290 research outputs found
Combining high pressure and coherent diffraction: a first feasibility test
We present a first experiment combining high pression and coherent X-ray
diffraction. By using a dedicated diamond anvil cell, we show that the degree
of coherence of the X-ray beam is preserved when the X-ray beam passes through
the diamond cell. This observation opens the possibility of studying the
dynamics of slow fluctuations under high pressure.Comment: 3 pages, 2 figures, GHPR 2009 conferenc
Fault-tolerant, high-level quantum circuits: Form, compilation and description
© 2017 IOP Publishing Ltd. Fault-tolerant quantum error correction is a necessity for any quantum architecture destined to tackle interesting, large-scale problems. Its theoretical formalism has been well founded for nearly two decades. However, we still do not have an appropriate compiler to produce a fault-tolerant, error-corrected description from a higher-level quantum circuit for state-of the-art hardware models. There are many technical hurdles, including dynamic circuit constructions that occur when constructing fault-tolerant circuits with commonly used error correcting codes. We introduce a package that converts high-level quantum circuits consisting of commonly used gates into a form employing all decompositions and ancillary protocols needed for fault-tolerant error correction. We call this form the (I)initialisation, (C)NOT, (M)measurement form (ICM) and consists of an initialisation layer of qubits into one of four distinct states, a massive, deterministic array of CNOT operations and a series of time-ordered X- or Z-basis measurements. The form allows a more flexible approach towards circuit optimisation. At the same time, the package outputs a standard circuit or a canonical geometric description which is a necessity for operating current state-of-the-art hardware architectures using topological quantum codes
High-pressure study of X-ray diffuse scattering in ferroelectric perovskites
We present a high-pressure x-ray diffuse scattering study of the ABO
ferroelectric perovskites BaTiO_3 and KNbO_3. The well-known diffuse lines are
observed in all the phases studied. In KNbO_3, we show that the lines are
present up to 21.8 GPa, with constant width and a slightly decreasing
intensity. At variance, the intensity of the diffuse lines observed in the
cubic phase of BaTiO_3 linearly decreases to zero at GPa. These
results are discussed with respect to x-ray absorption measurements, which
leads to the conclusion that the diffuse lines are only observed when the B
atom is off the center of the oxygen tetrahedron. The role of such disorder on
the ferroelectric instability of perovskites is discussed.Comment: 4 pages, Accepted in PR
High Pressure X-Ray Diffraction Study of UMn2Ge2
Uranium manganese germanide, UMn2Ge2, crystallizes in body-centered
tetragonal ThCr2Si2 structure with space group I4/mmm, a = 3.993A and c =
10.809A under ambient conditions. Energy dispersive X-ray diffraction was used
to study the compression behaviour of UMn2Ge2 in a diamond anvil cell. The
sample was studied up to static pressure of 26 GPa and a reversible structural
phase transition was observed at a pressure of ~ 16.1 GPa. Unit cell parameters
were determined up to 12.4 GPa and the calculated cell volumes were found to be
well reproduced by a Murnaghan equation of state with K0 = 73.5 GPa and K' =
11.4. The structure of the high pressure phase above 16.0 GPa is quite
complicated with very broad lines and could not be unambiguously determined
with the available instrument resolution
Structural and optical studies of FeSb2 under high pressure
Nanostructured orthorhombic FeSb2 and an amorphous phase were formed by
mechanical alloying starting from a mixture of high purity elemental Fe and Sb
powders. The effects of high pressures on structural and optical properties
were studied using X-ray diffraction (XRD) and Raman spectroscopy (RS). XRD
patterns showed the presence of the orthorhombic FeSb2 phase up to the maximum
pressure applied (28.2 GPa). The XRD patterns showed also an increase in the
amount of the amorphous phase with increasing pressure up to 23.3 GPa. At 14.3
GPa, together with the former phases, a new phase was observed and indexed to a
tetragonal FeSb2 phase, but its volume fraction is small at least up to 23.3
GPa. For the orthorhombic FeSb2 phase, the pressure dependence of the volume
fitted to a Birch-Murnaghan equation of state gave a bulk modulus = 74.2 +- 3.0
GPa and its pressure derivative = 7.5 +- 0.6. RS measurements were performed
from atmospheric pressure up to 45.2 GPa. For the orthorhombic FeSb2 phase, the
Raman active mode was observed up to the maximum pressure applied, while the
mode disappeared at 16.6 GPa. For pressures higher than 21 GPa, the Raman
active mode of a tetragonal FeSb2 phase was observed, confirming ab initio
calculations reported in the literature.Comment: 31 pages, 11 figures and 2 tables. Already submitted for publicatio
Dynamics of the magnetic and structural a -> e phase transition in Iron
We have studied the high-pressure iron bcc to hcp phase transition by
simultaneous X-ray Magnetic Circular Dichroism (XMCD) and X-ray Absorption
Spectroscopy (XAS) with an X-ray dispersive spectrometer. The combination of
the two techniques allows us to obtain simultaneously information on both the
structure and the magnetic state of Iron under pressure. The magnetic and
structural transitions simultaneously observed are sharp. Both are of first
order in agreement with theoretical prediction. The pressure domain of the
transition observed (2.4 0.2 GPa) is narrower than that usually cited in
the literature (8 GPa). Our data indicate that the magnetic transition slightly
precedes the structural one, suggesting that the origin of the instability of
the bcc phase in iron with increasing pressure is to be attributed to the
effect of pressure on magnetism as predicted by spin-polarized full potential
total energy calculations
Solid helium at high pressure: A path-integral Monte Carlo simulation
Solid helium (3He and 4He) in the hcp and fcc phases has been studied by
path-integral Monte Carlo. Simulations were carried out in the
isothermal-isobaric (NPT) ensemble at pressures up to 52 GPa. This allows one
to study the temperature and pressure dependences of isotopic effects on the
crystal volume and vibrational energy in a wide parameter range. The obtained
equation of state at room temperature agrees with available experimental data.
The kinetic energy, E_k, of solid helium is found to be larger than the
vibrational potential energy, E_p. The ratio E_k/E_p amounts to about 1.4 at
low pressures, and decreases as the applied pressure is raised, converging to
1, as in a harmonic solid. Results of these simulations have been compared with
those yielded by previous path integral simulations in the NVT ensemble. The
validity range of earlier approximations is discussed.Comment: 7 pages, 5 figure
Complex high-pressure polymorphism of barium tungstate
We have studied BaWO 4 under compression at room temperature by means of x-ray diffraction and Raman spectroscopy. When compressed with neon as a pressure-transmitting medium (quasihydrostatic conditions), we found that BaWO 4 transforms from its low-pressure tetragonal structure into a much denser monoclinic structure. This result confirms our previous theoretical prediction based on ab initio calculations that the scheelite to BaWO 4-II transition occurs at room temperature if kinetic barriers are suppressed by pressure. However, our experiment without any pressure- transmitting medium has resulted in a phase transition to a completely different structure, suggesting nonhydrostaticity may be responsible for previously reported rich polymorphism in BaWO 4. The crystal structure of the low- and high-pressure phases from the quasihydrostatic experiments has been Rietveld refined. Additionally, for the tetragonal phase the effects of pressure on the unit-cell volume and lattice parameters are discussed. Finally, the pressure evolution of the Raman modes of different phases is reported and compared with previous studies. © 2012 American Physical Society.This research was supported by Spanish MEC (Grant No. MAT2010-21270-C04-01/04), MALTA Consolider Ingenio 2010 (Grant No. CSD2007-00045), and Vicerrectorado de Investigacion y Desarrollo of the Universitat Politecnica de Valencia (Grants No. UPV2011-0914 PAID-05-11 and No. UPV2011-0966 PAID-06-11). XRD data were collected at HPCAT, Advanced Photon Source (APS), Argonne National Laboratory. HPCAT is supported by CIW, CDAC, UNLV, and LLNL through funding from DOE-NNSA, DOE-BES, and NSF. APS is supported by DOE-BES under Contract No. DEAC02-06CH11357.Gomis Hilario, O.; Sans, JA.; Lacomba-Perales, R.; Errandonea, D.; Meng, Y.; Chervin, JC.; Polian, A. (2012). Complex high-pressure polymorphism of barium tungstate. 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