Superconductivity up to 29 K in SrFe2As2 and BaFe2As2 at high pressures

We report the discovery of superconductivity at high pressure in SrFe2As2 and BaFe2As2. The superconducting transition temperatures are up to 27 K in SrFe2As2 and 29 K in BaFe2As2, making these the highest pressure-induced superconducting materials discovered thus far.Comment: Accepted in Journal of Physics: Condensed Matte

Towards resolution of the Fermi surface in underdoped high-Tc superconductors

We survey recent experimental results including quantum oscillations and complementary measurements probing the electronic structure of underdoped cuprates, and theoretical proposals to explain them. We discuss quantum oscillations measured at high magnetic fields in the underdoped cuprates that reveal a small Fermi surface section comprising quasiparticles that obey Fermi-Dirac statistics, unaccompanied by other states of comparable thermodynamic mass at the Fermi level. The location of the observed Fermi surface section at the nodes is indicated by a body of evidence including the collapse in Fermi velocity measured by quantum oscillations, which is found to be associated with the nodal density of states observed in angular resolved photoemission, the persistence of quantum oscillations down to low fields in the vortex state, the small value of density of states from heat capacity and the multiple frequency quantum oscillation pattern consistent with nodal magnetic breakdown of bilayer-split pockets. A nodal Fermi surface pocket is further consistent with the observation of a density of states at the Fermi level concentrated at the nodes in photoemission experiments, and the antinodal pseudogap observed by photoemission, optical conductivity, nuclear magnetic resonance Knight shift, as well as other complementary diffraction, transport and thermodynamic measurements. One of the possibilities considered is that the small Fermi surface pockets observed at high magnetic fields can be understood in terms of Fermi surface reconstruction by a form of small wavevector charge order, observed over long lengthscales in experiments such as nuclear magnetic resonance and x-ray scattering, potentially accompanied by an additional mechanism to gap the antinodal density of states.Comment: 33 pages, 15 figures (this version updated with new figures and additional text, as published

Unconventional quantum vortex matter state hosts quantum oscillations in the underdoped high-temperature cuprate superconductors.

A central question in the underdoped cuprates pertains to the nature of the pseudogap ground state. A conventional metallic ground state of the pseudogap region has been argued to host quantum oscillations upon destruction of the superconducting order parameter by modest magnetic fields. Here, we use low applied measurement currents and millikelvin temperatures on ultrapure single crystals of underdoped [Formula: see text] to unearth an unconventional quantum vortex matter ground state characterized by vanishing electrical resistivity, magnetic hysteresis, and nonohmic electrical transport characteristics beyond the highest laboratory-accessible static fields. A model of the pseudogap ground state is now required to explain quantum oscillations that are hosted by the bulk quantum vortex matter state without experiencing sizable additional damping in the presence of a large maximum superconducting gap; possibilities include a pair density wave.Royal Society Winton Programme for the Physics of Sustainability Engineering and Physical Sciences Research Council (EPSRC; studentship and grant numbers EP/R513180/1, EP/M506485/1 and EP/P024947/1) European Research Council under the European Unions Seventh Framework Programme (Grant Agreement numbers 337425 and 772891). EPSRC Strategic Equipment Grant EP/M000524/1 Leverhulme Trust by way of the award of a Philip Leverhulme Prize. National Key Research and Development Program of China (grant no. 2016YFA0401704). Work performed at the National High Magnetic Field Laboratory (NHMFL) supported by NSF Cooperative Agreement DMR-1157490, the State of Florida, and the Department of Energy (DOE) DOE Basic Energy Sciences project: ‘Science of 100 tesla’

Temperature dependence of the exchange splitting in ferromagnetic metals

The temperature dependence of the exchange splitting of the energy bands in ferromagnetic iron has been studied at low temperatures by carefully examining the de Haas-van Alphen frequency associated with the minority-spin electron 'lens' sheets of the Fermi surface as a function of temperature between 1° and 4°K. A high resolution phase measurement technique revealed that the variation of the lens frequency over this temperature range was less than one part in 10⁵ and was virtually identical to that measured for the corresponding electron lens in molybdenum. By contrast, a variation of one part in 10⁴ would be expected for the iron lens on the basis of a literal interpretation of the Stoner model, in which the exchange splitting of the energy bands is proportional to the magnetization at all temperatures. The absence of any significant change of the frequency with temperature gives strong evidence that the magnetization in iron decreases almost entirely by spin-wave excitations and that spin waves have negligible effect on the exchange splitting. The latter conclusion is consistent with a recent theory by Edwards for the electron-magnon interaction in itinerant-electron ferromagnets. On the basis of Edwards' theory the experimental technique described in this dissertation can be used to systematically study the single-particle magnetization in metallic ferromagnets without any interference from spin-wave excitations. In particular our experimental results yield an upper bound for the single-particle magnetization in iron at low temperatures. A new set of low-frequency de Haas-van Alphen oscillations has been studied in iron and the experimental results obtained so far suggest the oscillations originate from very small ellipsoidal surfaces centered on the points N of the Brillouin zone. This result points to a particular ordering of the energy bands at N, a feature of the band structure that cannot be predicted reliably from first principles.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Quantum critical phenomena in a compressible displacive ferroelectric

The dielectric and magnetic polarizations of quantum paraelectrics and paramagnetic materials have in many cases been found to initially increase with increasing thermal disorder and hence, exhibit peaks as a function of temperature. A quantitative description of these examples of “order-by-disorder” phenomena has remained elusive in nearly ferromagnetic metals and in dielectrics on the border of displacive ferroelectric transitions. Here, we present an experimental study of the evolution of the dielectric susceptibility peak as a function of pressure in the nearly ferroelectric material, strontium titanate, which reveals that the peak position collapses toward absolute zero as the ferroelectric quantum critical point is approached. We show that this behavior can be described in detail without the use of adjustable parameters in terms of the Larkin–Khmelnitskii–Shneerson–Rechester (LKSR) theory, first introduced nearly 50 y ago, of the hybridization of polar and acoustic modes in quantum paraelectrics, in contrast to alternative models that have been proposed. Our study allows us to construct a detailed temperature–pressure phase diagram of a material on the border of a ferroelectric quantum critical point comprising ferroelectric, quantum critical paraelectric, and hybridized polar-acoustic regimes. Furthermore, at the lowest temperatures, below the susceptibility maximum, we observe a regime characterized by a linear temperature dependence of the inverse susceptibility that differs sharply from the quartic temperature dependence predicted by the LKSR theory. We find that this non-LKSR low-temperature regime cannot be accounted for in terms of any detailed model reported in the literature, and its interpretation poses an empirical and conceptual challenge