu$^{2+}$ ions linked into magnetically square lattices by bridging pyrazine molecules (pz = $C_4H_4N_2$). The anions provide charge balance as well as isolation between the layers. Single crystal measurements of susceptibility and magnetization, as well as muon spin relaxation studies, reveal low ratios of Néel temperatures to exchange strengths ($4.25 / 17.5 = 0.243$, $3.80/15.3=0.248$, and $3.05/10.8=0.282$, respectively) while the ratio of the anisotropy fields $H_A$ (kOe) to the saturation field $H_\mathrm{SAT}$ (kOe) are small ($2.6/490 = 5.3\times10^{-3}$, $2.4/430=5.5\times10^{-3}$, and $0.07/300=2.3\times10^{-4}$, respectively), demonstrating close approximations to a 2D Heisenberg model. The susceptibilities of ClO$_4$ and BF$_4$ show evidence of an exchange anisotropy crossover (Heisenberg to $XY$) at low temperatures; their ordering transitions are primarily driven by the $XY$ behavior with the ultimate 3D transition appearing parasitically. The PF$_6$ compound remains Heisenberg-like at all temperatures, with its transition to the Néel state due to the interlayer interactions. Effects of field-induced anisotropy have been observed.

cond-mat.mtrl-sci↗
arXiv2009

Heavy holes: precursor to superconductivity in antiferromagnetic CeIn3

Numerous phenomenological parallels have been drawn between f- and d- electron systems in an attempt to understand their display of unconventional superconductivity. The microscopics of how electrons evolve from participation in large moment antiferromagnetism to superconductivity in these systems, however, remains a mystery. Knowing the origin of Cooper paired electrons in momentum space is a crucial prerequisite for understanding the pairing mechanism. Of especial interest are pressure-induced superconductors CeIn3 and CeRhIn5 in which disparate magnetic and superconducting orders apparently coexist - arising from within the same f-electron degrees of freedom. Here we present ambient pressure quantum oscillation measurements on CeIn3 that crucially identify the electronic structure - potentially similar to high temperature superconductors. Heavy pockets of f-character are revealed in CeIn3, undergoing an unexpected effective mass divergence well before the antiferromagnetic critical field. We thus uncover the softening of a branch of quasiparticle excitations located away from the traditional spin-fluctuation dominated antiferromagnetic quantum critical point. The observed Fermi surface of dispersive f-electrons in CeIn3 could potentially explain the emergence of Cooper pairs from within a strong moment antiferromagnet.

cond-mat.str-el↗
arXiv2009

Spin-density wave Fermi surface reconstruction in underdoped YBa2Cu3O6+x

We consider the reconstruction expected for the Fermi surface of underdoped YBa2Cu3O6+x in the case of a collinear spin-density wave with a characteristic vector Q=(pi[1+/-2 delta],pi), assuming an incommensurability delta~0.06 similar to that found in recent neutron scattering experiments. A Fermi surface possibly consistent with the multiple observed quantum oscillation frequencies is obtained. From the low band masses expected using this model as compared with experiment, a uniform enhancement of the quasiparticle effective mass over the Fermi surface by a factor of ~7 is indicated. Further predictions of the Fermi surface topology are made, which may potentially be tested by experiment to indicate the relevance of this model to underdoped YBa2Cu3O6+x.

cond-mat.str-el↗
arXiv2009

Quantum oscillations in antiferromagnetic CaFe2As2 on the brink of superconductivity

We report quantum oscillation measurements on CaFe2As2 under strong magnetic fields- recently reported to become superconducting under pressures of as little as a kilobar. The largest observed carrier pocket occupies less than 0.05 % of the paramagnetic Brillouin zone volume- consistent with Fermi surface reconstruction caused by antiferromagnetism. On comparing several alkali earth AFe2As2 antiferromagnets (with A=Ca,Sr and Ba), the dependence of both the Fermi surface cross-sectional area F_alpha and the effective mass m*_alpha of the primary observed pocket on the antiferromagnetic/structural transition temperature T_s is found to be consistent with quasiparticles in a conventional spin-density wave model. These findings suggest that a conventional spin-density wave exists within close proximity to superconductivity in this series of compounds, which may have implications for the microscopic origin of unconventional pair formation.

cond-mat.supr-con↗
arXiv2008

The role of f -electrons in the Fermi surface of the heavy fermion superconductor beta-YbAlB4

We present a detailed quantum oscillation study of the Fermi surface of the recently discovered Yb-based heavy fermion superconductor beta-YbAlB4 . We compare the data, obtained at fields from 10 to 45 Tesla, to band structure calculations performed using the local density approximation. Analysis of the data suggests that f-holes participate in the Fermi surface up to the highest magnetic fields studied. We comment on the significance of these findings for the unconventional superconducting properties of this material.

cond-mat.str-el↗
arXiv2008

Quantum oscillations in the parent magnetic phase of an iron arsenide high temperature superconductor

We report quantum oscillation measurements in SrFe2As2 - which is an antiferromagnetic parent of the iron-arsenide family of superconductors - known to become superconducting under doping and the application of pressure. The magnetic field and temperature dependences of the oscillations between 20 and 55 T in the liquid helium temperature range suggest that the electronic excitations are those of a Fermi liquid. We show that the observed Fermi surface comprising small pockets is consistent with the formation of a spin-density wave. Our measurements thus demonstrate that high Tc superconductivity can occur on doping or pressurizing a conventional metallic spin-density wave state.

cond-mat.supr-con↗
arXiv2008

Observation of a multiferroic critical end point

The study of abrupt increases in magnetization with magnetic field known as metamagnetic transitions has opened a rich vein of new physics in itinerant electron systems, including the discovery of quantum critical end points with a marked propensity to develop new kinds of order. However, the electric analogue of the metamagnetic critical end point, a "metaelectric" critical end point has not yet been realized. Multiferroic materials wherein magnetism and ferroelectricity are cross-coupled are ideal candidates for the exploration of this novel possibility using magnetic-field (\emph{H}) as a tuning parameter. Herein, we report the discovery of a magnetic-field-induced metaelectric transition in multiferroic BiMn$_{2}$O$_{5}$ in which the electric polarization (\emph{P}) switches polarity along with a concomitant Mn spin-flop transition at a critical magnetic field \emph{H}$_{\rm c}$. The simultaneous metaelectric and spin-flop transitions become sharper upon cooling, but remain a continuous crossover even down to 0.5 K. Near the \emph{P}=0 line realized at $μ_{0}$\emph{H}$_{\rm c}$$\approx$18 T below 20 K, the dielectric constant ($\varepsilon$) increases significantly over wide field- and temperature (\emph{T})-ranges. Furthermore, a characteristic power-law behavior is found in the \emph{P}(\emph{H}) and $\varepsilon$(\emph{H}) curves at \emph{T}=0.66 K. These findings indicate that a magnetic-field-induced metaelectric critical end point is realized in BiMn$_2$O$_5$ near zero temperature.

cond-mat.mtrl-sci↗