Neil Sullivan - Academia.edu (original) (raw)

Papers by Neil Sullivan

$Research paper thumbnail of Magnetic Susceptibility of a Dirty BEC System Ni $ _ {0.85} $ Cd $ _ {0.15} <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>−</mo><mn>4</mn><mi>S</mi><mi>C</mi><mo stretchy="false">(</mo><mi>N</mi><mi>H</mi></mrow><annotation encoding="application/x-tex">-4SC (NH </annotation></semantics></math>−4SC(NH _ {2}) _ {2} $$

report measurements of the magnetic susceptibility of a dirty BEC system of magnons: Ni 0.85 Cd 0... more report measurements of the magnetic susceptibility of a dirty BEC system of magnons: Ni 0.85 Cd 0.15-4SC(NH 2) 2 (The 15%Cd sitediluted DTN) was studied down to 10 mK. Below 100 mK, the critical fields of BEC H c do not obey the conventional 3D universality class H c (T)-H c (0) ∼ T α , where α = 1.5. The observed scaling relation of α = 1/1.15 below 100 mK is in agreement with the numerical simulation of Bose glass for this system.

$Research paper thumbnail of Investigation of the magnetic susceptibility of the disordered BEC system NiCl $ _ {0.85} $ Br $ _ {0.15} <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>−</mo><mn>4</mn><mi>S</mi><mi>C</mi><mo stretchy="false">(</mo><mi>N</mi><mi>H</mi></mrow><annotation encoding="application/x-tex">-4SC (NH </annotation></semantics></math>−4SC(NH _ {2}) _ {2} $ at ultralow-temperatures$

Submitted for the MAR11 Meeting of The American Physical Society Investigation of the magnetic su... more Submitted for the MAR11 Meeting of The American Physical Society Investigation of the magnetic susceptibility of the disordered BEC system NiCl 0.85 Br 0.15-4SC(NH 2) 2 at ultralow-temperatures LIANG YIN, CHAO HUAN, JIAN

$Research paper thumbnail of Magnetic Bose glass in Br-doped NiCl $ _ {2} <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>−</mo><mn>4</mn><mi>S</mi><mi>C</mi><mo stretchy="false">(</mo><mi>N</mi><mi>H</mi></mrow><annotation encoding="application/x-tex">-4SC (NH </annotation></semantics></math>−4SC(NH _ {2}) _ {2} $: Magneto-Electric Effect$

1.85 Br 0.15-4SC(NH 2) 2 is a candidate for Bose glass state of the magnetism at low temperatures... more 1.85 Br 0.15-4SC(NH 2) 2 is a candidate for Bose glass state of the magnetism at low temperatures, which is the bosonic analog of Anderson localization. Here we explore the glassy dynamics of the magnetization taking advantage of magneto-electric coupling in this material. We report measurements of the magnetoelectric effect in NiCl 1.85 Br 0.15-4SC(NH 2) 2 using two different experimental setups: (i) magnetic field-induced polarization and (ii) electric-field-induced magnetization. Both measurements show that the proposed Bose glass state in a high magnetic field is sensitive to the applied frequencies of AC magnetic and electric fields, with a maximum response between 700Hz and 1000Hz. This frequency-dependent behavior could provide information about the characteristic response time and the size of magnetic clusters.

Physical Review B, 1998

We report NMR studies of the orientational ordering of molecular hydrogen in films physisorbed on... more We report NMR studies of the orientational ordering of molecular hydrogen in films physisorbed on hexagonal boron nitride ͑BN͒. The local orientational ordering of the molecular axes is determined for both thick films ͑12 layers͒ and thin films ͑bilayers͒. The ordering observed is in both cases very different from that observed for bulk solid hydrogen. Quadrupolar glass ordering occurs for intermediate layers in thick films at low ortho concentrations 0.25ϽXϽ0.40 and Tр0.7 K. The ortho molecules remain rotationally disordered in the outer layers of the thick films and invariant to small changes in thickness. In the bilayers one observes a long range ordering for Tр1.95 K and Xр0.56, and a quadrupolar glass ordering for intermediate concentrations (0.37ϽXϽ0.55) and intermediate temperatures (0.7ϽTϽ1.1 K). ͓S0163-1829͑98͒09019-5͔

Advances in Cryogenic Engineering, 1994

The National High Magnetic Field Laboratory (NHMFL) develops and operates high magnetic field fac... more The National High Magnetic Field Laboratory (NHMFL) develops and operates high magnetic field facilities at its main location at Florida State University, Tallahassee, as well as a pulsed magnetic field facility at Los Alamos National Laboratory. A number of specialized facilities are also available to collaborators at the University of Florida for research at ultra-low temperatures, advanced magnetic resonance imaging, and materials sciences. The NHMFL is supported by the United States National Science Foundation (NSF) and by the State of Florida. It is a user facility available to qualified users through a peer review proposal process. The ., facilities and staffsupport research and development at the extremes of parameter space. A part of its activities is devoted to the advancement of the state of the art of superconducting, pulsed, resistive, and l',ybrid magnets. This involves cryogenic materials research, the development of high strength, high conductivity conductors, and the development of low and ultra low temperature systems.

Physical Review B, 1997

We report the results of NMR studies of the two-dimensional ͑2D͒ phase diagrams for molecular H 2... more We report the results of NMR studies of the two-dimensional ͑2D͒ phase diagrams for molecular H 2 films ͑monolayers and submonolayers͒ adsorbed on BN for ortho-H 2 concentrations 0.3ϽXϽ0.745 and temperatures 0.05ϽTϽ4.2 K. A 2D quadrupolar glass state has been identified for submonolayer coverages for 0.48ϽXϽ0.69 and 0.2рTр0.6 K. For submonolayer films ͑75% of full monolayer͒, an orientationally ordered phase ͑expected pinwheel structure͒ was observed for Xу0.69 with a transition to a second distinct ordered phase ͑expected herringbone structure͒ for Xр0.69. ͓S0163-1829͑97͒50702-8͔

It was reported by several research groups that the recently discovered possible supersolid state... more It was reported by several research groups that the recently discovered possible supersolid state of solid 4 He has strong correlations with the presence of 3 He impurities and other defects. These observations aroused new interest in the NMR study of solid 4 He doped with 3 He. However, the extremely low concentration of 3 He (less than 100 ppm) and the long nuclear spin lattice relaxation times in the interested regime lead to a challenging task to achieve detectable NMR signals. We present a new design for a low temperature preamplifier working with a crossed-coil NMR probe. This preamplifier reduces the noise temperature to 1 K at a sample temperature as low as 250 mK when it is thermally anchored to the mixing chamber of a dilution refrigerator. With an adapted design, we are able to cool down the sample to about 10 mK and keep the noise temperature well below 4 K.

Princeton University-The 12/5 state has attracted growing interest due to its superior potential ... more Princeton University-The 12/5 state has attracted growing interest due to its superior potential in performing universal topological quantum computation. Up to date, except for the observation of a well developed quantum Hall plateau at this filling, much less experimental work has been carried out and there is no experimental evidence to support this state being a paraferminoic or non-Abelian state. Here, we present our tilt magnetic field dependence results in examining its spin-polarization. It was observed that the diagonal resistance R xx at ν=12/5 shows a nonmonotonic dependence on tilt angle (θ). It first increases sharply with increasing θ, reaches a maximal value of ∼ 60 Ω around θ ∼14 o , and then decreases with θ further increased. Correlated with this R xx dependence, the 12/5 activation energy gap (∆ 12/5) also shows a non-monotonic θ dependence. ∆ 12/5 first decreases. Around 14 o , R xx becomes non-activated and a true activation energy gap is not obtainable. With further increasing θ, R xx becomes activated again and ∆ 12/5 increases with θ. This tilt dependence in R xx and ∆ 12/5 is similar to the composite fermion states at ν=2/5 and 8/5 in the lowest Landau level, which was interpreted as a spin transition. Our results thus call for more investigations on the nature of the 12/5 ground state.

We have carried out tilt magnetic field (B) studies of the =12/5 fractional quantum Hall state i... more We have carried out tilt magnetic field (B) studies of the =12/5 fractional quantum Hall state in an ultra-high quality GaAs quantum well specimen. Its diagonal magneto-resistance R xx shows a non-monotonic dependence on tilt angle (). It first increases sharply with increasing , reaches a maximal value of ~ 70  at  ~ 14 o , and then decreases at higher tilt angles. Correlated with this dependence of R xx on , the 12/5 activation energy ( 12/5) also shows a non-monotonic tilt dependence.  12/5 first decreases with increasing . Around  = 14 o ,  12/5 disappears as R xx becomes non-activated. With further increasing tilt angles,  12/5 reemerges and increases with . This tilt B dependence at =12/5 is strikingly different from that of the well-documented 5/2 state and calls for more investigations on the nature of its ground state.

Review of Scientific Instruments, 1998

We report measurements of the characteristics of a junction field effect transistor (JFET) in app... more We report measurements of the characteristics of a junction field effect transistor (JFET) in applied magnetic fields up to 30 T. For orientations parallel to the applied field, the device maintained a high transconductance (gfs) close to 80% of the zero field value. For field orientations perpendicular to the plane of current transport, gfs decreased by a factor of 6 at 30 T. This field dependence can be understood quantitatively in terms of the magnetoresistance associated with the cyclotron motion of current carriers within the conductive channels of the JFET. The results show that for suitable orientations the JFET may be used “in situ” for applications in very high fields.

Physics Letters A, 1999

We report the first audio frequency dielectric constant measurements of CO and N2 in their solid ... more We report the first audio frequency dielectric constant measurements of CO and N2 in their solid phases. We have observed several new features, including (1) strong hysteresis effects above an onset temperature T h ≃ 42 K in the β-phase of pure N2, (2) absence of the expected short range antiferroelectric dipolar ordering in CO down to 4.2 K, and (3) anomalous temperature dependence of the dielectric constant in the α-phases of both CO and N2. Quantum mechanical treatment of the molecular rotation explains some of the observed anomalies in the α-phase, but the strong hysteresis indicates that pure geometrical frustration plays a significant role in the β-phase.

Physical Review Letters, 2014

It is shown that dark matter axions cause an oscillating electric current to flow along magnetic ... more It is shown that dark matter axions cause an oscillating electric current to flow along magnetic field lines. The oscillating current induced in a strong magnetic field B0 produces a small magnetic field Ba. We propose to amplify and detect Ba using a cooled LC circuit and a very sensitive magnetometer. This appears to be a suitable approach to searching for axion dark matter in the 10 −7 to 10 −9 eV mass range.

Physical Review Letters, 2004

Physical Review Letters, 2007

The viscosity is measured for a Fermi liquid, a dilute 3 He-4 He mixture, under extremely high ma... more The viscosity is measured for a Fermi liquid, a dilute 3 He-4 He mixture, under extremely high magnetic field/temperature conditions (B ≤ 14.8 T, T ≥ 1.5 mK). The spin splitting energy µB is substantially greater than the Fermi energy kBTF ; as a consequence the polarization tends to unity and s-wave quasiparticle scattering is suppressed for T ≪ TF. Using a novel composite vibratingwire viscometer an enhancement of the viscosity is observed by a factor of more than 500 over its low-field value. Good agreement is found between the measured viscosity and theoretical predictions based upon a t-matrix formalism.

$Research paper thumbnail of NMR Study of the Dynamics of ^{3}He Impurities in the Proposed Supersolid State of Solid ^{4}He$

Physical Review Letters, 2011

The dynamics of 3 He atoms in solid 4 He have been investigated by measuring the NMR relaxation t... more The dynamics of 3 He atoms in solid 4 He have been investigated by measuring the NMR relaxation times T1, T2 in the region where a significant non-classical rotational inertia fraction (NCRIF) has been reported. For 3 He concentrations x3 = 16 ppm and 24 ppm, changes are observed for both the spin-lattice relaxation time T1 and the spin-spin relaxation time T2 at the temperatures corresponding to the onset of NCRIF and, at lower temperatures, to the 3 He-4 He phase separation. The magnitudes of T1 and T2 at temperatures above the phase separation agree roughly with existing theory based on the tunneling of 3 He impurities in the elastic strain field due to isotopic mismatch. However, a distinct peak in T1 and a less well-resolved feature in T2 are observed near the reported NCRIF onset temperature, in contrast to the temperature-independent relaxation times predicted by the tunneling theory.

Physical Review D, 1990

Ropld Communications are intended for important new results which deserve accelerated publication... more Ropld Communications are intended for important new results which deserve accelerated publication, and are therefore given priority in editorial processing and production A. Rapid Communication in Physical Review D should be no longer thanftve prt'« ed poges and must be accompanied by an abstract Pa. ge proofs are sent to authors, but because of the accelerated schedulep, ubli cation is generally not delayed for receipt of corrections unless requested by the author

$Research paper thumbnail of NMR investigation of the low-temperature dynamics of solid ^{4}He doped with ^{3}He impurities$

Physical Review B, 2013

The lattice dynamics of solid 4 He has been explored using pulsed NMR methods to study the motion... more The lattice dynamics of solid 4 He has been explored using pulsed NMR methods to study the motion of 3 He impurities in the temperature range where experiments have revealed anomalies attributed to superflow or unexpected viscoelastic properties of the solid 4 He lattice. We report the results of measurements of the nuclear spin-lattice and spin-spin relaxation times that measure the fluctuation spectrum at high and low frequencies, respectively, of the 3 He motion that results from quantum tunneling in the 4 He matrix. The measurements were made for 3 He concentrations 16 < x 3 < 2000 ppm. For 3 He concentrations x 3 = 16 ppm and 24 ppm, large changes are observed for both the spin-lattice relaxation time T 1 and the spin-spin relaxation time T 2 at temperatures close to those for which the anomalies are observed in measurements of torsional oscillator responses and the shear modulus. These changes in the NMR relaxation rates were not observed for higher 3 He concentrations.

Physical Review B, 2008

High sensitivity measurements of the dielectric constant of solid oxygen are reported for 4 Ͻ T Ͻ... more High sensitivity measurements of the dielectric constant of solid oxygen are reported for 4 Ͻ T Ͻ 54 K. The results show unexpectedly large hysteresis effects for the temperature dependence of the dielectric constant in the ␣ and ␤ phases of oxygen on thermal cycling below 44 K. The behavior is compared to that observed for solid N 2-Ar mixtures where the geometrical frustration of the molecular orientational ordering leads to pronounced memory effects. In contrast to solid N 2-Ar where the effects of frustration and disorder combine to form an orientational glass, there is no disorder present for pure O 2 and the hystereses are attributed to the strong frustration of the interactions.