57 research outputs found
MgB2 single crystals substituted with Li and with Li-C: Structural and superconducting properties
The effect of Li substitution for Mg and of Li-C co-substitution on the
superconducting properties and crystal structure of MgB2 single crystals has
been investigated. It has been found that hole doping with Li decreases the
superconducting transition temperature Tc, but at a slower rate than electron
doping with C or Al. Tc of MgB2 crystals with simultaneously substituted Li for
Mg and C for B decreases more than in the case where C is substituted alone.
This means that holes introduced by Li cannot counterbalance the effect of
decrease of Tc caused by introduction of electrons coming from C. The possible
reason of it can be that holes coming from Li occupy the pi band while
electrons coming from C fill the sigma band. The temperature dependences of the
upper critical field Hc2 for Al and Li substituted crystals with the same Tc
show a similar dHc2/dT slope at Tc and a similar Hc2(T) behavior, despite of
much different substitution level. This indicates that the mechanism
controlling Hc2 and Tc is similar in both hole and electron doped crystals.
Electrical transport measurements show an increase of resistivity both in Li
substituted crystals and in Li and C co-substituted crystals. This indicates
enhanced scattering due to defects introduced by substitutions including
distortion of the lattice. The observed behavior can be explained as a result
of two effects, influencing both Tc and Hc2. The first one is doping related to
the changes in the carrier concentration, which may lead to the decrease or to
the increase of Tc. The second one is related to the introduction of new
scattering centers leading to the modification of the interband and/or
intraband scattering and therefore, to changes in the superconducting gaps and
to the reduction of Tc.Comment: 22 pages, 17 figures, submitted to PR
The effect of magnetic impurities in a two-band superconductor: A point-contact study of Mn-substituted MgB2 single crystals
We present the first results of directional point-contact measurements in
Mg_{1-x}Mn_{x}B_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3
K. The order parameters Delta_{sigma} and Delta_{pi} were obtained by fitting
the conductance curves with the two-band Blonder-Tinkham-Klapwijk (BTK) model.
Both Delta_{pi} and Delta_{sigma} decrease with the critical temperature of the
junctions T_c^A, but remain clearly distinct up to the highest Mn content. Once
analyzed within the Eliashberg theory, the results indicate that spin-flip
scattering is dominant in the sigma band, as also confirmed by first-principle
band structure calculations.Comment: 4 pages, 5 eps figures. New theoretical results added, text and some
figures changed. References adde
Crystal growth, structural studies and superconducting properties of beta-pyrochlore KOs2O6
Single crystals of KOs2O6 have been grown in a sealed quartz ampoule.
Detailed single crystal X-ray diffraction studies at room temperature show
Bragg peaks that violate Fd-3m symmetry. With a comparative structure
refinement the structure is identified as non-centrosymmetric (F-43m). Compared
to the ideal beta-pyrochlore lattice (Fd-3m), both Os tetrahedral and O
octahedral network exhibit breathing mode like volume changes accompanied by
strong anisotropic character of the K channels. The crystals show metallic
conductivity and a sharp transition to the superconducting state at Tc = 9.65
K. Superconducting properties have been investigated by magnetization
measurements performed in a temperature range from 2 to 12 K and in magnetic
fields from 0 to 60 kOe. The temperature dependence of the upper critical field
Hc2(T) has been determined and the initial slope (dHc2/dT)Tc = -33.3 kOe/K has
been obtained near Tc. The upper critical field at zero temperature was
estimated to be Hc2(0) \cong 230 kOe, which is a value close to the Pauli
paramagnetic limiting field Hp(0)\cong 250 kOe. Then, the Ginzburg-Landau (GL)
coherence length xi GL(0) \approx 3.8 nm was calculated, and the Maki parameter
alpha \approx \sqrt 2 was obtained, suggesting the possibility that KOs2O6
might behave unconventionally at low temperatures and high magnetic fields
High magnetic field scales and critical currents in SmFeAs(O,F) crystals: promising for applications
Superconducting technology provides most sensitive field detectors, promising
implementations of qubits and high field magnets for medical imaging and for
most powerful particle accelerators. Thus, with the discovery of new
superconducting materials, such as the iron pnictides, exploring their
potential for applications is one of the foremost tasks. Even if the critical
temperature Tc is high, intrinsic electronic properties might render
applications rather difficult, particularly if extreme electronic anisotropy
prevents effective pinning of vortices and thus severely limits the critical
current density, a problem well known for cuprates. While many questions
concerning microscopic electronic properties of the iron pnictides have been
successfully addressed and estimates point to a very high upper critical field,
their application potential is less clarified. Thus we focus here on the
critical currents, their anisotropy and the onset of electrical dissipation in
high magnetic fields up to 65 T. Our detailed study of the transport properties
of optimally doped SmFeAs(O,F) single crystals reveals a promising combination
of high (>2 x 10^6 A/cm^2) and nearly isotropic critical current densities
along all crystal directions. This favorable intragrain current transport in
SmFeAs(O,F), which shows the highest Tc of 54 K at ambient pressure, is a
crucial requirement for possible applications. Essential in these experiments
are 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with
sub-\mu\m^2 cross-section, with current along and perpendicular to the
crystallographic c-axis and very good signal-to-noise ratio (SNR) in pulsed
magnetic fields. The pinning forces have been characterized by scaling the
magnetically measured "peak effect"
Strong magnetic pair breaking in Mn substituted MgB_2 single crystals
Magnetic ions (Mn) were substituted in MgB_2 single crystals resulting in a
strong pair-breaking effect. The superconducting transition temperature, T_c,
in Mg_{1-x}Mn_xB_2 has been found to be rapidly suppressed at an initial rate
of 10 K/%Mn, leading to a complete suppression of superconductivity at about 2%
Mn substitution. This reflects the strong coupling between the conduction
electrons and the 3d local moments, predominantly of magnetic character, since
the nonmagnetic ion substitutions, e.g. with Al or C, suppress T_c much less
effectively (e.g. 0.5 K/%Al). The magnitude of the magnetic moment, derived
from normal state susceptibility measurements, uniquely identifies the Mn ions
to be divalent, and to be in the low-spin state (S = 1/2). This has been found
also in X-ray absorption spectroscopy measurements. Isovalent Mn^{2+}
substitution for Mg^{2+} mainly affects superconductivity through spin-flip
scattering reducing T_c rapidly and lowering the upper critical field
anisotropy H_{c2}^{ab}/H_{c2}^c at T = 0 from 6 to 3.3 (x = 0.88% Mn), while
leaving the initial slope dH_{c2}/dT near T_c unchanged for both field
orientations.Comment: 9 pages, 9 figure
Development of a Linear Ion Trap Mass Spectrometer (LITMS) Investigation for Future Planetary Surface Missions
Future surface missions to Mars and other planetary bodies will benefit from continued advances in miniature sensor and sample handling technologies that enable high-performance chemical analyses of natural samples. Fine-scale (approx.1 mm and below) analyses of rock surfaces and interiors, such as exposed on a drill core, will permit (1) the detection of habitability markers including complex organics in association with their original depositional environment, and (2) the characterization of successive layers and gradients that can reveal the time-evolution of those environments. In particular, if broad-based and highly-sensitive mass spectrometry techniques could be brought to such scales, the resulting planetary science capability would be truly powerful. The Linear Ion Trap Mass Spectrometer (LITMS) investigation is designed to conduct fine-scale organic and inorganic analyses of short (approx.5-10 cm) rock cores such as could be acquired by a planetary lander or rover arm-based drill. LITMS combines both pyrolysis/gas chromatograph mass spectrometry (GCMS) of sub-sampled core fines, and laser desorption mass spectrometry (LDMS) of the intact core surface, using a common mass analyzer, enhanced from the design used in the Mars Organic Molecule Analyzer (MOMA) instrument on the 2018 ExoMars rover. LITMS additionally features developments based on the Sample Analysis at Mars (SAM) investigation on MSL and recent NASA-funded prototype efforts in laser mass spectrometry, pyrolysis, and precision subsampling. LITMS brings these combined capabilities to achieve its four measurement objectives: (1) Organics: Broad Survey Detect organic molecules over a wide range of molecular weight, volatility, electronegativity, concentration, and host mineralogy. (2) Organic: Molecular Structure Characterize internal molecular structure to identify individual compounds, and reveal functionalization and processing. (3) Inorganic Host Environment Assess the local chemical/mineralogical makeup of organic host phases to help determine deposition and preservation factors. (4) Chemical Stratigraphy Analyze the fine spatial distribution and variation of key species with depth
Carbon substitution in MgB2 single crystals: structural and superconducting properties
Growth of carbon substituted magnesium diboride Mg(B1-xCx)2 single crystals
with 0<x<0.15 is reported and the structural, transport, and magnetization data
are presented. The superconducting transition temperature decreases
monotonically with increasing carbon content in the full investigated range of
substitution. By adjusting the nominal composition, Tc of substituted crystals
can be tuned in a wide temperature range between 10 and 39 K. Simultaneous
introduction of disorder by carbon substitution and significant increase of the
upper critical field Hc2 is observed. Comparing with the non-substituted
compound, Hc2 at 15K for x=0.05 is enhanced by more then a factor of 2 for H
oriented both perpendicular and parallel to the ab-plane. This enhancement is
accompanied by a reduction of the Hc2-anisotropy coefficient gamma from 4.5
(for non-substituted compound) to 3.4 and 2.8 for the crystals with x = 0.05
and 0.095, respectively. At temperatures below 10 K, the single crystal with
larger carbon content shows Hc2 (defined at zero resistance) higher than 7 and
24 T for H oriented perpendicular and parallel to the ab-plane, respectively.
Observed increase of Hc2 cannot be explained by the change in the coherence
length due to disorder-induced decrease of the mean free path only.Comment: 22 pages, 9 figures, 4 table
Bose-Einstein condensation of strongly correlated electrons and phonons in cuprate superconductors
The long-range Froehlich electron-phonon interaction has been identified as
the most essential for pairing in high-temperature superconductors owing to
poor screening, as is now confirmed by optical, isotope substitution, recent
photoemission and some other measurements. I argue that low energy physics in
cuprate superconductors is that of superlight small bipolarons, which are
real-space hole pairs dressed by phonons in doped charge-transfer Mott
insulators. They are itinerant quasiparticles existing in the Bloch states at
low temperatures as also confirmed by continuous-time quantum Monte-Carlo
algorithm (CTQMC) fully taking into account realistic Coulomb and long-range
Froehlich interactions. Here I suggest that a parameter-free evaluation of Tc,
unusual upper critical fields, the normal state Nernst effect, diamagnetism,
the Hall-Lorenz numbers and giant proximity effects strongly support the
three-dimensional (3D) Bose-Einstein condensation of mobile small bipolarons
with zero off-diagonal order parameter above the resistive critical temperature
Tc at variance with phase fluctuation scenarios of cuprates.Comment: 35 pages, 10 figures, to appear in the special volume of Journal of
Physics: Condensed Matte
Single crystals of LnFeAsO1-xFx (Ln=La, Pr, Nd, Sm, Gd) and Ba1-xRbxFe2As2: growth, structure and superconducting properties
A review of our investigations on single crystals of LnFeAsO1-xFx (Ln=La, Pr,
Nd, Sm, Gd) and Ba1-xRbxFe2As2 is presented. A high pressure technique has been
applied for the growth of LnFeAsO1-xFx crystals, while Ba1-xRbxFe2As2 crystals
were grown using quartz ampoule method. Single crystals were used for
electrical transport, structure, magnetic torque and spectroscopic studies.
Investigations of the crystal structure confirmed high structural perfection
and show less than full occupation of the (O, F) position in superconducting
LnFeAsO1-xFx crystals. Resistivity measurements on LnFeAsO1-xFx crystals show a
significant broadening of the transition in high magnetic fields, whereas the
resistive transition in Ba1 xRbxFe2As2 simply shifts to lower temperature.
Critical current density for both compounds is relatively high and exceeds
2x109 A/m2 at 15 K in 7 T. The anisotropy of magnetic penetration depth,
measured on LnFeAsO1-xFx crystals by torque magnetometry is temperature
dependent and apparently larger than the anisotropy of the upper critical
field. Ba1-xRbxFe2As2 crystals are electronically significantly less
anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the
existence of two energy gaps in LnFeAsO1-xFx. Scanning Tunneling Spectroscopy
reveals in addition to a superconducting gap, also some feature at high energy
(~20 meV).Comment: 27 pages, 19 figures, 2 tables, accepted to the special issue of the
Physica C on superconducting pnictide
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