Boris Khaykovich

'Inverse' melting of a vortex lattice

Inverse melting is the process in which a crystal reversibly transforms into a liquid or amorphous phase when its temperature is decreased. Such a process is considered to be very rare, and the search for it is often hampered by the formation of non-equilibrium states or intermediate phases. Here we report the discovery of first-order inverse melting of the lattice formed by magnetic flux lines in a high-temperature superconductor. At low temperatures, disorder in the material pins the vortices, preventing the observation of their equilibrium properties and therefore the determination of whether a phase transition occurs. But by using a technique to ‘dither’ the vortices, we were able to equilibrate the lattice, which enabled us to obtain direct thermodynamic evidence of inverse melting of the ordered lattice into a disordered vortex phase as the temperature is decreased. The ordered lattice has larger entropy than the low-temperature disordered phase. The mechanism of the first-order phase transition changes gradually from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures.

Lindemann criterion and vortex-matter phase transitions in high-temperature superconductors

The vortex-matter in superconductors is generally believed to exist in two main phases, vortex-solid and vortex-liquid. Recent investigations of the phase diagram of anisotropic high-temperature superconductors indicate, however, the existence of at least three distinct phases: relatively ordered quasi-lattice, highly-disordered entangled vortex-solid, and a liquid phase. A theoretical analysis in terms of an extended Lindemann criterion provides a quantitative description of the resulting vortex-matter phase boundaries and the behavior of the transition lines with varying anisotropy and disorder.

Enhancement of long-range magnetic order by magnetic field in superconducting La 2 CuO 4 + y

We report a detailed study, using neutron scattering, transport and magnetization measurements, of the interplay between superconducting (SC) and spin density wave (SDW) orders in

Compact x-ray source based on burst-mode inverse Compton scattering at 100 kHz

{\mathrm{La}}_{2}{\mathrm{CuO}}_{4+y}.

Field-induced transition between magnetically disordered and ordered phases in underdoped La 2 − x Sr x Cu O 4

Both kinds of order set in below the same critical temperature. However, the SDW order grows with applied magnetic field, whereas SC order is suppressed. Most importantly, the field dependence of the SDW Bragg peak intensity has a cusp at zero field, as predicted by a recent theory of competing SDW and SC orders. This leads us to conclude that there is a repulsive coupling between the two order parameters. The question of whether the two kinds of order coexist or microscopically phase separate is discussed.

From x-ray telescopes to neutron scattering: Using axisymmetric mirrors to focus a neutron beam

A design for a compact x-ray light source (CXLS) with flux and brilliance orders of magnitude beyond existing laboratory scale sources is presented. The source is based on inverse Compton scattering of a high brightness electron bunch on a picosecond laser pulse. The accelerator is a novel high-efficiency standing-wave linac and rf photoinjector powered by a single ultrastable rf transmitter at X-band rf frequency. The high efficiency permits operation at repetition rates up to 1 kHz, which is further boosted to 100 kHz by operating with trains of 100 bunches of 100 pC charge, each separated by 5 ns. The entire accelerator is approximately 1 meter long and produces hard x rays tunable over a wide range of photon energies. The colliding laser is a

Demonstration of achromatic cold-neutron microscope utilizing axisymmetric focusing mirrors

\mathrm{Yb}\ensuremath{\mathbin:}\mathrm{YAG}

Effect of a magnetic field on long-range magnetic order in stage-4 and stage-6 superconducting La 2 CuO 4 + y

solid-state amplifier producing 1030 nm, 100 mJ pulses at the same 1 kHz repetition rate as the accelerator. The laser pulse is frequency-doubled and stored for many passes in a ringdown cavity to match the linac pulse structure. At a photon energy of 12.4 keV, the predicted x-ray flux is

Structural transition in NaxCoO2 with x near 0.75 due to Na rearrangement

5\ifmmode\times\else\texttimes\fi{}1{0}^{11}\text{ }\text{ }\mathrm{photons}/\mathrm{second}

Structure of cholesterol helical ribbons and self-assembling biological springs

in a 5% bandwidth and the brilliance is