Publications


Spin Jahn-Teller antiferromagnetism in CoTi$_2$O$_5$

Physical Review B American Physical Society (0)

FKK Kirschner, RD Johnson, F Lang, DD Khalyavin, P Manuel, T Lancaster, D Prabhakaran, SJ Blundell

We have used neutron powder diffraction to solve the magnetic structure of orthorhombic CoTi$_2$O$_5$, showing that the long-range ordered state below 26 K identified in our muon-spin rotation experiments is antiferromagnetic with propagation vector ${\bf k}=(\pm \frac{1}{2}, \frac{1}{2}, 0)$ and moment of 2.72(1)$\mu_{\rm B}$ per Co$^{2+}$ ion. This long range magnetic order is incompatible with the experimentally determined crystal structure because the imposed symmetry completely frustrates the exchange coupling. We conclude that the magnetic transition must therefore be associated with a spin Jahn-Teller effect which lowers the structural symmetry and thereby relieves the frustration. These results show that CoTi$_2$O$_5$ is a highly unusual low symmetry material exhibiting a purely spin-driven lattice distortion critical to the establishment of an ordered magnetic ground state.


Electric field control of spins in molecular magnets

Physical Review Letters American Physical Society (0)

A ARDAVAN, J LIU, J Mrozek, W MYERS, G Timco, R Winpenny, B Kinzel, W Plass


Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

arXiv:1511.04972v1 (0)

T Lancaster, F Xiao, Z Salman, IO Thomas, SJ Blundell, FL Pratt, SJ Clark, T Prokscha, A Suter, SL Zhang, AA Baker, T Hesjedal

We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40 nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around T ≈ 40 K and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields.


Spin waves and revised crystal structure of honeycomb iridate Na2IrO3

ArXiv (0)

SK Choi, R Coldea, AN Kolmogorov, T Lancaster, II Mazin, SJ Blundell, PG Radaelli, Y Singh, P Gegenwart, KR Choi, S-W Cheong, PJ Baker, C Stock, J Taylor

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zig-zag magnetic order. The onset of long-range magnetic order below 15.3 K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90 deg Ir-O-Ir bonds required for dominant Kitaev exchange.


Intrinsic magnetic order in Cs2AgF4 detected by muon-spin relaxation

PHYSICAL REVIEW B 75 (0) 22

T Lancaster, SJ Blundell, PJ Baker, W Hayes, SR Giblin, SE McLain, FL Pratt, Z Salman, EA Jacobs, JFC Turner, T Barnes

We present the results of a muon-spin relaxation study of the high-T-c analog material Cs2AgF4. We find unambiguous evidence for magnetic order, intrinsic to the material, below T-C=13.95(3) K. The ratio of interplane to intraplane coupling is estimated to be vertical bar J(’)/J vertical bar=1.9x10(-2), while fits of the temperature dependence of the order parameter reveal a critical exponent beta=0.292(3), implying an intermediate character between pure two- and three-dimensional magnetism in the critical regime. Above T-C we observe a signal characteristic of dipolar interactions due to linear F-mu(+)-F bonds, allowing the muon stopping sites in this compound to be characterized.


Observation of quantized circulation in superfluid B3

Physical Review Letters 66 (1991) 329-332

JC Davis, JD Close, R Zieve, RE Packard

We report the first observation of quantized circulation in superfluid B3. The apparatus consists of a straight vibrating wire immersed in liquid He3, which is cooled by a rotating nuclear demagnetization cryostat. The experiment is carried out at about 250 K. The superfluid at this temperature is in the ballistic quasiparticle regime. Circulation around the wire is found to be stable only when it takes on the values -h/2m3, 0, and +h/2m3, where h is Plancks constant and m3 is the mass of the He3 atom. This experiment confirms that superfluid B3 is a Cooper-paired superfluid with a macroscopic quantum wave function. © 1991 The American Physical Society.


Phase-slips in the flow of superfluid <sup>4</sup>He through a submicron orifice

Physica B: Physics of Condensed Matter 165-166 (1990) 753-754

A Amar, JC Davis, RE Packard, RL Lozes

We have constructed a 6 Hz Helmholtz oscillator to study phase-slip phenomena in superfluid 4He. This method was pioneered by Zimmermann (1-2) and by Avenel and Varoquaux (3-6). A superconducting displacement transducer employing a D.C. SQUID and capable of resolving a displacement of 2 x 10-13 m in one second is used. We report preliminary flow studies at 0.3K using a slit with cross-sectional dimensions of 0.4μ × 4.6μ and 0.08μ length. We detect sudden dissipation events, the smallest being consistent with phase-slips of 2π . © 1990.


Continuously pumped rotating millikelvin cryostat

Physica B: Physics of Condensed Matter 165-166 (1990) 57-58

JD Close, RJ Zieve, JC Davis, RE Packard

We report the construction of a rotating nuclear demagnetization cryostat which can be continuously operated. The refrigerator employs a quadruple concentric rotating vacuum seal. The minimum 3He temperature of this cryostat in the stationary state is 165 μK. In the rotating state there is a relatively large heat leak which depends both on the rotation rate and on the field in the main demagnetization magnet. © 1990.


LLNL multi-user tandem laboratory

Nuclear Inst. and Methods in Physics Research, A 268 (1988) 344-349

ID Proctor, JC Davis, MR Eaton, JL Garibaldi, TL Moore, BJ Schumacher, JR Southon, TA Zimmerman

The Physics Department at Lawrence Livermore National Laboratory (LLNL) is building a new tandem Van de Graaff laboratory for nuclear physics and applied physics and technology programs. The laboratory has been funded by a coalition of users including several LLNL divisions, Sandia National Laboratories Livermore, and the University of California. The tandem is the former University of Washington injector FN. The accelerator is upgraded with a Pelletron charging system, Dowlish spiral inclined field beam tubes and SF6 insulation. The laboratory incorporates several novel design concepts. Initial operation will be in June, 1987 with full operation in October, 1987. Design features, radiological controls, computer assisted operation, and experimental facilities of the laboratory are discussed. © 1988.


Evidence for thermally activated dissipation in flowing superfluid <sup>3</sup>He

Journal of Low Temperature Physics 71 (1988) 141-149

JP Pekola, JC Davis, RE Packard

We report on observations of the onset of dissipation in superfluid 3He flowing through a single cylindrical channel of diameter d = 0.7 μm and length L = 6.0 μm. We propose a model in which thermally activated phase slips in the order parameter can account for the observed onset of dissipation. © 1988 Plenum Publishing Corporation.


Superfluidity of He3 films

Physical Review Letters 60 (1988) 302-304

JC Davis, A Amar, JP Pekola, RE Packard

We have developed a new technique to study superfluidity in saturated liquid-He3 films. Using the enhanced sensitivity of this new method, we directly observe the suppression of the film's transition temperature TcF as the film thickness decreases. We also measure a well-defined maximum critical film current which scales with temperature as (1-TTcF)a, where a=1.50.17. © 1988 The American Physical Society.


Suppression of the critical current and the superfluid transition temperature of<sup>3</sup>He in a single submicron cylindrical channel

Journal of Low Temperature Physics 67 (1987) 47-63

JP Pekola, JC Davis, Z Yu-Qun, RNR Spohr, PB Price, RE Packard

We report on an investigation into confined geometry effects and critical currents of superfluid3He in a single circular cylindrical channel. The diameter of the channel, 0.7 μm, is of the order of the (temperature-dependent) coherence length and its aspect ratio is ∼10. The reduction of the critical temperature demonstrates diffuse scattering on the solid walls of the microchannel. Using the Ginzburg-Landau formulation, we derive a model for the critical current and the critical temperature in a small, infinitely long, cylindrical channel with a circular cross section. The measured reductions of these quantities are in reasonable agreement with the predictions of the model. © 1987 Plenum Publishing Corporation.


Observations on the flow properties of <sup>3</sup>he films below 1 mk

Japanese Journal of Applied Physics 26 (1987) 147-148

JC Davis, A Amar, JP Pekola, RE Packard

We have performed an experiment designed to observe superfluid film flow in 3He. Our apparatus measures flow rates as small as 4ȕ10-5mm3/s onto a horizontal surface located a distance h above a reservoir. Results indicate that when h is several millimeters no flow is detected. However when h is less than 2.5 mm from the horizontal surface, film flow is observed at temperatures well below Tc. © 1987 The Japan Society of Applied Physics.


Suppression of the critical current and transition temperature of superfluid <sup>3</sup>he in a single submicron cylindrical channel

Japanese Journal of Applied Physics 26 (1987) 113-114

JP Pekola, JC Davis, RE Packard

The flow of superfluid 3He through a single cylindrical channel of 0.7 micron diameter is studied. suppresion of the transition temperature and of the depairing critical current density are observed indicating that the boundary condition at the wall is that for diffuse quasiparticle scattering. The suppresions agree reasonably well with model calculations for this geometry using the diffuse scattering boundary conditions. Some evidence for dissipation of superflow due to thermal activation of 2π phase slips of the order parameter inside the channel is seen. © 1987 The Japan Society of Applied Physics.


Adjustments in the n-p singlet effective range

Physics Letters B 27 (1968) 636-637

JC Davis, HH Barschall

Neutron energies below 5 MeV at which neutron-proton total cross sections have been measured in earlier experiments were determined. Revised values of the singlet effective range are presented. © 1968.


Electric field control of spins in molecular magnets

Physical Review Letters American Physical Society (0)

A Ardavan, J Liu, J Mrozek, W Myers, G Timco, R Winpenny, B Kinzel, W Plass


Photo-molecular high temperature superconductivity

Physical Review X American Physical Society (0)

M Buzzi, D Nicoletti, M Fechner, N Tancogne-Dejean, MA Sentef, A Georges, T Biesner, E Uykur, M Dressel, A Henderson, T Siegrist, JA Schlueter, K Miyagawa, K Kanoda, M-S Nam, A Ardavan, J Coulthard, J Tindall, F Schlawin, D Jaksch, A Cavalleri


Quantum coherent spin-electric control in molecular nanomagnets

ArXiv (0)

J Liu, J Mrozek, Y Duan, A Ullah, JJ Baldoví, E Coronado, A Gaita-Ariño, A Ardavan

Electrical control of spins at the nanoscale offers significant architectural advantages in spintronics, because electric fields can be confined over shorter length scales than magnetic fields. Thus, recent demonstrations of electric-field (E-field) sensitivities in molecular spin materials are tantalising, raising the viability of the quantum analogues of macroscopic magneto-electric devices. However, the E-field sensitivities reported so far are rather weak, prompting the question of how to design molecules with stronger spin-electric couplings. Here we show that one path is to identify an energy scale in the spin spectrum that is associated with a structural degree of freedom with a significant electrical polarisability. We study an example of a molecular nanomagnet in which a small structural distortion establishes clock transitions (i.e. transitions whose energy is to first order independent of magnetic field) in the spin spectrum; the fact that this distortion is associated with an electric dipole on the molecule allows us to control the clock transition energy to an unprecedented degree. We demonstrate coherent electrical control of the quantum spin state and exploit it to manipulate independently the two magnetically-identical but inversion-related molecules in the unit cell of the crystal. Our findings pave the way for the use of molecular spins in quantum technologies and spintronics.

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