arXiv subjects

N. Harrison

Publications and source records attributed to N. Harrison.

At least 55 records · Page 3Linked to original sources

arXiv2016

Target and beam-target spin asymmetries in exclusive pion electroproduction for $Q^2>1$ GeV$^2$. II. $e p \rightarrow e π^0 p$

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive $π^0$ electroproduction reaction $γ^* p \to p π^0$, expanding an analysis of the $γ^* p \to n π^+$ reaction from the same experiment. The results were obtained from scattering of 6 GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Lab. The kinematic range covered is $1.1<W<3$ GeV and $1<Q^2<6$ GeV$^2$. Results were obtained for about 5700 bins in $W$, $Q^2$, \cthcm, and $ϕ^*$. The beam-target asymmetries were found to generally be greater than zero, with relatively modest \phicmsp dependence. The target asymmetries exhibit very strong \phicmsp dependence, with a change in sign occurring between results at low $W$ and high $W$, in contrast to $π^+$ electroproduction. Reasonable agreement is found with phenomenological fits to previous data for $W<1.6$ GeV, but significant differences are seen at higher $W$. When combined with cross section measurements, as well as $π^+$ observables, the present results will provide powerful constraints on nucleon resonance amplitudes at moderate and large values of $Q^2$, for resonances with masses as high as 2.4 GeV.

nucl-ex↗

arXiv2016

Measurement of Target and Double-spin Asymmetries for the $\vec e\vec p\to eπ^+ (n)$ Reaction in the Nucleon Resonance Region at Low $Q^2$

We report measurements of target- and double-spin asymmetries for the exclusive channel $\vec e\vec p\to eπ^+ (n)$ in the nucleon resonance region at Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS). These asymmetries were extracted from data obtained using a longitudinally polarized NH$_3$ target and a longitudinally polarized electron beam with energies 1.1, 1.3, 2.0, 2.3 and 3.0 GeV. The new results are consistent with previous CLAS publications but are extended to a low $Q^2$ range from $0.0065$ to $0.35$ (GeV$/c$)$^2$. The $Q^2$ access was made possible by a custom-built Cherenkov detector that allowed the detection of electrons for scattering angles as low as $6^\circ$. These results are compared with the unitary isobar models JANR and MAID, the partial-wave analysis prediction from SAID and the dynamic model DMT. In many kinematic regions our results, in particular results on the target asymmetry, help to constrain the polarization-dependent components of these models.

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arXiv2016

Target and Beam-Target Spin Asymmetries in Exclusive $π^+$ and $π^-$ Electroproduction with 1.6 to 5.7 GeV Electrons

Beam-target double spin asymmetries and target single-spin asymmetries in exclusive $π^+$ and $π^-$ electroproduction were obtained from scattering of 1.6 to 5.7 GeV longitudinally polarized electrons from longitudinally polarized protons (for $π^+$) and deuterons (for $π^-$) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The kinematic range covered is $1.1 1.5$ GeV. Very large target-spin asymmetries are observed for $W>1.6$ GeV. When combined with cross section measurements, the present results will provide powerful constraints on nucleon resonance amplitudes at moderate and large values of $Q^2$, for resonances with masses as high as 2.3 GeV.

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arXiv2016

Magnetic anisotropy in the frustrated spin chain compound $β$-TeVO$_4$

Isotropic and anisotropic magnetic behavior of the frustrated spin chain compound $β$-TeVO$_4$ is reported. Three magnetic transitions observed in zero magnetic field are tracked in fields applied along different crystallographic directions using magnetization, heat capacity, and magnetostriction measurements. Qualitatively different temperature-field diagrams are obtained below 10 T for the field applied along $a$ or $b$ and along $c$, respectively. In contrast, a nearly isotropic high-field phase emerges above 18 T and persists up to the saturation that occurs around 22.5 T. Upon cooling in low fields, the transitions at $T_{\rm N1}$ and $T_{\rm N2}$ toward the spin-density-wave and stripe phases are of the second order, whereas the transition at $T_{\rm N3}$ toward the helical state is of the first order and entails a lattice component. Our microscopic analysis identifies frustrated $J_1-J_2$ spin chains with a sizable antiferromagnetic interchain coupling in the $bc$ plane and ferromagnetic couplings along the $a$ direction. The competition between these ferromagnetic interchain couplings and the helical order within the chain underlies the incommensurate order along the $a$-direction, as observed experimentally. Although a helical state is triggered by the competition between $J_1$ and $J_2$ within the chain, the plane of the helix is not uniquely defined because of competing magnetic anisotropies. Using high-resolution synchrotron diffraction and $^{125}$Te nuclear magnetic resonance, we also demonstrate that the crystal structure of $β$-TeVO$_4$ does not change down to 10 K, and the orbital state of V$^{4+}$ is preserved.

cond-mat.mtrl-sci↗

arXiv2016

Broken rotational symmetry on the Fermi surface of a high-T$_\mathrm{c}$ superconductor

Broken fourfold rotational (C$_4$) symmetry is observed in the experimental properties of several classes of unconventional superconductors. It has been proposed that this symmetry breaking is important for superconducting pairing in these materials, but in the high superconducting transition temperature (high-T$_{\mathrm{c}}$) cuprates this broken symmetry has never been observed on the Fermi surface. We have measured a pronounced anisotropy in the angle dependence of the interlayer magnetoresistance of the underdoped high-T$_{\mathrm{c}}$) superconductor YBa$_2$Cu$_3$O$_{6.58}$, directly revealing broken C$_4$ symmetry on the Fermi surface. Moreover, we demonstrate that this Fermi surface has C$_2$ symmetry of the type produced by a uniaxial or anisotropic density-wave phase. This establishes the central role of C$_4$ symmetry breaking in the Fermi surface reconstruction of YBa$_2$Cu$_3$O$_{6+δ}$, and suggests a striking degree of universality among unconventional superconductors.

cond-mat.supr-con↗

arXiv2016

Single reconstructed Fermi surface pocket in an underdoped single layer cuprate superconductor

The observation of a reconstructed Fermi surface via quantum oscillations in hole-doped cuprates opened a path towards identifying broken symmetry states in the pseudogap regime. However, such an identification has remained inconclusive due to the multi-frequency quantum oscillation spectra and complications accounting for bilayer effects in most studies. We overcome these impediments with high resolution measurements on the structurally simpler cuprate HgBa2CuO4+d (Hg1201), which features one CuO2 plane per unit cell. We find only a single oscillatory component with no signatures of magnetic breakdown tunneling to additional orbits. Therefore, the Fermi surface comprises a single quasi-two-dimensional pocket. Quantitative modeling of these results indicates that biaxial charge-density-wave within each CuO2 plane is responsible for the reconstruction, and rules out criss-crossed charge stripes between layers as a viable alternative in Hg1201. Lastly, we determine that the characteristic gap between reconstructed pockets is a significant fraction of the pseudogap energy.

cond-mat.supr-con↗

arXiv2016

The number of holes contained within the Fermi surface volume in underdoped high temperature superconductors

We bring resolution to the longstanding problem relating Fermi surface reconstruction to the number of holes contained within the Fermi surface volume in underdoped high Tc superconductors. On considering uniaxial and biaxial charge-density wave order, we show that there exists a relationship between the ordering wave vector, the hole doping and the cross-sectional area of the reconstructed Fermi surface whose precise form depends on the volume of the starting Fermi surface. We consider a `large' starting Fermi surface comprising 1+p hole carriers, as predicted by band structure calculations, and a `small' starting Fermi surface comprising p hole carriers, as proposed in models in which the Coulomb repulsion remains the dominant energy. Using the reconstructed Fermi surface cross-sectional area obtained in quantum oscillation experiments in YBa2Cu3O6+x and HgBa2CuO4+x and the established methods for estimating the chemical hole doping, we find the ordering vectors obtained from x-ray scattering measurements to show a close correspondence with those expected for the small starting Fermi surface. We therefore show the Coulomb repulsion to remain largely unscreened throughout the entire underdoped regime where the pseudogap exists and further show that the quantum oscillation frequency and charge-density wave vectors provide accurate estimates for the number of holes contributing to the Fermi surface volume.

cond-mat.str-el↗

arXiv2015

Magnetization of underdoped YBa$_2$Cu$_3$O$_{y}$ above the irreversibility field

Torque magnetization measurements on YBa$_2$Cu$_3$O$_{y}$ (YBCO) at doping $y=6.67$($p=0.12$), in DC fields ($B$) up to 33 T and temperatures down to 4.5 K, show that weak diamagnetism persists above the extrapolated irreversibility field $H_{\rm irr} (T=0) \approx 24$ T. The differential susceptibility $dM/dB$, however, is more rapidly suppressed for $B\gtrsim 16$ T than expected from the properties of the low field superconducting state, and saturates at a low value for fields $B \gtrsim 24$ T. In addition, torque measurements on a $p=0.11$ YBCO crystal in pulsed field up to 65 T and temperatures down to 8 K show similar behaviour, with no additional features at higher fields. We discuss several candidate scenarios to explain these observations: (a) superconductivity survives but is heavily suppressed at high field by competition with CDW order; (b) static superconductivity disappears near 24 T and is followed by a region of fluctuating superconductivity, which causes $dM/dB$ to saturate at high field; (c) the stronger 3D ordered CDW that sets in above 15 T may suppress the normal state spin susceptibility sufficiently to give an apparent diamagnetism of the magnitude observed.

cond-mat.supr-con↗

arXiv2015

Successive magnetic field-induced transitions and colossal magnetoelectric effect in Ni$_{3}$TeO$_{6}$

We report the discovery of a metamagnetic phase transition in a polar antiferromagnet Ni$_3$TeO$_6$ that occurs at 52 T. The new phase transition accompanies a colossal magnetoelectric effect, with a magnetic-field-induced polarization change of 0.3 $μ$C/cm$^2$, a value that is 4 times larger than for the spin-flop transition at 9 T in the same material, and also comparable to the largest magnetically-induced polarization changes observed to date. Via density-functional calculations we construct a full microscopic model that describes the data. We model the spin structures in all fields and clarify the physics behind the 52 T transition. The high-field transition involves a competition between multiple different exchange interactions which drives the polarization change through the exchange-striction mechanism. The resultant spin structure is rather counter-intuitive and complex, thus providing new insights on design principles for materials with strong magnetoelectric coupling.

cond-mat.mtrl-sci↗

arXiv2015

Tunable excitonic insulator in quantum limit graphite

Half a century ago, Mott noted that tuning the carrier density of a semimetal towards zero produces an insulating state in which electrons and holes form bound pairs. It was later argued that such pairing persists even if a semiconducting gap opens in the underlying band structure, giving rise to what has become known as the strong coupling limit of an `excitonic insulator.' While these `weak' and `strong' coupling extremes were subsequently proposed to be manifestations of the same excitonic state of electronic matter, the predicted continuity of such a phase across a band gap opening has not been realized experimentally in any material. Here we show the quantum limit of graphite, by way of temperature and angle-resolved magnetoresistance measurements, to host such an excitonic insulator phase that evolves continuously between the weak and strong coupling limits. We find that the maximum transition temperature T_EI of the excitonic phase is coincident with a band gap opening in the underlying electronic structure at B_0= 46 +/- 1 T, which is evidenced above T_EI by a thermally broadened inflection point in the magnetoresistance. The overall asymmetry of the observed phase boundary around B_0 closely matches theoretical predictions of a magnetic field-tuned excitonic insulator phase in which the opening of a band gap marks a crossover from predominantly momentum-space pairing to real-space pairing.

cond-mat.mtrl-sci↗

arXiv2015

Fragile charge order in the non-superconducting ground state of the underdoped high temperature superconductors

The normal state in the hole underdoped copper oxide superconductors has proven to be a source of mystery for decades. The measurement of a small Fermi surface by quantum oscillations on suppression of superconductivity by high applied magnetic fields, together with complementary spectroscopic measurements in the hole underdoped copper oxide superconductors, point to a nodal electron pocket from charge order in YBa2Cu3O6+x. Here we report quantum oscillation measurements in the closely related stoichiometric material YBa2Cu4O8, which reveal similar Fermi surface properties to YBa2Cu3O6+x, despite an absence of charge order signatures in the same spectroscopic techniques such as x-ray diffraction that revealed signatures of charge order in YBa2Cu3O6+x. Fermi surface reconstruction in YBa2Cu4O8 is suggested to occur from magnetic field enhancement of charge order that is rendered fragile in zero magnetic fields because of its potential unconventional symmetry, and/or its occurrence as a subsidiary to more robust underlying electronic correlations.

cond-mat.supr-con↗

arXiv2015

Normal-state nodal electronic structure in underdoped high-Tc copper oxides

An outstanding problem in the field of high-transition-temperature (high Tc) superconductivity is the identification of the normal state out of which superconductivity emerges in the mysterious underdoped regime. The normal state uncomplicated by thermal fluctuations is effectively accessed by the use of applied magnetic fields sufficiently strong to suppress long-range superconductivity at low temperatures. Proposals in which the normal ground state is characterised by small Fermi surface pockets that exist in the absence of symmetry breaking have been superseded by models based on the existence of a superlattice that breaks the translational symmetry of the underlying lattice. Recently, a charge superlattice model that positions a small electron-like Fermi pocket in the vicinity of the nodes (where the superconducting gap is minimum) has been proposed a replacement for the prevalent superlattice models that position the Fermi pocket in the vicinity of the pseudogap at the antinodes (where the superconducting gap is maximum). Although some ingredients of symmetry breaking have been recently revealed by crystallographic studies, their relevance to the electronic structure remains unresolved. Here we report angle-resolved quantum oscillation measurements in the underdoped copper oxide YBa2Cu3O6+x. These measurements reveal a normal ground state comprising electron-like Fermi surface pockets located in the vicinity of the superconducting gap minima (or nodes), and further point to an underlying superlattice structure of low frequency and long wavelength with features in common with the charge order identified recently by complementary spectroscopic techniques.

cond-mat.supr-con↗

arXiv2015

Unconventional Fermi surface in an insulating state

Insulators occur in more than one guise, a recent finding was a class of topological insulators, which host a conducting surface juxtaposed with an insulating bulk. Here we report the observation of an unusual insulating state with an electrically insulating bulk that simultaneously yields bulk quantum oscillations with characteristics of an unconventional Fermi liquid. We present quantum oscillation measurements of magnetic torque in high purity single crystals of the Kondo insulator SmB6, which reveal quantum oscillation frequencies characteristic of a large three-dimensional conduction electron Fermi surface similar to the metallic rare earth hexaborides such as PrB6 and LaB6. The quantum oscillation amplitude strongly increases at low temperatures, appearing strikingly at variance with conventional metallic behaviour.

cond-mat.str-el↗

arXiv2015

Precise Determination of the Deuteron Spin Structure at Low to Moderate $Q^2$ with CLAS and Extraction of the Neutron Contribution

We present the final results for the deuteron spin structure functions obtained from the full data set collected with Jefferson Lab's CLAS in 2000-2001. Polarized electrons with energies of 1.6, 2.5, 4.2 and 5.8 GeV were scattered from deuteron ($^{15}$ND$_3$) targets, dynamically polarized along the beam direction, and detected with CLAS. From the measured double spin asymmetry, the virtual photon absorption asymmetry $A_1^d$ and the polarized structure function $g_1^d$ were extracted over a wide kinematic range (0.05 GeV$^2 < Q^2 <$ 5 GeV$^2$ and 0.9 GeV $< W <$ 3 GeV). We use an unfolding procedure and a parametrization of the corresponding proton results to extract from these data the polarized structure functions $A_1^n$ and $g_1^n$ of the (bound) neutron, which are so far unknown in the resonance region, $W < 2$ GeV. We compare our final results, including several moments of the deuteron and neutron spin structure functions, with various theoretical models and expectations as well as parametrizations of the world data. The unprecedented precision and dense kinematic coverage of these data can aid in future extractions of polarized parton distributions, tests of perturbative QCD predictions for the quark polarization at large $x$, a better understanding of quark-hadron duality, and more precise values for higher-twist matrix elements in the framework of the Operator Product Expansion.

nucl-ex↗

arXiv2015

Quantum oscillation signatures of nodal spin-orbit coupling in underdoped bilayer high Tc cuprates

The highest superconducting transition temperatures in the cuprates are achieved in bilayer and trilayer systems, highlighting the importance of intralayer interactions for high Tc. It has been argued that interlayer hybridization vanishes along the nodal directions by way of a specific pattern of orbital overlap. Recent quantum oscillation measurements in bilayer cuprates have provided evidence for a residual bilayer-splitting at the nodes that is sufficiently small to enable magnetic breakdown tunneling at the nodes. Here we show that several key features of the experimental data can be understood in terms weak spin-orbit interactions naturally present in bilayer systems, whose primary effect is to cause the magnetic breakdown to be accompanied by a spin flip. These features can now be understood include the equidistant set of three quantum oscillation frequencies, the asymmetry of the quantum oscillation amplitudes in c-axis transport compared to ab-plane transport, and the anomalous magnetic field angle dependence of the amplitude of side frequencies suggestive of small effective g-factors. We suggest that spin-orbit interactions in bilayer systems can further affect the structure of the nodal quasiparticle spectrum in the superconducting phase.

cond-mat.supr-con↗

arXiv2015

Quasiparticle mass enhancement approaching optimal doping in a high-Tc superconductor

In the quest for superconductors with high transition temperatures (T$_\mathrm{c}$s), one emerging motif is that unconventional superconductivity is enhanced by fluctuations of a broken-symmetry phase near a quantum-critical point. While recent experiments have suggested the existence of the requisite broken symmetry phase in the high-T$_\mathrm{c}$ cuprates, the signature of quantum-critical fluctuations in the electronic structure has thus far remained elusive, leaving their importance for high-T$_\mathrm{c}$ superconductivity in question. We use magnetic fields exceeding 90 tesla to access the underlying metallic state of the cuprate YBa2Cu3O6+$_δ$ over an unprecedented range of doping, and magnetic quantum oscillations reveal a strong enhancement in the quasiparticle effective mass toward optimal doping. This mass enhancement is a characteristic signature of quantum criticality, and identifies a quantum-critical point at p$_{crit}$ $\approx$ 0.18. This point also represents the juncture of the vanishing pseudogap energy scale and the disappearance of Kerr rotation, the negative Hall coefficient, and the recently observed charge order, suggesting a mechanism of high-T$_\mathrm{c}$ that is strongest when these definitive experimental signatures of the underdoped cuprates converge at a quantum critical point.

cond-mat.supr-con↗

arXiv2015

Determination of the Beam-Spin Asymmetry of Deuteron Photodisintegration in the Energy Region $E_γ=1.1-2.3$ GeV

The beam-spin asymmetry, $Σ$, for the reaction $γd\rightarrow pn$ has been measured using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility (JLab) for six photon-energy bins between 1.1 and 2.3 GeV, and proton angles in the center-of-mass frame, $θ_{c.m.}$, between $25^\circ$ and $160^\circ$. These are the first measurements of beam-spin asymmetries at $θ_{c.m.}=90^\circ$ for photon-beam energies above 1.6 GeV, and the first measurements for angles other than $θ_{c.m.}=90^\circ$. The angular and energy dependence of $Σ$ is expected to aid in the development of QCD-based models to understand the mechanisms of deuteron photodisintegration in the transition region between hadronic and partonic degrees of freedom, where both effective field theories and perturbative QCD cannot make reliable predictions.

nucl-ex↗

arXiv2015

First Measurement of the Polarization Observable E in the $\vec p(\vec γ,π^+)n$ Reaction up to 2.25 GeV

First results from the longitudinally polarized frozen-spin target (FROST) program are reported. The double-polarization observable E, for the reaction $\vec γ\vec p \to π^+n$, has been measured using a circularly polarized tagged-photon beam, with energies from 0.35 to 2.37 GeV. The final-state pions were detected with the CEBAF Large Acceptance Spectrometer in Hall B at the Thomas Jefferson National Accelerator Facility. These polarization data agree fairly well with previous partial-wave analyses at low photon energies. Over much of the covered energy range, however, significant deviations are observed, particularly in the high-energy region where high-L multipoles contribute. The data have been included in new multipole analyses resulting in updated nucleon resonance parameters. We report updated fits from the Bonn-Gatchina, Jülich, and SAID groups.

nucl-ex↗