Selected publications

2021_CWH_NatPhys_teaserImageSplit superconducting and time-reversal symmetry-breaking transitions in Sr2RuO4 under stress

V. Grinenko, S. Ghosh, R. Sarkar, J.-C. Orain, A. Nikitin, M. Elender, D. Das, Z. Guguchia, F. Brückner, M.E. Barber, J. Park, N. Kikugawa, D.A. Sokolov, J.S. Bobowski, T. Miyoshi, Y. Maeno, A.P. Mackenzie, H. Luetkens, C.W. Hicks & H.-H. Klauss
Nature Physics 17, 748 (2021)

The muon spin rotation measurements report here have strongly supported the idea that the superconductivity Sr2RuO4 breaks time reversal symmetry (for example, with a spontaneous net angular momentum). Superconductivity that breaks time reversal symmetry is very rare, because it implies a frustration of the pairing interaction. It is a particularly intriguing possibility for Sr2RuO4 because it might only be possible with an altogether new form of pairing interaction; the fact though that we might need a new pairing interaction to explain this possible feature of Sr2RuO4 means that the standard of proof is high. In this experiment, we showed that the time-reversal symmetry-breaking transition splits from Tc when Sr2RuO4 is placed under uniaxial stress, which shows decisively that this transition cannot be some type of artefact of the superconducting transition.

Scientific diagram showing sizable Suppression of Thermal Hall Effect upon Isotopic Substitution in SrTiO3Sizable Suppression of Thermal Hall Effect upon Isotopic Substitution in SrTiO3

S. Sim, H. Yang, H.-L. Kim, M.J. Coak, M. Itoh, Y. Noda & J.-G. Park
Physical Review Letters 126, 015901 (2021)

Using our newly-developed thermal Hall effect instrumentation, with record-breaking resolution, we measured the thermal Hall effect in single crystals of both pristine and isotopically substituted strontium titanate. Substituting the 16O atoms for 18O drives SrTiO3 ferroelectric – its behaviour is otherwise dominated by disordered quantum critical fluctuations (Coak et al., PNAS 117, 12707). Despite the system being completely non-magnetic, we showed an applied magnetic field to ‘bend’ the flow of heat (the thermal Hall effect) across a crystal of SrTiO3 – but only in the quantum critical regime.

Scientifc diagram showing Emergent Magnetic Phases in Pressure-Tuned van der Waals Antiferromagnet FePS3Emergent Magnetic Phases in Pressure-Tuned van der Waals Antiferromagnet FePS3

M.J. Coak, D.M. Jarvis, H. Hamidov, A.R. Wildes, J.A.M. Paddison, C. Liu, C.R.S. Haines, N.T. Dang, S.E. Kichanov, B.N. Savenko, S. Lee, M. Kratochvílová, S. Klotz, T.C. Hansen, D.P. Kozlenko, J.-G. Park & S.S. Saxena
Physical Review X 11, 011024 (2021)

This work describes the first high-pressure neutron diffraction study on the TMPS3 family of van-der-Waals materials, compounds under intense research scrutiny worldwide. Through the use of revolutionary new pressure techniques we have been able to map the evolution of magnetic ordering in FePS3 through its insulator-metal transition and into the unconventional metallic state, where we observe novel short-range magnetic order. This phase most likely forms either a precursor or a suppressing factor to superconductivity – future work will seek to uncover which.

2021_CWH_FeSe_teaserImageRelationship between transport anisotropy and nematicity in FeSe

J. Bartlett, A. Steppke, S. Hosoi, H. Noad, J. Park, C. Timm, T. Shibauchi, A.P. Mackenzie & C.W. Hicks
Phys. Rev. X 11, 021038 (2021)

Electronic nematicity is a spontaneous reduction in the rotational symmetry of a system due to electronic interactions. At high temperatures, FeSe is tetragonal, but then at the nematic transition temperature Ts = 90 K a spontaneous anisotropy between the x- and y-axis conductivities appears. Why this occurs is not known, and one of the motivations to understand is that high-Tc superconductivity may be linked with nematicity. Here, we applied large strains to FeSe, in spite of it being a material that peels apart easily. We showed that the dependence of the resistive anisotropy on the strain-induced nematicity might not be consistent with one of the popular theories of its origin, that it is driven by spin fluctuations.

2021_EMF_PLAA proposal for detection of absolute rotation using superconductors and large voltages

E.M. ForganC.M. MuirheadA.I.M. RaeC.C. Speake
Physics Letters A 386, 126994 (2021)

Consider rotating an electrostatically charged metal cylinder about its axis. The charge on the cylinder constitutes a solenoidal current, so the rotation induces a small magnetic field along the axis of the cylinder. This magnetic field is proportional to the absolute rotation rate of the cylinder relative to the rest of the Universe, so in principle it is possible by this means to measure the rotation rate of the Earth without looking out at the stars. In this paper, we consider how to do this in practice, making use of the properties of superconductors in screening magnetic fields, flux quantisation and SQUID detectors of very small fields. Collaborators in Padua (where Galileo worked) are constructing apparatus to test this idea.

Scientific diagram of electronically driven spin-reorientation transition of the correlated polar metal Ca3Ru2O7Electronically driven spin-reorientation transition of the correlated polar metal Ca3Ru2O7

I. Marković, M.D. Watson, O.J. Clark, F. Mazzola, E. Abarca Morales, C.A. Hooley, H. Rosner, C.M. Polley, T. Balasubramanian, S. Mukherjee, N. Kikugawa, D. Sokolov, A.P. Mackenzie & P.D.C. King
PNAS 117, 15524 (2020)

The orientation of ordered magnetic moments is typically set by details of the crystalline environment in which they reside. Here, we reported the discovery of a magnetic anisotropy which is instead driven by a reconfiguration of the underlying fermiology of the system. In the oxide material Ca3Ru2O7 the interplay of spin–orbit coupling, local polar distortions of the crystal structure, and pronounced electronic correlations leads to a coupled structural and spin-reorientation transition. Our photoemission results revealed a concomitant destruction of a large Fermi surface mediated by a hidden Rashba-type spin-orbit coupling which is enabled by the bulk structural distortions and unlocked by the reorientation of the spins on Ru sites. Our findings suggest materials design approaches for manipulating magnetic textures and open pathways to creating large magnetoelectric-type couplings in solids.



A tunable stress dilatometer and measurement of the thermal expansion under uniaxial stress of Mn3Sn

M. Ikhlas, K.R. Shirer, P.-Y. Yang, A.P. Mackenzie, S. Nakatsuji, & C.W. Hicks
Appl. Phys. Lett. 117, 233502 (2020)

Piezoelectric-based uniaxial pressure cell with integrated force and displacement sensors

M.E. Barber, A. Steppke, A.P. Mackenzie & C.W. Hicks
Rev. Sci. Inst. 90, 023904 (2019)

In the above two articles we describe methods for performing stress-strain measurements on correlated electron materials. Stress-strain measurements are standard in mechanical engineering, but it is an unexplored area of experimentation in condensed matter physics. Reasons include the need to operate at cryogenic temperatures, and the large strains that are sometimes required to qualitatively the alter the electronic structure of a material. The potential, however, is huge, because strain can be measured to extremely high precision. In these papers, we show that even a subtle change in magnetic structure generates measurable anomalies in the stress-strain relationship, and in these papers we show how to do these measurements.

Set of four science diagrams showing Weyl-like points from band inversions of spin-polarised surface states in NbGeSbWeyl-like points from band inversions of spin-polarised surface states in NbGeSb

I. Marković, C. A. Hooley, O. J. Clark, F. Mazzola, M. D. Watson, J. M. Riley, K. Volckaert, K. Underwood, M. S. Dyer, P. A. E. Murgatroyd, K. J. Murphy, P. Le F`evre, F. Bertran, J. Fujii, I. Vobornik, S. Wu, T. Okuda, J. Alaria & P. D. C. King
Nature Communications 10, 5485 (2019)

The routes by which different underlying symmetries can lead to a variety of topologically protected electronic states in solids have garnered great attention in the past years. Our study of the spin- and momentum-resolved electronic structure of the surface states in the square net system NbGeSb illustrated how surface states, as well as bulk states, can provide a rich environment for realising, and indeed extending, the variety of electronic structures that can be generated from band inversions in solids. We demonstrate how strong spin-orbit coupling leads to the development of pronounced Rashba-like spin splitting of intertwined surface states. Combined with symmetry protections, this leads to a rich topological crossing structure of the spin-polarised surface states, hosting 2D Weyl-like fermions.

2018_CWH_YBCO_teaserImageUniaxial pressure control of competing orders in a high-temperature superconductor

H.-H. Kim, S.M. Souliou, M.E. Barber, E. Lefrançois, M. Minola, M. Tortora, R. Heid, N. Nandi, R.A. Borzi, G. Garbarino, A. Bosak, J. Porras, T. Loew, M. König, P.M. Moll, A.P. Mackenzie, B. Keimer, C.W. Hicks & M. Le Tacon
Science 362, 1040 (2018)

Although we would all like to see a room-temperature superconductor, to understand the origins of high-temperature cuprate superconductivity it is often more valuable to try to suppress the superconductivity, and to see what other electronically ordered states can emerge. Here, we show that in-plane uniaxial stress can weaken the superconductivity of YBa2Cu3O6.67 and induce charge density wave order. The proximity of charge density wave order to superconductivity may provide a hint on the structure of the superconducting state.


Strong peak in Tc of Sr2RuO4 under uniaxial pressure

A. Steppke, L. Zhao, M.E. Barber, T. Scaffidi, F. Jerzembeck, H. Rosner, A.S. Gibbs, Y. Maeno, S.H. Simon, A.P. Mackenzie & C.W. Hicks
Science 355, eaaf9398 (2017)

In this paper we showed just how powerful a technique uniaxial stress can be. Under uniaxial compression, one of the Fermi surfaces of Sr2RuO4 passes through a Lifshitz transition (a change in Fermi surface topology), and as it does so the density of states peaks strongly. This happens when the lattice is compressed by about 0.5%– to see what that means technically, imaging taking a metre stick and pushing on it so hard that it compresses, elastically, by 5 mm. Right at the Lifshitz transition, there is a portion of Fermi surface where the Fermi velocity is almost zero. Tuning to this point causes Tc to more than double. Figure: Tc versus lattice strain in Sr2RuO4.

2017_MJ_DIMPYMagnetic-order crossover in coupled spin ladders

M. Jeong, H. Mayaffre, C. Berthier, D. Schmidiger, A. Zheludev & M. Horvatic
Phys. Rev. Lett. 118, 167206 (2017)

In the spin ladder compound known as DIMPY, we found a new type of crossover at very low temperature (~100 mK), below the ordering temperature of ≈300 mK. We interpret this as a dimensional crossover, which is unusual to observe below the ordering temperature. It maybe a consequence of an unusually large hierarchy in the exchange coupling network, due to frustrated interaction between the ladders. 

2016_MJ_DIMPYDichotomy between attractive and repulsive Tomonaga-Luttinger liquids in spin ladders

M. Jeong, D. Schumidifier, H. Mayaffre, M. Klanjsek, C. Berthier, W. Knafo, G. Ballon, B. Vignolle,   S. Krämer, A. Zheludev & M. Horvatic
Phys. Rev. Lett. 117, 106402 (2016)

We found the first qualitative difference between the attractive and repulsive Tomonaga-Luttinger liquid using spin ladder materials. The results are in perfect agreement with the theoretical calculations using the DMRG data. The methodology developed here may apply broadly to one dimensional quantum spin systems. 

2015_EMF_NatThe microscopic structure of charge density waves in underdoped YBa2Cu3O6.54 revealed by X-ray diffraction

E.M. ForganE. Blackburn, A.T. Holmes, A.K.R. Briffa, J. Chang, L. Bouchenoire, S.D. Brown, R. Liang, D. Bonn, W.N. Hardy, et al
Nature Communications 6, 10064 (2015)

Using a very versatile diffractometer at the ESRF synchrotron, we measure the intensities of the spots due to the spatially modulated ionic displacements. These intensities are about 10-6 of those due to the main crystal lattice. We deduce that the CDW action is in the CuO2 bilayers where superconductivity resides, and corresponds to a spatially varying doping in that region.

2013_MJ_PRLAttractive Tomonaga-Luttinger liquid in a quantum spin ladder

M. Jeong, H. Mayaffre, C. Berthier, D. Schmidiger, A. Zheludev & M. Horvatic
Phys. Rev. Lett. 111, 106404 (2013)

Using NMR relaxation measurements, we demonstrate a realisation of 'attractive' Tomonaga-Luttinger liquid (unlike more natural repulsive ones realised in electronic systems such as carbon nanotube) in a spin ladder compound. The field variation of the strength of the attractive interaction is also demonstrated.  

2012_EMF_NatPhysDirect observation of competition between superconductivity and charge density wave order in YBa2Cu3O6.67

J. Chang, E. BlackburnA.T. Holmes, N.B. Christensen, J. Larsen, J. Mesot, R. Liang, D.A. Bonn, W.N. Hardy, A. Watenphul, M. v. Zimmermann, E.M. Forgan & S.M. Hayden
Nature Physics 8, 871 (2012)

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.

2010_EMF_NatMagnetic flux lines in type-II superconductors and the ‘hairy ball’ theorem

M. Laver & E.M. Forgan
Nature Communications 1, 45 (2010)

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.

2010_TF_NJPObservations of Pauli paramagnetic effects on the flux line lattice in CeCoIn5

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
New Journal of Physics 12, 023026 (2010)

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 Bc2, 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.

2008_EMF_Science_teaserImageSuperconducting vortices in CeCoIn5: toward the Pauli-limiting field

A.D. Bianchi, M. Kenzelmann, L. DeBeer-Schmitt, J.S. WhiteE.M. Forgan, J. Mesot, M. Zolliker, J. Kohlbrecher, R. Movshovich, E.D. Bauer, J.L. Sarrao, Z. Fisk, C. Petrovic & M.R. Eskildsen
Science 319, 177 (2008)

In this paper we demonstrated the effects of Pauli paramagnetism on the structure of the magnetic flux lines in the mixed state of a superconductor. CeCoIn5 is a d-wave superconductor and the Cooper pairs consist of heavy electrons with antiparallel spins. The alignment of the spins by an applied magnetic field magnetises the flux line cores and limits the upper critical field. We also see the effects of the d-wave pairing on the shape of the flux line lattice in this material.

2002_WFV_JLPQuantum Turbulence

W.F. Vinen & J.J. Niemela
J. Low Temp. Physics 128, 167 (2002)

This review discusses turbulence in superfluids, which on a long length scale is similar to classical turbulence, but on a short length scale is altered by the quantisation of vorticity. Superfluids can host counterflow turbulence, in which the normal and superfluid portions flow in opposite directions, and which has no counterpart in classical turbulence.

1998_Sr2RuO4_teaserImageObservation of a square flux-line lattice in the unconventional superconductor Sr2RuO4

T.M. RisemanP.G. KealeyE.M. ForganA.P. MackenzieL.M. GalvinA.W. Tyler, S.L. Lee, C. Ager, D. Mck. Paul, C.M. Aegerter, R. Cubitt, Z.Q. Mao, T. Akima & Y. Maeno
Nature 396, 242 (1998)

The flux lattice of most superconductors is triangular, which is the arrangement that minimises the field energy. Here, using small-angle neutron scattering, it was shown that Sr2RuO4 has a square flux lattice. This result shows that the superconducing gap in Sr2RuO4 varies strongly around the Fermi surfaces, which is a signature of unconventional superconductivity. This gap anisotropy is strong enough to overcome the natural tendency towards a triangular lattice.

1987_CEG_YBCO_teaserImageFlux quantization in a high-Tc superconductor

C.E. GoughM.S. ColcloughE.M. Forgan, R.G. Jordan, M. Keene, C.M. Muirhead, A.I.M. Rae, N. Thomas, J.S. Abell & S. Sutton
Nature 326, 855 (1987)

Within a fortnight of the publication of superconductivity above liquid nitrogen temperatures in yttrium-based cuprates, we were measuring the quantum of flux in this system, and found that the value was h/2e. This was the first demonstration that high-Tc superconductivity is due to paired electrons, just as in conventional superconductors. It was some time before it became clear that the pairing is d-wave, and not the original BCS s-wave, while the exact mechanism causing the pairing is still a matter of controversy more than 30 years later. Figure: output of a conventional SQUID magnetometer, measuring quanta of flux jumping in and out of a donut of Y-Ba-Cu-O high-Tc superconductor. The “Millikan-style” graph demonstrates the size of the quantum of flux in this material.

1984_CMM_PhilTransThe nucleation of vorticity by ions in superfluid 4He

C.M. MuirheadW.F. Vinen & R.J. Donnelly
Phil. Trans. R. Soc. Lond. A 311, 433 (1984)

When ions move through superfluid 4He, they can nucleate vortices, if their velocity exceeds a critical value. One possibility is that in the illustration here, that a whole vortex ring is created that trails the ion. Much more likely though is a vortex that begins and ends on the solid helium that is attached to the ion.

1966_Vinen_PMThe motion of flux lines in type II superconductors

P. Nozières & W.F. Vinen
The Philosophical Magazine, 14, 130 (1966)

Vortices in type-II superconductors experience a Magnus force identical to that found in liquid helium. In combination with friction between the vortex core and the lattice, the Magnus force causes vortices to move sideways when strong current is applied to a type II superconductor.

1961_WHV_ProcRoySocThe detection of single quanta of circulation in liquid helium II

W.F. Vinen
Proc. Roy. Soc. A260, 218 (1961)

Before being appointed as a professor in Birmingham, Joe Vinen made the first direct demonstration of a macroscopic quantum effect in any superfluid system, by observing quantised vortices trapped on a fine wire in rotating superfluid 4He.