Ahmadwand H., Salamati, H., Kameli P., Razavi F. S.
Ni/NiO nanoparticles were synthesized using a mixture of nickel acetate and polyvinyl acetate as precursors, followed by heating between 350 to 450 C. The prepared samples were investigated using XRD, TEM and SQUID magnetometry. Depending on the heating temperature, the average sizes of Ni and NiO crystallites were found to be in the ranges of 25 to 37 nm and 10 to 26 nm, respectively. The analysis of XRD patterns shows that the volume percentage of Ni can be easily controlled by the heating temperature. Magnetization measurements show that the samples consist of ferromagnetic moments which are blocked at low temperatures and a disordered NiO component which causes spin-glass like behavior.
Crandles D. A. , DesRoches B., Razavi F. S. ,
Abstract: Room temperature ferromagnetic hysteresis is observed in commercial SrTiO3 substrates purchased from a variety of suppliers. It is shown that the ferromagnetic signal comes from the unpolished surfaces. Surface impurity phases cannot be detected using either x-ray diffraction or energy dispersive x-ray spectra on the unpolished surfaces. However, a possible correlation between surface disorder (x-ray diffraction peak linewidth) and ferromagnetism is observed. Ar ion bombardment (10-90 keV) can be used to produce surface layer disorder but is not found to induce ferromagnetism. Annealing of the substrates at temperatures ranging from 600 to 1100 degrees C is found to alter the hysteresis curves differently depending on whether the annealing is performed in air or in vacuum. Identical annealing behavior is observed if the substrates are artificially spiked with iron. This suggests that the ferromagnetic hysteresis of as purchased SrTiO3 could be due to Fe contamination of the unpolished surfaces. In addition, it is shown that no ferromagnetism is observed in samples that contain 10(19)-10(21) cm(-3) of oxygen vacancies if all the faces are polished. (C) 2010 American Institute of Physics.
JOURNAL OF APPLIED PHYSICS Volume: 108 Issue: 5 Article Number: 053908 Published: SEP 1 2010
Ma RC , Mansour AI , Egilmez M , Winterfield CE , Fan I , Chow KH , Jung J , Prabhakaran D , Razavi F
Abstract: A superconducting ring could be used to simulate the supercurrent conduction, and its interaction with magnetic vortices in a superconducting solenoid, allowing one to investigate the nature of the vortex structure and its pinning in the presence of the persistent supercurrent. The dissipation of the persistent supercurrent has been studied in a ring-shaped high purity single crystal of Bi2Sr2CaCu2Ox in order to obtain the information about the exponent mu, a parameter in the scaling relation between the effective energy barrier against vortex motion U-eff and the persistent current density J. The measurements of the persistent supercurrent decay show a transition from a strongly nonlogarithmic to a logarithmic decay regime with an increasing temperature. In response to a small increase in the concentration of oxygen vacancies mu decreases in the logarithmic decay regime but remains almost constant in the nonlogarithmic one. (C) 2010 American Institute of Physics.
JOURNAL OF APPLIED PHYSICS 107 083909 2010.
Razavi FS , Gharetape SJ , Crandles DA , Christiani G , Kremer RK , Habermeier HU
Highly conducting interfaces of transition metal oxide heterolayers could provide a promising access to transition metal oxide based electronic devices similar to those based on semiconductor heterostructures. Recently, metallic conductivity has been reported at the LaAlO3/SrTiO3 heterointerface. Here we report the observation of diodelike behavior and the formation of a random diode network in thin films of LaVO3 deposited on as-polished SrTiO3 substrates. Depending on films annealing conditions we observed linear or nonlinear I-V characteristic below 50 K.
APPLIED PHYSICS LETTERS , 96:042110 (2010).
Reuvekamp P , Razavi FS , Hoch C , Kim JS , Kremer RK 1, Simon A
To prevent the loss of K in growing single crystals of Ba1-x K (x) Fe2As2 we developed a rapid-heating Sn-flux method. Large single crystals with the optimal superconducting transition temperature T (C)a parts per thousand 38 K were obtained and their structural, chemical and superconducting properties were investigated. Additionally, the effect of post-growth annealing on these crystals at different temperatures was examined. Scanning electron microscopy microprobe studies on a crystal with the composition goal of Ba0.25K0.75Fe2As2 revealed a well defined separation of two phases with compositions that are suggestive of rational ratios of the K and Ba content.
JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 22 353 2009.
The synthesis of a new 3-ethynylthienyl-substituted QsaIH ligand (QsalH is the short form for N-(8-quinolyl)salicylaldimine) (ThEAsalH 3), and the preparation, electronic, and magnetic properties of three homoleptic and cationic iron(3+) complexes containing this ligand with PF6- 4, SCN- 5, and ClO4- 6 counteranions are reported. In all three complexes a spin-crossover is observed in the solid state by variable temperature magnetic susceptibility measurements and Mossbauer spectroscopy, indicating that the synthetic modification of the QsalH ligand has not significantly altered the electronics at the metal center. This includes the observation of a very rare S = 5/2 to 312 spin-crossover in a non-porphyrin iron(3+) complex 5. The molecular structure and magnetic properties of an unusual iron(2+) complex 7 generated by reduction of complex 6 serendipitously during a recrystallization attempt in aerobic acetone solution is also reported: Complexes 4-6 feature iron(3+) reduction and oxidation of the thiophene ring at potentials of approximately -0.7 and +1.2 V (vs Fc), respectively.
INORGANIC CHEMISTRY, 48 , 699 2009.
A test was made of the ability of Kramers-Kronig-constrained variational dielectric fitting to extract the optical conductivity of a thin film from reflectance data containing structure due to both thin film and substrate. The reflectance of a series of well-characterized thin filius of SrRuxMg1-xO3 and SrRuxO3 with a variety of thicknesses (approximate to 56-300nm) and dc resistivities (approximate to 250-2200 mu Omega cm) was measured. The low frequency values of the extracted optical conductivities agree with the dc measurements. however, removal of features due to the substrate improves with increasing film thickness. (c) 2008 Optical Society of America
APPLIED OPTICS, 47, 4205, 2008.
: We present a combined experimental and theoretical study of the effects of pressure on T-c of the hexagonal layered superconductors nH-CaAlSi (n=1, 5, and 6), where nH denotes the different stacking variants that were recently discovered. Experimentally, the pressure dependence of T-c has been investigated by measuring the magnetic susceptibility of single crystals up to 10 kbars. In contrast to previous results on polycrystalline samples, single crystals with different stacking sequences display different pressure dependences of T-c. 1H-CaAlSi shows a decrease in T-c with pressure, whereas 5H- and 6H-CaAlSi exhibit an increase in T-c with pressure. Ab initio calculations for 1H-, 5H-, and 6H-CaAlSi reveal that an ultrasoft phonon branch associated with out-of-plane vibrations of the Al-Si layers softens with pressure, leading to a structural instability at high pressures. For 1H-CaAlSi, the softening is not sufficient to cause an increase in T-c, which is consistent with the present experiments but adverse to previous reports. For 5H and 6H, the softening provides the mechanism to understand the observed increase in T-c with pressure. Calculations for hypothetical 2H and 3H stacking variants reveal qualitative and quantitative differences
PHYSICAL REVIEW B ,77 144502, 2008.
: A new bis tridentate ligand 2,2'-bipyridine-3,3'-[2-pyridinecarboxamide] H2L1 Which can bind transition metal ions has been synthesized via the condensation of 3,3'-diamino-2,2'-bipyridine together with 2-pyridine carbonyl chloride. Two copper(II) coordination compounds have been prepared and characterized: [Cu-2(L-1)(hfac)(2)]center dot 3CH(3)CN center dot H2O (1) and [Cu-2(L-1)Cl-2]center dot CH3CN (2). The single-crystal X-ray structures reveal that complex 1 crystallizes in the triclinic space group P (1) over bar, with the unit cell parameters a = 12.7185(6) angstrom, b = 17.3792(9) angstrom, c = 19.4696(8) angstrom, alpha = 110.827(2)degrees, beta = 99.890(3)degrees, gamma = 97.966(3)degrees, V = 3868.3(3) angstrom(3), Z = 4, R = 0.0321 and R-W = 0.0826. Complex 2 crystallizes in the monoclinic space group P-21/n with the unit cell parameters a = 12.8622(12) angstrom, b = 9.6100(10) angstrom, c = 19.897(2) angstrom, beta = 102.027(3)degrees, V = 2405.3(4) angstrom(3), Z = 4, R = 0.0409 and R-W = 0.1005. In both complexes the ligand is in the dianionic form and coordinates the divalent Cull ions via one amido and two pyridine nitrogen donor atoms. In 1, the coordination geometry around both Cull ions is best described as distorted trigonal bipyramidal where the remaining two coordination sites are satisfied by hexafluoroacetylaceton ate counterions. In 2 both Cull ions adopt a (4 + 1) distorted square pyramidal geometry. One copper forms a longer apical bond to an adjacent carbonyl oxygen atom, whereas the second copper is chelated to a neighboring Cu-Cl chloride ion to afford a mu-Cl-bridged dimerized [Cu-2(L-1)Cl-2](2) complex. The magnetic susceptibility data for 1 (2-270 K), reveal the occurrence of weak antiferromagnetic interactions between the Cull ions. In contrast, variable-temperature magnetic susceptibility measurements for 2 reveal more complex magnetic properties, with the presence of a weak antiferromagnetic exchange (J = -10.1 K) between the copper ions in each dinuclear copper complex and a stronger ferromagnetic exchange interaction (J = 32.9 K) between the Cull ions of the Cu(mu-Cl)(2)Cu climeric bridging units.
INORGANIC CHEMISTRY , 46, 8560, 2007 .
J. S. Kim, L. Boeri, J. R. O'Brien, F. S. Razavi, , R. K. Kremer
Abstract: We report the discovery of superconductivity below 1.65(6) K in Sr-intercalated graphite SrC6, by susceptibility and specific heat (C-p) measurements. In comparison with CaC6, we found that the anisotropy of the upper critical fields for SrC6 is much reduced. The C-p anomaly at T-c is smaller than the BCS prediction, indicating an anisotropic superconducting gap for SrC6 similar to CaC6. The significantly lower T-c of SrC6 as compared to CaC6 can be understood in terms of "negative" pressure effects, which decreases the electron-phonon coupling for both in-plane intercalant and the out-of-plane C phonon modes. We observed no superconductivity for BaC6 down to 0.3 K.
PHYSICAL REVIEW LETTERS 99 (2): Art. No. 027001 JUL 13 2007
J. Wang , B. Slater, A. Alberola , H. Stoeckli-Evans , F. S. Razavi , M. Pilkington
Abstract: A novel dimeric Mn-II complex {[Mn(N3O2)]Cl(OH2)}(2)2Cl (2) of a macrocyclic Schiff base ligand derived from the condensation of 2,2', 6,6'-tetraacetyl-4,4'-bipyridine with 3,6-dioxaoctane-1,8-diamine in the presence of a stoichiometric amount of MnII has been prepared and characterized. The X-ray analysis of 2 reveals that the two Mn ions assume a pentagonal-bipyramidal geometry, with the macrocycle occupying the pentagonal plane and the axial positions being filled by a halide ion and a H2O molecule. Magnetic susceptibility data (2-270 K) reveal the occurrence of weak antiferromagnetic interactions between covalently tethered Mn-II-Mn-II dimeric units.
INORGANIC CHEMISTRY 46 (12): 4763-4765 JUN 11 2007
U. Gebhardt , N. Y. Kasper, A. Vigliante, P. Wochner , H. Dosch , F. S. Razavi , H_U. Habermeier
Abstract: We present an extended synchrotron x-ray scattering study of the structure of thin manganite films grown on SrTiO3(001) substrates and reveal a new kind of misfit strain relaxation process which exploits twinning to adjust lattice
mismatch. We show that this relaxation mechanism emerges in thin films as one-dimensional twinning waves which freeze out into a twin domain pattern as the manganite film continues to grow. A quantitative microscopic model which uses a matrix formalism is able to reproduce all x-ray features and provides a detailed insight into this novel relaxation mechanism. We further demonstrate how this twin angle pattern affects the transport properties in these functional films.
PHYSICAL REVIEW LETTERS 98 (9): Art. No. 096101 MAR 2 2007
J. S. Kim , L. Boeri, R. K. Kremer, F. S. Razavi
Abstract: The effect of pressure on the superconducting transition temperature (T-c) of the Ca-intercalated graphite compound CaC6 has been investigated up to similar to 16 kbar. T-c is found to increase under pressure with a large relative ratio Delta T-c/T-c of approximate to 0.4%/kbar. Using first-principles calculations, we show that the positive effect of pressure on T-c can be explained within the scope of electron-phonon theory due to the presence of a soft phonon branch associated with in-plane vibrations of the Ca atoms. The implications of the present findings for the current debate about the superconducting mechanism in graphite intercalation compounds are discussed.
PHYSICAL REVIEW B 74 (21): Art. No. 214513 DEC 2006
J. S. Kim , R. K. Kremer, L. Boeri, F. S. Razavi
Abstract: The superconducting state of Ca-intercalated graphite CaC6 has been investigated by specific heat measurements. The characteristic anomaly at the superconducting transition (T-c=11.4 K) indicates clearly the bulk nature of the superconductivity. The temperature and magnetic field dependence of the electronic specific heat are consistent with a fully gapped superconducting order parameter. The estimated electron-phonon coupling constant is lambda=0.70 +/- 0.04, suggesting that the relatively high T-c of CaC6 can be explained within the intermediate coupling BCS approach.
PHYSICAL REVIEW LETTERS 96 (21): Art. No. 217002 JUN 2 2006 .
F. S. Razavi, G. V. S. Rao, H. Jalili, H.-U. Habermeier,
Abstract: We have studied the resistivity of a hole-doped 95 nm thin film of La0.88Sr0.1MnO3 sample as a function of pressure up to 2.0 GPa. For pressures below 0.81 GPa, the sample showed that the resistivity decreased at all temperatures and the metal insulator transition temperature T-MI increased to higher temperatures similar to the pressure results observed previously in bulk samples and thin films of manganites. However, for pressures above 0.81 GPa, we observed a rapid increase in resistivity and a rapid decrease of T-MI as a function of pressure. The rate of change of T-MI with pressure is -133.5 K/GPa which is an order of magnitude larger than that of bulk samples and thin films of manganite with an opposite sign. We interpreted our results as a consequence of a phase change in the film by pressure from having strain induced periodic microtwinned regions below 0.81 GPa to a strained free region and formation of coherent twins above this pressure in 95 nm thin film of La0.88Sr0.1MnO3.
APPLIED PHYSICS LETTERS 88 (17): Art. No. 174103 APR 24 2006 .
S. Jacob, T. Roch, F.S. Razavi, G.M. Gross, and H.-U. Habermeier.
The discovery of the colossal magnetoresistance and pressure effects introduced new questions
concerning the complex interplay among lattice structure, magnetism, and transport in doped
perovskite manganites. To observe the interplay between the effect of pressure and strain due to
the substrate in the La2/3Ca1/3MnO3 compound, we produced
epitaxial films with the thickness
varied between 20 and 100 nm on SrTiO3 substrates, using the pulse laser
deposition technique
and under the same epitaxial growth conditions. We measured magnetic transition temperature
T-C and resistivity rho, as a function of pressure up to 8 kbar. Using these results, the
dependence of bond compression on pressure, and the lattice distortion induced by the
substrate, are discussed. We also report a universal behavior from the plot of d ln T-C/dP and
drho/dP versus T-C for these films, as well as previously published results on ceramic and thin
films of this compound. (C) 2002 American Institute of Physics.
J. Appl. Phys., 91, 2232-5, 2002.
The superconducting transition temperature Tc of polycrystalline samples of MgB2
was studied under hydrostatic and quasi-hydrostatic pressures up to 1 and 12 GPa, respectively.
The rate of Tc
change with hydrostatic pressure, (dT(c)/dP), was found to be -1.18 K(GPa)(-1). Quasi-hydrostatic
measurements yield a slightly higher value: (dT(c)/dP) = -1.2 K(GPa)(-1). Analysis of the
electron-phonon coupling parameter lambda and its logarithmic volume derivative using the
McMillan expression for Tc yields results that fall within the range of a conventional s-wave BCS
superconductor with intermediate coupling, in agreement with several experimental and theoretical
studies that have already been reported. Based on electronic structure calculations performed by us
as well as those reported in the literature, it is concluded that the change in the coupling parameter
lambda is dominated by the change of the average phonon frequency with pressure.
feasible.
Physica C, 366, 73-9, 2002.
An interesting memory effect occurs when La1-xSrxMnO3 (x~1/8)
is repeatedly exposed to x
rays. While the "dark" conductivity remains unaffected by the irradiation history, the
conductivity is markedly enhanced upon exposure to x rays at low temperatures. Immediately
after renewed exposure, it recovers the value attained at the end of the previous
exposure. We provide a qualitative explanation of this unusual effect in terms of three
distinct states with different orbital correlations.
Phys. Rev. B, 64, 100404, 2001.
X-ray scattering experiments have demonstrated that for thin lms (100 Angstrom to 500 Angstrom)
of La0.9Sr0.1SMnO3 epitaxially grown on SrTiO3
(100) substrates, the accommodation of the lattice mismatch between lm and substrate gives rise
to a modulated structure. The non-bulk like structural properties of these thin lms are the
origin of anomalies in the electronic and magnetic behavior. Studies of the lattice as a function
of temperature clearly reveal a magnetostrictive effect close to the Curie temperature. For
thicker lms the strain relief mechanism is replaced by formation of coherent microtwins and
some of the bulk properties are recovered, but some differences are still observed.
Europhys Lett., 54, 619-25, 2001.
Lanthanum manganites belong to the perovskite type Mott insulators. They show a doping
dependent drastic modification of conductivity including a metal-insulator transition
associated with ferromagnetic ordering. We investigated La2/3Cal/3MnO3
thin films, a member of this class of material showing a colossal magnetoresistance.
Application of internal and external pressure can be used to modify the electronic and
magnetic properties of this material. It has been shown that, in
La2/3Cal/3MnO3, as the band width for the itinerant e,
electrons broadens, it causes an enhancement of the ferromagnetic coupling and consequently an
increase of the metal-insulator transition T-MI, which is close to the ferromagnetic transition
temperature T-MI. Thin films were deposited on SrTiO3 (STO) single crystals using
the pulsed laser deposition technique. In this study we prepared films with different T-MI by
choosing an appropriate depositing temperature, annealing condition and thickness range.
We studied the effect of hydrostatic pressures on the in-situ annealed films in order to
investigate the effect of external pressure on T-MI and resistivity as a function of annealing.
Due to lattice mismatch between film and substrate we introduced controllable biaxial strain.
The increse of lattice parameters and T-MI shows the relaxation process by varying the film
thickness and by annealing thin layers. Our results are a contribution to the clarification of
the interplay between the microscopic structure of Mn-O-Mn units with the macroscopic transport
properties.
J. Alloy Compd., 317, 141-4, 2001.
We have grown single crystalline doped LaMnO3 thin films using the pulsed laser
deposition technique and analysed the interrelation of epitaxial strain with transport
properties. An unexpected possibility of tailoring the metal-insulator transition
temperature, T-MI, has been developed and correlated with biaxially epitaxial strain. Whereas
in the case of La2/3Cal/3MnO3 a reduction of T-MI is observed,
we demonstrate an unexpected epitaxial strain-induced metal-insulator transition for
La.88Sr.lMnO3 thin films. The results are explained by biaxial
tensile and compressive stress of the films, respectively.
Physica C, 341, 777-8, 2000.
Introducing biaxial strain in doped lanthanum manganite thin films is an excellent tool to
modify the electrical and transport properties systematically by controlled changes of the
microstructure on an atomic scale. We deposited La2/3Cal/3MnO3
thin films onto SrTiO3 single-crystal substrates
under reproducible conditions in an optimized pulsed laser deposition process.
A thickness range of 40 to 500 nm was chosen for the manganite layers to investigate the
variation of the average lattice parameters with thickness using X-ray diffraction. We re-
investigated the samples after annealing to determine the structural aspects of the
relaxation behavior of the layers. A second set of samples was prepared with an analogous
in situ annealing process. AFM was applied to examine the morphology and grain size of the
layers. Due to a thermally induced strain relaxation process the thicker films show a
gradual approach of the out-of-plane lattice constant to the bulk ceramic value.
Further relaxation could be achieved with an annealing at 900 degrees C for 1 h. Lattice
parameter changes with varying thickness and annealing processes are correlated with transport
and magnetic properties.
J. Magn. Magn. Mater., 211, 22-7, 2000.
The lattice mismatch of doped manganites and conventional oxide substrate materials offers an
interesting opportunity to tailor the epitaxial strain of thin films systematically. This can be
used to study the interrelation of the microscopic arrangement of the atoms and the
magnetotransport properties. In our experiment La0.67Ca0.33MnO3
thin films with varying thickness (40-520 nm) were grown on SrTiO3 (0 0 1) single-
crystal substrates using the pulsed laser deposition technique. The thickness-dependence
epitaxial strain leads to a systematic modification of the characteristic parameters of the
magnetization curve and the dependence of resistivity on temperature. We analyse the magnetic
and transport data of as-grown films and compare them to results of measurements performed
on identical films subjected to a Ih post-deposition annealing process at 900 degrees C in
flowing oxygen.
J. Magn. Magn. Mater., 211, 41-6, 2000.
Using the technique of pulsed laser deposition, we have studied the correlation of the film deposition
conditions with the microstructure and magnetotransport properties of La-2/3 Ca1/3MnO3 thin
films. Detailed TEM microstructure studies revealed the dependence of the local microstructure with
the deposition temperature. The TEM studies reveal a distortion of the La2/3Ca1/3MnO3 lattice
leading to a breakdown of the symmetry from orthorhombic to monoclinic. At high deposition
temperatures (T < 800 degrees C), the substrate/film interface is perfectly coherent, suggesting that
the film is strained due to the lattice mismatch. Effects of hydrostatic pressure on the resistivity of the
films reveal an increase in temperature of the metal-insulator transition and a colossal change in
resistivity qualitatively comparable to the magnetoresistance effect. The data are interpreted in terms
of microstructure sensitive changes of the contraction and alignment of the Mn-O-Mn bonds.
J. Superconductivity., 12 (1), 285-8, 1999.
The macroscopic properties of the superconducting phase YPd2B2C in the
multiphase compound YPd5B3C0.3 have been investigated. The
onset of superconductivity was observed at 22.6 K, zero resistance at 21.2 K, the lower critical
field H-cl at 0 K was estimated to be H-cl(0) similar to 310 G and the compound was found to be
an extreme type-II superconductor with the upper critical field in excess of 50 000 G below
15 K. At T=15 K, the Ginzburg-Landau parameter was determined to be kappa > 9 corresponding to a
coherence length xi similar to 80 Angstrom and magnetic penetration depth of 800 Angstrom. The
superconducting transition temperature T-c(P) was also measured under hydrostatically applied
pressure. T-c(P) of YPd2B2C decreases linearly with dT(c)/dP similar to
-9 x10(-2) K/kbar. The discussion of T-c(P) will focus on the influence pressure has on the
phonon spectrum and the density of states near the Fermi level.
Physica C, 321, 108, 1999.
The specific heat of the single crystals of UPd2Si2 and
UNi2Si2
has been studied using the step heating method for the temperature range between 2 and 250 K.
Successive phase transitions at high temperatures were observed for both samples, which are
consistent with previously reported magnetization and neutron scattering results. No evidence
of any phase transition was observed at temperatures below 70 K, which has been
previously reported. The Debye temperature and the electronic specific heat coefficient gamma
of UPd2Si2 are found to
be 117 K and 19 mJ/mol K2, respectively. In low temperatures between 2 and 4 K, an
upturn in specific heat with T2 dependence
was observed. Several possibilities for this upturn in the specific heat are discussed.
Phys. Rev. B, 58, 113-6, 1998.
We present the temperature dependence of the acoustic sound velocity for a CuGeO3 single crystal
measured under pressure up to 10 kbar. Anomalous elastic behavior along the c-axis, observed
around 100 K, is accounted for by the existence of a magnetoelastic coupling. We show that this
coupling is weakly affected by pressure. Moreover, the temperature dependence of the sound
velocity below the spin-Peierls phase transition is analyzed using a Landau-type model from
which we obtain the temperature and pressure dependences of the order parameter. We find that
the magnetic spin-Peierls energy gap increases with pressure at a rate of d Delta /dP=0.11+or-00.01
meV/kbar.
Phys. Rev. B, 58, 882-6, 1998.
It is well established that the ternary intermetallic family of compounds UT2Si2
, where T denotes
a transition metal, shows a rich variety of electronic and magnetic ground-state properties. In
order to better understand the magnetic properties of these materials, we have investigated the
behavior of the electrical resistivity of UNi22Si2
and UPd2Si2 as a function of temperature
and pressure. In the case of UNi2Si2, the proposed pressure-temperature
phase diagram is very
similar to its magnetic-field-temperature phase diagram. The pressure-temperature phase diagram
obtained for both compounds is also compared to predictions made using a mean-field Landau-type
analysis.
Phys. Rev. B, 57< 5220-4, 1998.
The superconducting transition temperature of hexagonal and orthorhombic
ZrRuP was studied under hydrostatic and quasi-hydrostatic pressures up to 7
GPa. It was found that the superconducting transition temperature decreases
with the pressure slightly for both structures of this compound. The rate
of Tc change with pressure is (dTc/dP) = -53mK (GPa)-1 for hexagonal
structure and (dTc/dP) = -98 mK (GPa)-1 for orthorhombic structure. We
calculated the logarithmic volume derivative of the electron-phonon
coupling parameter using the McMillan equation. It was found that under
pressure the change in the coupling parameter depends mainly on the change
of the phonon frequency with the pressure.
Phys. C, 292, 79-82, 1997.
The results of the magnetoresistance on two thin-film samples of
YBa2Cu3O7/PrBa2Cu3O7
superlattices have been reported for the temperatures near
and below the superconducting transition temperature. The thermally activated
flux-creep motion theory was used to describe the magnetoresistance results at
temperatures much lower than Tc. The existence of the superconducting
fluctuation near Tc as well as the two-dimensional behavior of superlattices
were studied by comparing the magnetoresistance results and Larkin's
quasi-two-dimensional (2D) superconducting fluctuation theory. A good fit
between experimental results and theory was obtained and contrary to previous
results, no crossover between quasi-2D and 3D was observed.
Phys. Rev. B, 55, 6631-5, 1997.
The magnetic phase diagram of Si-doped (0.7 and 1.0%)
CuGeO3 single crystals was obtained from an ultrasonic
velocity measurement. The magnetic phase diagram is similar
to what is generally found in pure spin-Peierls (SP)
systems: a SP instability is identified although an
additional antiferromagnetic phase is observed at the lowest
|temperatures. The diagram is weakly anisotropic: a spin-flop
(SF) phase is observed for the magnetic field aligned along
the c axis and a hard axis along a is deduced from the
angular dependence of the SF field. These results are
complemented by superconducting quantum interference device
magnetization measurements.
Phys. Rev. B, 52, R6971, 1995.
The superconducting properties of metallic glasses Zr
75(NixFe1-x)25 (x=0, 0.25, 0.50, 0.75,
1.0) were studied both at ambient pressure and under quasi-
hydrostatic pressure up to 8 GPa (80 kbar). To obtain
information on the spin fluctuation parameter lambdaSF
the following measurements were carried out: the dc
magnetic susceptibility, the upper critical field at the
ambient pressure superconducting transition temperature
Tc/(P=1 bar), mass density, and room temperature
resistivity. From the pressure dependence of the
superconducting transition temperature Tc(P) in
Cu25Zr75 glass, we estimate the volume derivative of
ph/ in this alloy using the McMillan equation. This is used
as a reference value for the volume derivative of lambdae-ph in
the Zr75(NixFe1-x)25
alloys. Using the modified McMillan equation, which includes
spin fluctuation effects and the above reference value for
the volume derivative of the e-ph coupling parameter, we
determine lambdae-ph, the electron-spin fluctuation
coupling parameter lambdaSF and the rate of
lambdaSF/d ln V) as a function of Fe concentration in
these alloys. Both spin fluctuations and the rate of
suppression of spin fluctuations with pressure
(d ln lambdaSF/d ln V) are found to increase with higher Fe
concentration in these alloys.
Phys. Rev. B, 52, 9674, 1995.
Compositional dependence of thin films of BiPbSrCaCuO on target to
substrate distance
Using magnetron sputtering techniques, several thin films of
superconducting BiPbSrCaCuO were fabricated by varying the distance
between the substrate (single crystal of MgO with polished (100) plane) and the
targets. During the deposition the gas pressure was kept constant at 0.3
mbar (1 mbar = 0.1 kPa) and the substrate temperature was kept at 700
degrees C. An energy-dispersive X-ray fluorescence was designed using a radioisotope source with a secondary target and a Si(Li) X-ray spectrometer and it was
used to measure the atomic composition of the film quantitatively. It was
found that the Ca concentration relative to Sr increases linearly as the
distance between the substrate and the targets increases. However, both
Cu and Bi show a more complex variation of concentration with distance.
The X-ray diffraction results also indicated that the films are grown
epitaxially along the C axis, which showed a semiconducting behavior with
TC, zero below 60 K.
Can. J. Phys., 73, 35, 1995.
Planar tunnel junctions have been realized on liquid-etched
Bi2Sr2CaCu2O8+x (BSCCO) single
crystals with natural barriers and Pb, Pb-Bi, Au crystals
with natural barriers and Pb, Pb-Bi, Au
counterelectrodes. At T=4.2 K and I-V characteristics of
these junctions often show multiple switches that disappear
at the Tc of the BSCCO compound. The dV/dI vs. V
curves show gaplike structures at about +or-22 mV and second
features at about +or-40 mV. These are very similar to the
double structure observed in YBa2Cu3O7
(YBCO) junctions. The background resistance above the
gaplike features is flat in the majority of the junctions,
while about 20% of the junctions show decreasing resistance
with bias. For Pb and PbBi counterelectrodes, negligible
leakage currents at low temperatures indicate good quality
tunnel barriers, continuous and pin-hole free. The phonon
structures of these materials indicate a single-step
tunneling process. The behavior of the zero-bias resistance
dip observed in some junctions is discussed.
IEEE Trans. Appl. Supercon., 3, 1-3, 1540,
1993.
Tc superconductor Bi2Sr2CaCu2O8+delta
X-band (approximately 9.4 GHz) EPR and thermal-expansion
measurements have been performed on a single crystal of the
high-Tc superconducting material
Bi2Sr2CaCu2O8+delta in the
temperature range 77- 295 K.
Cu2+ EPR spectra due to at least four different
Cu2+ centers located at about 0.3 T, were observed at
room temperature. The EPR spectrum disappeared almost
completely below the superconducting transition temperature
84 K. The temperature variation of the EPR line intensity
indicated the occurrence of a second-order phase transition
at 84 K. Thermal-expansion measurements confirmed the
occurrence of this phase transition. The thermal-expansion
coefficients in the phases of the crystal above and below 84
K have been estimated.
Solid State Commun., 81, No. 6, 1992.
Bi-Sr-Ca-Cu-O thin films were deposited on (100)-oriented
MgO single crystals using the pulsed-laser deposition
technique and targets of nominal composition Bi2Sr
2CaCu2O 8+delta (i.e.((Bi):(Sr):(Ca):(Cu)=)2:2:1:2) with
the mass density larger
than 90% of the theoretical value. To obtain an in-situ
process of producing single-phase 2:2:1:2 films, the authors
varied three parameters, namely the substrate temperature,
the oxygen pressure and the distance between the target and
substrate. The results of surface studies using scanning
electron microscopy, energy-dispersive X-ray analysis, the
superconducting transition temperature and the critical
current density are discussed.
Mat. Sc. Eng., B13, 1992.
Using KrF-excimer laser and high mass density target of
(Bi,Pb) 2Sr2Ca2Cu3O 10+ delta
(Bi-2223), the authors have fabricated 'in-situ' thin films
of this high Tc superconductor. By varying the
substrate temperature, the oxygen pressure and the distance
between target and substrate, they were able to optimize the
'in-situ' process for c-axis oriented thin films of Bi-
compounds. As deposited films of Bi-2223 show Tc, zero
at about 60 K and following annealing, they show Tc, zero at 105 K.
These films were examined using the
scanning electron microscope (SEM) and X-ray diffraction
(XRD). The results of magnetic susceptibility, electrical
resistivity and critical current density measurements will
be discussed.
Phys. C, 180, No. 1-4, 81, 1991.
The superconducting transition temperature Tc of
metallic glasses Fe20/Zr80/ Fe25/Zr75,
and Cu25Zr75 were measured under
quasihydrostatic pressure up to 10 GPa. The volume
(pressure) dependence of the electron-phonon coupling
parameter, lambdae-ph, for Cu25Zr75 was
calculated using the McMillan equation. Using this volume
dependence of lambdae-ph and the modified McMillan
equation, which incorporates spin fluctuations, the authors
determined the volume dependence of the spin-fluctuation
parameter, lambdaSF, in Ni25Zr75 and
Fe100-xZrx (x=80, 75). It was found that with
increasing pressure spin fluctuations are suppressed at a
faster rate in Fe100-xZrx as the Fe
concentration is increased. The rate of suppression of spin
fluctuations with pressure was also found to be higher in
the Fe-Zr glasses than in Ni-Zr glasses of similar
composition.
Phys. Rev. B, 43, 3649, 1991.
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