1.    Curriculum Vitae

 

Name:                       Roger A. Cowley

 

Personal:                    Born:               24th February 1939

                                    Married:           Sheila Wells 4th April 1964

                                    Children:           Sandra and Kevin

 

Degrees:                     B.A. Cambridge (I. Physics) 1960

                                    Ph.D. Cambridge (Physics) 1963

 

Employment:               Research Fellow, Trinity Hall, Cambridge 1962-64

                                    Research Officer, Atomic Energy of Canada Ltd, 1964-70

                                    Professor of Physics, University of Edinburgh, 1970-88

                                    Visiting Scientist Brookhaven National Laboratory 1974-1986

                                    Dr Lee's Professor of Experimental Philosophy, University of Oxford, 1988-2007

                                    Fellow of Wadham College, Oxford 1988-2007

                                    Consultant to DIAMOND (1 day/week), 2008-

                                    Emeritus Professor of Physics at Oxford University 2008-

                                    Emeritus Fellow of Wadham College, Oxford 2008-

 

Administration and some Committees:

                                    Head of Physics Department in Edinburgh (7 years)

                                    Chairman of Physics in Oxford (8 years)

                                    Member of SERC Committees for Physics, Neutron Beam Research and Synchrotron Radiation

                                    Member of Science Advisory Committees for ILL, ESRF, SNQ and ISIS

                                    Member of Physics Panel of EEC CODEST Committee

                                    Founding Editor in Chief: Journal of Physics Condensed Matter.

Editor of Journal of Physics, Solid State Communications, Zeitschrift fur Physik and Reports of Progress in Physics

                                    General Board of Oxford University (4 years)

                                    Chairman of Oxford University IT Committee

                                    Member and Chairman of IUPAP Commission C9

Member of International Committees to Review Physics in Norway, Switzerland, the ETH, Uppsala University and the NRC Laboratories in Ottawa.

                                    Council of Royal Society of London

 

Students and Postdoctoral Fellows:

                                    25 postgraduate students have been supervised and have successfully obtained their doctorates, while 21 Post-doctoral Fellows have been employed on research grants. 16 of these now have permanent posts at Universities or International Research Facilities.

 

Publications:

            The research results have been published in over 370 publications and in one book as well as being presented at international conferences and at seminars and lectures.

 

Grants :

Grants from UK Research Councils have been held continuously since 1970 until 2008 as well as grants from Europe, Canada, the USA and UK industry as well as grants for beam time at international neutron and synchrotron radiation facilities in Europe, Canada and the USA. A  Leverhulme Fellowship for retired academics was obtained from 2008-2010

 

 

2.     Scientific Research:

 

            Most of the research has been concerned with the use of neutron and x-ray scattering techniques to study condensed matter. Associated theoretical work has also been performed and many of the experiments have been designed to test current theories or have been analysed using the latest theoretical approach. Some of the x-ray measurements were performed  with  equipment in the University but most of the  experiments were performed at central facilities. As a result they inevitably involved collaborations with many colleagues and I would like to thank, in particular, A D Bruce, W J L Buyers, R J Birgeneau, G Dolling, D F McMorrow, G Shirane, R C C Ward and M R Wells and A D B Woods as well as many students and post-doctoral staff. A brief summary of the results and a list of some of the relevant references is given below: 

 

            1          The determination of phonon dispersion curves and inter-atomic forces in ionic crystals, semiconductors and metals.  These led on to the use of the shell model for ionic crystals and to the use of the pseudo-potential method for metals. The effect of the anharmonic effects on the scattering, thermal expansion, thermal conductivity and optical properties2 was then developed with specific applications3 to ionic crystals, semiconductors and metals.

 

2                    Structural phase transitions are associated with an unstable mode of vibration.  The first measurements of a soft mode were made for SrTiO31,5 and the research has continued the study with experiments and theory on ferroelectrics, perovskites, and incommensurate materials9.  The critical phenomena have been studied together with the deviations caused by the so-called central peak of the two-time scale problem and by the two-length scale problem13.

 

            3          The excitations in liquid helium 4 and liquid helium 3 have been studied as a function of temperature and pressure6,7. These measurements included the first comprehensive study of helium 4 which included the first observation of the impulse approximation scattering, the nature of the multi-phonon background and the anomalous form of the acoustic modes. The measurements made on helium 3 were the first for this difficult material and showed the zero sound peak  and how it decayed into a continuum. Measurements were also made for helium3 / helium 4 mixtures.

 

            4          The magnetic spin wave excitations and the exchange interactions have been determined in a variety of materials.  This led particularly to the understanding of the effects of crystal fields and how the orbital and spin excitations are coupled4 in CoF2, the observation of the spin wave phonon interaction in UO2  and showing that the tow-spin wave neutron scattering in MnF2 was very different from the light scattering in shape. The spin wave scattering cross-section for polarised neutrons was studied together with the canted magnetic structure of amorphous ferromagnets.

 

5                    Magnetic systems are ideal for testing the statistical mechanics and excitations of simple systems. Studies have been made of the Ising model in one, two and three dimensions and of the Heisenberg model in three dimensions. This was followed by a detailed study of random systems in different dimensions and with different interactions. The percolation problem was studied for both Ising and Heisenberg systems in both two and three dimensions and the results showed that the temperature scale was controlled by the one-dimensional weak links in the percolating cluster8. Random antiferromagnetic magnetic fields can be generated by applying a uniform field to a random antiferromagnet and this was used to show that the behaviour was very different from that expected. The results10 showed that below a critical temperature the behaviour was non-ergodic in both two and three dimensional systems as also observed in spin glass systems.

 

6                    The anomalous effects of the quantum fluctuations on low dimensional systems have been studied for magnetic chains of both S=1/2, and S=1. The excitations in the S=1/2 were observed for the first time and shown to arise from pairs of spinons instead of single spin waves as observed in higher dimensional systems14. At low temperatures in materials that order three dimensionally there is a cross-over from spinon behaviour to spin wave behaviour that was explored. In the case of S=1 the excitations are well-defined and an exhaustive study was made of the temperature dependence. In addition to expectations a continuum contribution was observed at low temperatures but this is not yet understood..

 

7                    The magnetic structures of the rare earth metals were determined showing the importance of lock-in effects12 particularly for Ho and Er. In the former case the nature of the q=1/6 lock-in transition was explored and shown to consist of two transitions while the changes in the magnetic structure were determined when a magnetic field was applied. In the case of Er the structure in the incommensurate phase was shown to be a wobbling cycloid13 and a new interaction was proposed to explain this structure.

 

8                    Superlattices and alloys containing different rare earth metals and the magnetic structures have been determined and the nature of the magnetic interactions elucidated. Detailed measurements have been made of superlattices containing the rare earth Ho with non-magnetic materials such as Y, Lu and Sc as well as the magnetic materials Dy, Gd, Er and Pr. The results showed that there was a coherent magnetic structure if the structure was the same as Ho and the lattice mis-match was not too large. This is because the conduction electrons can then easily penetrate from one material to the other. The phase diagrams of several alloy systems: Ho/Y, Ho/Lu, Ho/Dy, Ho/Er, Ho/Tm, and Pr/Dy, have been determined16. The Ho/Er system has a complex phase diagram with 6 phases and a penta-critical point. Similar measurements have been made with the Laves phase materials. DyFe2/YFe2 superlattices are interesting because the rare earth ion aligns anti-parallel to the Fe atoms. The result is that there are a large number of complex phases, including exchange-spring phases, spin flop phases as well as ferromagnetic phases, which depend on the exact composition, the applied magnetic field and the temperature. o/

           

9                    X-ray scattering techniques have been developed to study thin films, surfaces and interfaces11. The first measurements were made of surface truncation rods by using an in-house spectrometer in Edinburgh and this has been followed by experiments that have determined the interface roughness15 and the structure of the superlattices including the density of dislocations as well as the structure of GaSb, Si and Ge and U20 grown on different materials.

 

10                The magnetic excitations of high temperature superconductors have been measured and shown for different oxygen concentrations in YBCO to change dramatically. For lightly doped materials the excitations are similar to those of the insulating parent but there is heavy damping at high energies possibly associated with the pseudo-gap. At higher concentrations there is a resonance the origin of which is at yet unknown19. Clear evidence has been observed that shows the pseudo-gap in high temperature superconductors is present in the neutron scattering and plays an important role.

 

References

1         R.A. Cowley.

Temperature Dependence of a Transverse Optic Mode in Strontium Titanate.

Physical Review Letters, 9, 159-161 (1962).

 

2         R.A. Cowley.

Lattice Dynamics of an Anharmonic Crystal.

Advances in Physics 12, 421-480 (1963).

 

3         R.A. Cowley.

Zero Sound, First Sound and Second Sound of Solids.

Proceedings of the Physical Society, 90, 1127-1147 (1967).

 

4         R.A. Cowley, P. Martel and R.W.H. Stevenson.

           Spin-Wave and Exciton Dispersion Relations of Cobalt Fluoride.

           Physical Review Letters, 18 162-164  (1967).

 

5         R.A. Cowley, W.J.L. Buyers and G. Dolling.

            Relationship of Normal Modes of Vibration of Strontium Titanate and its Antiferroelectric Phase Transition at 1100K.

           Solid State Communications, 7, 181-184 (1969).

 

6         A.D.B. Woods and R.A. Cowley.

            Structure and Excitations of Liquid Helium.

Reports on Progress in Physics, 36, 1135-1231 (1973).

 

7         W.G. Stirling, R. Scherm, P.A. Hilton and R.A. Cowley.

            Neutron Inelastic Scattering from Liquid Three.

Journal of Physics C: Solid State Physics  9 1643-1663 (1976).

 

8         R.A. Cowley, R.J. Birgeneau, G. Shirane and E.C. Svensson.

Critical Scattering near the Percolation Transition in MnCZn1-CF2

Physical Review Letters 39, 894-898 (1977).

 

9         A.D. Bruce and R.A. Cowley.

Structural Phase Transitions.

 London: Taylor and Francis, (1981).

 

10        R.J. Birgeneau, R.A. Cowley, G. Shirane and H. Yoshizawa

 Temporal Phase transition in the Three-Dimensional Ising Model

 Physical Review Letters, 54  2147-2150, (1985).

 

11      S. R. Andrews and R. A. Cowley

Scattering of x-rays from crystal surfaces

Journal of Physics C: Solid State Physics 18, 6427-6439, (1985).

 

12            R.A. Cowley, D.A. Jehan, D.F. McMorrow and G.J. McIntyre

                Evidence for a Devil's Staircase in Holmium Produced by an Applied Magnetic Field

                Physical Review Letters 56, 1521-1524, (1991)

 

13            R.A. Cowley and J. Jensen

                Magnetic Structures and Interactions in Erbium5

                Journal of Physics: Condensed. Matter 4, 9673-9696, (1992)

 

14            D A Tennant, T G Perring, R A Cowley and S E Nagler

                Unbound spinons in the S=½ antiferromagnetic chain KCuF3

                Physical Review Letters 70, 4003-4006, (1993)

 

15            P P Swaddling, D F McMorrow, R A Cowley, R C C Ward and M R Wells

                Determination of the Interfacial Roughness Exponent in Rare Earth Superlattices

                Physical Review Letters 73, 2232 – 2235, (1994)

 

16       R A Cowley, J A Simpson, C. Bryn-Jacobsen, R C C Ward, M R Wells and D F McMorrow

Magnetic Phase Diagram of Ho/Er alloys

Physical Review B 57, 8394-8407 (1998)

 

17         R A  Cowley

Quantum entanglement and neutron scattering experiments
Journal of Physics: Condensed Matter, 15, 4143-4152 (2003)

 

18            S.N. Gvasaliya, B. Roessli, R. A. Cowley, P. Huber and S. G. Lushnikov

Quasi-elastic scattering, random fields and phonon-coupling effects in PbMg1/3Nb2/3O3

Journal of Physics: Condensed Matter 17, 4343-4359 (2005)

 

19         C Stock, W J L Buyers, R A Cowley, P S Clegg, R Coldea, C D Frost, R Liang, D. Peets, D Bonn, W N Hardy, R J Birgeneau

From Incommensurate to dispersive spin-fluctuations: the high energy inelastic spectrum in superconducting YBa2Cu3O6.5

Physical Review. B, 71, 024522, (2005)

 

20       R. C C Ward, R A Cowley, N. Ling, W Goetze, and W G Stirling

                   The structure of epitaxial layers of uranium

                   Journal of Physics: Condensed Matter 20 135003 (2008)

 

 

3.   Current Research

 

a.         Relaxor Ferroelectrics

 

Relaxor ferroelectrics have a peak in the dielectric constant but no spontaneous electric polarisation and no change in the crystal structure. One of the most studied materials is PMN (PbMn2/3Nb1/3O3) for which the crystal structure at high temperatures is the cubic perovskite structure like BaTiO3, with a random arrangement of Mn and Nb ions on the Ti site. The dielectric constant peaks at about 400K but there is no change in the structure down to the lowest temperatures. Many groups have studied the neutron scattering and the most unusual feature is that below 600K the scattering shows the waterfall effect which is that as the wave-vector decreases the transverse optic mode becomes overdamped and it is not possible to determine its energy. Because the dielectric constant peaks at 400K most groups have assumed that this sets the energy scale for the dielectric critical fluctuations but our high resolution neutron scattering experiments has shown1 that the dielectric fluctuations occur with an energy of about 0.3meV.  This low energy scattering can be observed below 600K and decreases in energy on cooling to 400K. Below 400K there is elastic scattering but this is not a Bragg peak but has a finite wave-vector width. We have suggested that this is due to the random electric fields from the B site disorder preventing the development of long range dielectric order. Current measurements at PSI are being performed to understand the waterfall effect, to understand the change in the structure below 400K by measuring the elastic scattering from the micro-domains and to study the behaviour when PMN is mixed with PbTiO3 when a ferroelectric transition can be observed2.

 

b.         Spin and Orbital Excitations in CoO            

 

CoO is a transition metal compound with a transition to an antiferromagnetic structure at 290K. Above this transition, the crystal is a cubic face centred structure. The phonons and magnetic excitations were measured using neutron scattering by us3 in 1967. At the time we considered that we had correctly measured the phonon spectra but were uncertain whether our measurements of the magnetic excitations were correct. The problem is complicated because it is uncertain whether the magnetic ground state of the Co ions is given by the weak crystal field model or by the strong crystal field model. Furthermore in either model the  spin-orbit splitting is about the same energy as the Neel temperature and also because in the face centred structure the magnetic ordering is determined by the second nearest neighbour exchange interactions rather than the nearest neighbours. This leads to domain structure for the magnetism with 12 different domain orientations and with excitations that have strongly coupled spin and orbital components. We have now remeasured the magnetic spin waves at Chalk River and have found that in 1967 we did not obtain a correct magnetic model for the system, but unfortunately although we could show that our earlier results were incorrect, we were not able to obtain a satisfactory solution. Recently we have measured the magnetic excitations for CoO using the MAPS spectrometer at ISIS and have measured the low temperature scattering for about 30 different sample orientations. The data now require analysis using the HORACE programme at ISIS and we have begun a detailed analysis of the results so as to see if we can determine the magnetic excitations and interactions and the magnetic structure of the Co ions.

           

c          Neutron Scattering from Hydrogen

 

The scattering from water at high energies (several eV) was studied using the indirect time-of-flight spectrometer, VESUVIO at ISIS. The measurements found that the integrated intensity was less than expected (60%) and similar measurements were made for other samples containing hydrogen, such as polymers and metal hydrides, with similar results. The results were initially interpreted as showing that there was quantum entanglement of the hydrogen atoms in water at room temperature but this result was questioned4 and it is now generally agreed that this is not a correct explanation. The question however arises as to the origin of the effect and in particular whether it arises from an experimental error or whether another theoretical explanation is required. We did careful measurements using VESUVIO and a sample of polythene and found a similar decrease in intensity5 although somewhat smaller than the initial measurements (75%). So far all the measurements have been performed on VESUVIO because it has not so far been possible to perform them on another spectrometer. Recently we have made measurements on the direct time-of-flight spectrometer MARI with incident energies up to 500eV.  This opens up the possibility of measuring the scattering from hydrogen on a different type of instrument and hence to find whether the origin of the discrepancy in the intensity is an experimental artefact or requires a new and different theory.

 

d          Thin Layers and Superlattices

 

The Laves phases such as DyFe2 are cubic ferromagnets with transition temperatures at about 600K. We have shown that it is possible to grow superlattices on a Nb substrate when the materials grow with a (110) axis as the growth axis and on a Mo substrate when the growth axis is the (111) axis. We are studying superlattices of DyFe2/YFe2 and ErFe2/YFe2 for which the /preferred moment directions are quite different due to the different distortions of the cubic structure. The experiments are being performed on a reflectometer at the ILL and the experiments enable us to measure the changes in the structures as an external magnetic field is applied. The hope is to be able to control the properties so as to be able to use the magneto-structural distortions for sensors in the future.

In collaboration with the Chemistry Department we are also studying the growth of dots of In2O3 grown on top of ZrO2.. Both of these materials have cubic structures and they can be grown with the growth direction along the (001) axis. The unexpected feature is that the growth of the In2O3 is not homogeneous but that they grow as dots with a square base which then reduces in size to give a series of regular pyramids. The sides of the base are the (110) directions, while the height of the pyramid is about 600Ǻ and the faces of the pyramid are mostly (111) planes. We have measured how the strain varies through the pyramids and intend to perform further measurement both at DIAMOND and in-house to study the development of these pyramids for different growth times.

           

            References

       1          N. Gvasaliya, B. Roessli, R. A. Cowley, P. Huber and S. G. Lushnikov,

Quasi-elastic scattering, random fields and phonon-coupling effects in Pb Mg1/3Nb2/3O3

                   Journal of Physics: Condensed Matter, 17 4343 (2005)

 

2          S. N. Gvasaliya, B. Roessli, R. A. Cowley, S. Kojima and S. G. Lushnikov.

Neutron Scattering Studies of the Mechanism of Ferroelectricity in 68%PbMg1/3Nb2/3O3-32%PbTiO3

Journal of Physics: Condensed Matter 19 016219 1-14, (2007)

           

       3          J. Sakurai, W.J.L. Buyers, R.A. Cowley and G. Dolling.

                    Crystal Dynamics and Magnetic Excitations in Cobaltous Oxide.

                    Physical Review, 167, 510-518 (1968).

 

       4          R. A. Cowley,

                   Quantum entanglement and neutron scattering experiments

                   Journal of Physics: Condensed Matter 15 4143 (2003)

 

       5          R.  A. Cowley and J. Mayers                           

                   Anomalous neutron scattering from hydrogen

                   Journal of Physics: Condensed Matter 18, 5291-5301 (2006)

      

       6          M J Bentall, R A Cowley, R C C Ward, M R Wells and A Stunault

The Structure of rare earth thin films: holmium and gadolinium on yttrium

Journal of Physics: Condensed Matter, 15, 7155-7174 (2003).

 

 

 

 

X-ray scattering measurements from U thin films grown on Nb (red) or Gd (blue) showing that different structures are obtained

 

Diagram showing the different behaviour of the spin waves as a function of the temperature and the wave-vector as determined from an ISIS experiment on Rb2MnF4

 

 

 

 

 

            The intensity of the hydrogen scattering from polythene. The results were obtained with the direct geometry spectrometer MARI at ISIS with 4 different energies between 16eV and 140eV. Theory suggest that the intensity should be constant for angles below 900 and zero above, independent of the incident neutron energy.

 

 

 

 

 

 

 

The neutron scattering from a relaxor ferroelectric PMN. The red line shows the critical scattering which is present below 600K and which is associated with ferroelectric fluctuations