Publications: Recent Highlights

Updated 28 January 2015.

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Title: Field-tunable spin-density-wave phases in Sr3Ru2O7

Author(s): C. Lester, S. Ramos, R. S. Perry, T. P. Croft, R. I. Bewley, T. Guidi, P. Manuel, D. D. Khalyavin, E. M. Forgan and S. M. Hayden
Source: Nature Materials (2015), arXiv
DOI: 10.1038/nmat4181

There has been a huge amount of interest over the years in the properties at low temperatures and high magnetic fields of the strontium ruthenate, Sr3Ru2O7. This material shows a “metamagnetic phase transition” (a rapid increase in magnetisation) very close to absolute zero in a magnetic field ~ 8 Tesla applied along the crystal c-direction. Near this transition, the electrical resistivity rises suddenly, and shows very curious behaviour when the field is tilted slightly away from the c-direction: the resistivity remains high in the tilting direction but drops down to normal in the other. Many people have suggested possible reasons for this “nematic” behaviour (which means a sudden change from an isotropic electron fluid to an anisotropic one – which is analogous to similar behaviour in the formation of nematic liquid crystals, which are used in LCD displays). Finally, we have found an answer which was unsuspected until the experiments were done. Despite the “nearly ferromagnetic” properties of the sample, the cause is an antiferromagnetic ordering of the ruthenium electron spins which goes along a direction controlled by the tilted field and causes the high resistivity.

Left: The proposed magnetic structure for a single domain of the spin density wave phase. Right: The upper panel shows the antiferromagnetic Bragg spots from the two domains. In the lower panel, the field has been tilted, suppressing one of the spin density wave domains.

Title: Simple Bayesian method for improved analysis of quasi-two-dimensional scattering data

Author(s): A. T. Holmes
Source: Physical Review B 90, 024514 (2014), arXiv
DOI: 10.1103/PhysRevB.90.024514

An Editors' Suggestion that featured on the front page of the Phys. Rev. B website. This paper presents a new method for the analysis of small angle neutron scattering data from quasi-two-dimensional systems such as flux lattices, Skyrmion lattices, and aligned liquid crystals. A significant increase in signal to noise ratio and a natural application of the Lorentz factor can be achieved by taking advantage of the knowledge that all relevant scattering is centered on a plane in reciprocal space. The Bayesian form ensures that missing information is treated in a controlled way and can be subsequently included in the analysis. A simple algorithm based on Gaussian probability assumptions is provided which provides very satisfactory results. Finally, it is argued that a generalized model-independent Bayesian data analysis method would be highly advantageous for the processing of neutron and x-ray scattering data. These methods are available as an add-on to the GRASP software developed by Charles Dewhurst. If you would like a copy of the add-on, please contact Alex Holmes.

Title: Coherent magnetic diffraction from the uranium M4 edge in the multi-k magnet, USb

Author(s): J. A. Lim, E. Blackburn, G. Beutier, F. Livet, N. Magnani, A. Bombardi, R. Caciuffo and G. H. Lander
Source: Journal of Physics: Conference Series 519, 012010 (2014)
DOI: 10.1088/1742-6596/519/1/012010

The magnetic Bragg peaks of the canonical 3-k antiferromagnet USb were looked at using a highly coherent x-ray beam at I16, Diamond, using a pinhole. The speckle pattern observed on the Bragg peaks appears to be related to the domain structure of the material. By monitoring the time dependence of the speckle pattern (x-ray photon correlation spectroscopy - XPCS), the slow magnetic dynamics in this material were probed.

Upper: Typical speckle pattern obtained from the magnetic Q = (0, 0, 3) reflection at the M4 resonant edge of uranium. The exposure time was 1 s and 25 x 25 m slits were used. The black dashed lines show the region for the intensity pro file in the lower panel. Lower: The intensity profile (solid line) of a slice through the speckle pattern showing intensity fluctuations that arise from the beam coherence. The red curve (Gaussian) shows the expected non-coherent intensity profi le.

Title: Neutron spin echo spectroscopy under 17T magnetic field at RESEDA

Author(s): J. Kindervater, N. Martin, W. Häußler, M. Krautloher, C. Fuchs, S. Mühlbauer, J. A. Lim, E. Blackburn, P. Böni and C. Pfleiderer
Source: EPJ Web of Conferences 83, 03008 (2015) - Open Access

The Birmingham 17T magnet has now been combined with the neutron spin-echo technique, at FRM-II, using the MIEZE method. This stands for Modulated Intensity with Zero Effort. A proof of principle of experiment has been carried out, showing that fields up to 17 T can be used successfully at the RESEDA spectrometer at FRM-II.

A typical MIEZE echo measured with the longitudinal MIEZE setup on RESEDA. The echo was recorded under an applied field of 17 T at a frequency of 77 kHz. The line is a cosine fit to the data.

Title: Inelastic x-ray study of phonon broadening and charge-density wave formation in ortho-II ordered YBa2Cu3O6.54

Author(s): E. Blackburn, J. Chang, A. H. Said, B. M. Leu, R. Liang, D. A. Bonn, W. N. Hardy, E. M. Forgan and S. M. Hayden
Source: Physical Review B 88, 054506 (2013), arXiv
DOI: 10.1103/PhysRevB.88.054506

Following up on our previous papers (see below) using 100 keV x-rays to observe diffraction peaks from charge density waves in the cuprate superconductor YBa2Cu3Oy, we have used inelastic x-ray scattering at the HERIX beamline at the Advanced Photon Source, Argonne National Lab. to probe the energy width of the charge density wave signal - this was found to be less than the resolution of the beamline (1.5 meV). We also saw signs of broadening of a particular transverse acoustic (TA) phonon at the same position in momentum space as the charge density wave.

The broadening of the phonon at the momentum transfer associated with the charge density wave. At 155 K, the onset temperature for the charge density wave, two phonon modes are visible; they are both resolution limited. At 55 K, the onset temperature of superconductivity, at least one of the phonon modes has broadened significantly.

Title: X-Ray Diffraction Observations of a Charge-Density-Wave Order in Superconducting Ortho-II YBa2Cu3O6.54 Single Crystals in Zero Magnetic Field

Author(s): E. Blackburn, J. Chang, M. Huecker, A. T. Holmes, N. B. Christensen, Ruixing Liang, D. A. Bonn, W. N. Hardy, U. Ruett, O. Gutowski, M. v. Zimmermann, E. M. Forgan and S. M. Hayden
Source: Phys. Rev. Lett. 110, 137004 (2013), arXiv
DOI: 10.1103/PhysRevLett.110.137004

Superconductivity often emerges in the proximity of, or in competition with, symmetry-breaking ground states such as antiferromagnetism or charge density waves. Several cuprates, including some that are superconductors, show spin and charge density wave order. However, these states have not been observed in all cuprates, calling into question the generality of the observation. In this paper, we observe charge density waves in the ortho-II doping, where they had previously not been seen. This doping is perhaps the most heavily studied of the underdoped variants, allowing direct comparison with, for example, recent ultrasound studies.

A comparison of spin and charge incommensurabilities in the underdoped cuprates. The side panels provide a zoom on the YBCO data and a comparison of the position in the Brillouin zone of the spin and charge modulations.

Title: Unexpected phase locking of magnetic fluctuations in the multi-k magnet USb

Author(s): J. A. Lim, E. Blackburn, N. Magnani, A. Hiess, L.-P. Regnault, R. Caciuffo, and G. H. Lander
Source: Physical Review B 87, 064421 (2013), arXiv
DOI: 10.1103/PhysRevB.87.064421

Uranium antimonide is a well known multi-k antiferromagnet. This means that the magnetic moments order in a complex non-linear arrangement. Somewhat surprisingly, the spin waves soften and become quasielastic well below the AFM ordering temperature, with no corresponding magnetic or structural transition. It had been suggested that this was due to de-phasing of the different magnetic components. Here, we show that this is not the case, and more surprisingly, the paramagnetic correlations also maintain the same clear phase correlations well above the ordering transition.

Title: Direct observation of competition between superconductivity and charge density wave order in YBa2Cu3O6.67

Author(s): J. Chang, E. Blackburn, A. T. Holmes, N. B. Christensen, J. Larsen, J. Mesot, Ruixing Liang, D. A. Bonn, W. N. Hardy, A.Watenphul, M. v. Zimmermann, E. M. Forgan and S. M. Hayden
Source: Nature Physics 8, 871 (2012), arXiv
DOI: 10.1038/nphys2456

Superconductivity often emerges in the proximity of, or in competition with, symmetry-breaking ground states such as antiferromagnetism or charge density waves. Several cuprates, including some that are superconductors, show spin and charge density wave order. However, these states have not been observed in all cuprates, calling into question the generality of the observation. In this paper, we observe charge density waves in a member of the YBCO family YBa2Cu3O6.67.

Using the Birmingham 17 T cryomagnet on beamline BW5 at HASYLAB, Hamburg, we show that they are in competition with the superconductivity.

This paper was also discussed in a Nature Physics News and Views by Eduardo Fradkin and Steven Kivelson. It has also featured on

Intensity of the Bragg peak from the charge density wave order as a function of field and temperature.

Title: A 17 T horizontal field cryomagnet with rapid sample change designed for beamline use

Author(s): A. T. Holmes, G. R. Walsh, E. Blackburn, E. M. Forgan and M. Savey-Bennett
Source: Review of Scientific Instruments 83, 023904 (2012)
DOI: 10.1063/1.3688657

Cryomagnets are common sample environment equipment on neutron and synchrotron beamlines, and we commissioned a 17 T horizontal field cryomagnet from Cryogenic Ltd, providing the largest steady state magnetic field currently available for small angle neutron scattering. This magnet has been used at the Institut Laue-Langevin, the Paul Scherrer Institut, and HASYLAB at DESY.

Rotation stage holder showing attocube® with 99.99% Al sample holder, heater, thermometer, and thermal link in foreground, for mounting inside the cryomagnet.

Title: Magnetic flux lines in type-II superconductors and the ‘ hairy ball ’ theorem

Author(s): Mark Laver & Edward. M. Forgan
Source: Nature Communications 1, 45 (2010)
Document Type: Article

Many prominent phenomena originate from geometrical effects rather than from local physics. For example, the ‘ hairy ball ’ (HB) theorem asserts that a hairy sphere cannot be combed without introducing at least one singularity, and is fulfilled by the atmospheric circulation with the existence of stratospheric polar vortices and the fact that there is always at least one place on Earth where the horizontal wind is still. In this study, we examine the consequences of the HB theorem for the lattice of flux lines that form when a magnetic field is applied to a type-II superconducting crystal. We find that discontinuities must exist in lattice shape as a function of field direction relative to the crystal. Extraordinary, ‘ unconventional ’ flux line lattice shapes that spontaneously break the underlying crystal symmetry are thus remarkably likely across all type-II superconductors, both conventional and unconventional.

Flux line lattice (FLL) structures and the hairy cube. FLL structures observed as a function of field direction in niobium at T = 4.5 K and μ0H = 150 mT. For clarity, only one of the two triangular half-unit-cells is shown, except at the < 110 > and < 111 > directions. Thick blue lines denote the field directions where there are sudden changes in the preferred FLL orientation.

Title: Observations of Pauli paramagnetic effects on the flux line lattice in CeCoIn5

Author(s): J S White, P Das, M R Eskildsen, L DeBeer-Schmitt, E M Forgan, A D Bianchi, M Kenzelmann, M Zolliker, S Gerber, J L Gavilano, J Mesot, R Movshovich, E D Bauer, J L Sarrao & C Petrovic.
Source: New Journal of Physics 12, 023026 (2010)
Document Type: Article

From small-angle neutron scattering studies of the flux line lattice in CeCoIn5, with magnetic field applied parallel to the crystal c-axis, we obtain the field and temperature dependence of the spatial variation of the field in the mixed state. This extends our earlier work (Bianchi et al. Science 319 177 (2008)) to temperatures up to 1250 mK. Over the entire temperature range, paramagnetic magnetisation in the flux line cores results in an increase of the visibility of the flux lines with field. This is the opposite behaviour to that of conventional superconductors. Near Hc2, the field variation decreases again, and our results indicate that this fall-off extends outside the proposed Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) region. Instead, we attribute the decrease to a paramagnetic suppression of Cooper pairing throughout the bulk, arising from the conflict between the anti-parallel alignment of the electron spins in this d-wave superconductor and the parallel alignment favoured by the field.

Field-dependence of the square of fundamental Fourier component of spatial variation of magnetic field for temperatures up to 1250 mK. The dashed lines are guides to the eye.

Title: Exploring the Fragile Antiferromagnetic Superconducting Phase in CeCoIn5

Author(s): E. Blackburn, P. Das, M. R. Eskildsen, E. M. Forgan, M. Laver, C. Niedermayer, C. Petrovic, and J. S. White
Source: Physical Review Letters 105, 187001 (2010), arXiv

CeCoIn5 is a well-known heavy-fermion superconductor, and close to the high magnetic field edge of the superconducting region, magnetic order develops within the superconductor, disappearing precipitously when the superconductivity is destroyed. Here we investigate the changes to the order as the magnetic field direction is rotated away from the basal plane of the crystal.

Title: Properties of superconducting planar resonators at millikelvin temperatures

Author(s): T. Lindström, J. E. Healey, M. S. Colclough, C. M. Muirhead, and A. Ya. Tzalenchuk
Source: Phys. Rev. B 80, 132501 (2009)
Alternative link here.

Superconductors in the form of submicron films deposited onto substrates of silicon and sapphire are being increasingly used at millikelvin temperatures in to study fundamental quantum mechanics, and in a number of novel applications such as quantum bits for potential computing applications. A major limitation on the performance of such devices is the presence of localized 2-level systems in the substrate on which the superconducting film is deposited. By patterning the thin film into a microwave resonator structure and studying the behaviour down to temperatures below 50 millikelvin, we can obtain valuable information about the nature of the 2-level systems.

Frequency shift as a function of the normalized frequency at several different powers for LE resonators R2–R7 on SiO2/Si (top); and R8–R10 sapphire (bottom). The solid lines show fits to theory. The log term dominates at small ℏω/kBT whereas g(T) becomes important in the opposite regime. Successive curves differ in power by 10 dB.

Title: Delocalization and hybridization enhance the magnetocaloric effect in Cu-doped Ni2MnGa

Author(s): S. Roy, E. Blackburn, S. M. Valvidares, M. R. Fitzsimmons, S. C. Vogel, M. Khan, I. Dubenko, S. Stadler, N. Ali, S. K. Sinha and J. B. Kortright
Source: Physical Review B, 79, 235127 (2009)
News article: magnetic fridge
More information: CM website

In the giant magnetocaloric effect, a changing magnetic field in a material causes its temperature to drop precipitously. A nickel-manganese-gallium alloy was found to show a large effect when copper was added. To find out why this alloy works so well, we used x-ray magnetic circular dichroism and x-ray absorption spectroscopy to explore changes in the alloy as the composition changed.

Title: Magnetic field tuning of coplanar waveguide resonators

Author(s): J. E. Healey, T. Lindström, M. S. Colclough, C. M. Muirhead, and A. Ya. Tzalenchuk
Source: Appl. Phys. Lett. 93, 043513 (2008)
DOI: 10.1063/1.2959824

We describe measurements on microwave coplanar resonators designed for quantum bit experiments. Resonators have been patterned onto sapphire and silicon substrates, and quality factors in excess of a million have been observed. The resonant frequency shows a high sensitivity to magnetic field applied perpendicular to the plane of the film, with a quadratic dependence for the fundamental, second, and third harmonics. Frequency shift of hundreds of linewidths can be obtained with no change in the quality factor.

Sketch of a coplanar resonator (CPR). Angles indicate the in-plane θ and out-of-plane φ orientation of the magnetic field with respect to the substrate. Inset: photograph of the CPR, comprising a center conductor of length 11 mm and width W=10 μm separated from ground planes by an S =5 μm gap. The center conductor is capacitively coupled to the feed lines by gap G of 4–8 μm at each end. The meander line (see inset) is used to confine the whole structure within the 10 mm long substrate.

Change in resonant frequency with perpendicular magnetic field for the fundamental, first and second harmonics. The data is highly reproducible with changing magnetic field. The inset shows the dependence of fundamental frequency shift on the angle φ at 0.2mT.

Title: Pinned Co spins in a polycrystalline Permalloy/CoO exchange biased bilayer

Author(s): E. Blackburn, C. Sanchez-Hanke, S. Roy, D. J. Smith, J.-I. Hong, K. T. Chan, A. E. Berkowitz and S. K. Sinha
Source: Physical Review B – Rapid Communications, 78, 180408(R) (2008)
More information: CM website

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