arXiv subjects

R. Puzniak

Publications and source records attributed to R. Puzniak.

At least 37 records · Page 2Linked to original sources

arXiv2010

Th substituted SmFeAsO: structural details and superconductivity with Tc above 50 K

Superconducting poly- and single-crystalline samples of Sm1-xThxFeAsO with partial substitution of Sm3+ by Th4+ were synthesized and grown under high pressure and their structural, magnetic and transport properties are studied. The superconducting Tc reaches values higher than 50 K. Bulk superconducting samples (x = 0.08, 0.15, 0.3) do not show any signs of a phase transition from tetragonal to orthorhombic crystal structure at low temperatures. With Th substitution the unit cell parameters a and c shrink and the fractional atomic coordinate of the As site zAs remains almost unchanged, while that of Sm/Th zSm/Th increases. Upon warming from 5 K to 295 K the expansion of the FeAs layer thickness is dominant, while the changes in the other structural building blocks are smaller by a factor of 1/5, and they compensate each other, since the As-Sm/Th distance appears to contract by about the same amount as the O-Sm/Th expands. The poly- and single-crystalline samples are characterized by a full diamagnetic response in low magnetic field, by a high intergrain critical-current density for polycrystalline samples, and by a critical current density of the order of 8 x 105 A/cm2 for single crystals at 2 K in fields up to 7 T. The magnetic penetration depth anisotropy γλ increases with decreasing temperature, a similar behavior to that of SmFeAsO1-xFy single crystals. The upper critical field estimated from resistance measurements is anisotropic with slopes of 5.4 T/K (H//ab plane) and 2.7 T/K (H//c axis), at temperatures sufficiently far below Tc. The upper critical field anisotropy γH is in the range of 2, consistent with the tendency of a decreasing γH with decreasing temperature, already reported for SmFeAsO1-xFy single crystals.

cond-mat.supr-con↗

arXiv2010

Anisotropic superconducting properties of single-crystalline FeSe0.5Te0.5

Iron-chalcogenide single crystals with the nominal composition FeSe$_{0.5}$Te$_{0.5}$ and a transition temperature of $T_{c}\simeq14.6$ K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal X-ray and neutron powder diffraction, SQUID and torque magnetometry, and muon-spin rotation. Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe_1.045Se_0.406Te_0.594. Additionally, a minor hexagonal Fe_7Se_8 impurity phase was identified. The magnetic penetration depth λat zero temperature was found to be 491(8) nm in the ab-plane and 1320(14) nm along the c-axis. The zero-temperature value of the superfluid density ρ_s(0) λ^-2(0) obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and iron-pnictides. The temperature dependences of both λ_ab and λ_c are well described by a two-gap s+s-wave model with the zero-temperature gap values of Δ_S(0)=0.51(3) meV and Δ_L(0)=2.61(9) meV for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter γ_λ(T)=λ_c(T)/λ_{ab}(T) increases with decreasing temperature, in agreement with γ_λ(T) observed in the iron-pnictide superconductors.

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arXiv2010

Contact superconductivity in In-PbTe junctions

The authors report on electron transport studies on superconductor-semiconductor hybrid structures of indium and n-type lead telluride, either in the form of quantum wells or bulk crystals. In-PbTe contacts form by spontaneous alloying, which occurs already at room temperature. The alloyed phase penetrates deeply into PbTe and forms metallic contacts even in the presence of depletion layers at the semiconductor surface. Although the detailed structure of this phase is unknown, we observe that it exhibits a superconducting transition at temperatures below 10 K. This causes such substantial reduction of the contact resistances that they even become comparable to those predicted for ideal superconductor-normal conductor contacts. Most importantly, this result indicates that the interface phase in the superconducting state becomes nearly homogeneous - in contrast to the structure expected for alloyed contacts. We suggest that the unusual interface superconductivity is linked to the unique properties of PbTe, namely, its huge static dielectric constant. Apparently the alloyed interface phase contains superconducting precipitates randomly distributed within the depletion layers, and their Coulomb charging energies are extremely small. According to the existing models of the granular superconductivity, even very weak Josephson coupling between the neighboring precipitates gives rise to the formation of a global superconducting phase which explains our observations.

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arXiv2010

Anisotropy of the superconducting state parameters and intrinsic pinning in low-level Pr-doped YBa2Cu3O(7-delta) single crystals

The influence of low-level Pr substitution in Y1-xPrxBa2Cu3O(7-delta) single crystals on the anisotropy of the superconducting state parameters was investigated by torque magnetometry in magnetic fields of up to 9 T. An averaged anisotropy parameter, γ, of 7.4 was found and no influence of the Pr ion concentration on γwas observed up to a Pr content of 2.4%. A pronounced maximum at angles between 0.5 and 1 degree out of the ab-plane was observed in all crystals in the irreversible angular dependence of the torque. This maximum is attributed to intrinsic pinning associated with kinked vortices. The variation of the irreversible torque with the substitution level indicates the influence of the Pr content on pinning within the CuO2 planes, even though the anisotropy of the superconducting state parameters is not affected.

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arXiv2010

Influence of Mg deficiency in MgB2 single crystals on crystal structure and superconducting properties

The effects of high-temperature vacuum-annealing induced Mg deficiency in MgB2 single crystals grown under high pressure were investigated. As the annealing temperature was increased from 800 to 975 C, the average Mg content in the MgB2 crystals systematically decreased, while Tc remains essentially unchanged and the superconducting transition slightly broadens from 0.55 K to 1.3 K. The reduction of the superconducting volume fraction was noticeable already after annealing at 875 C. Samples annealed at 975 C are partially decomposed and the Mg site occupancy is decreased to 0.92 from 0.98 in as-grown crystals. Annealing at 1000 C completely destroys superconductivity. X-ray diffraction analysis revealed that the main final product of decomposition is polycrystalline MgB4 and thus the decomposition reaction of MgB2 can be described as 2MgB2(s) = MgB4(s) + Mg(g). First-principles calculations of the Mg1-x(VMg)xB2 electronic structure, within the supercell approach, show a small downshift of the Fermi level. Holes induced by the vacancies go to both sigma and pi bands. These small modifications are not expected to influence Tc, in agreement with observations. The significant reduction of the superconducting volume fraction without noticeable Tc reduction indicates the coexistence, within the same crystal, of superconductive and non-superconductive electronic phases, associated with regions rich and poor in Mg vacancies.

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arXiv2009

Bulk Superconductivity at 2.6 K in Undoped RbFe_2As_2

The iron arsenide RbFe_2As_2 with the ThCr_2Si_2-type structure is found to be a bulk superconductor with T_c=2.6 K. The onset of diamagnetism was used to estimate the upper critical field H_c2(T), resulting in dH_c2/dT=-1.4 T/K and an extrapolated H_c2(0)=2.5 T. As a new representative of iron pnictide superconductors, superconducting RbFe_2As_2 contrasts with BaFe_2As_2, where the Fermi level is higher and a magnetic instability is observed. Thus, the solid solution series (Rb,Ba)Fe_2As_2 is a promising system to study the crossover from superconductivity to magnetism.

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arXiv2009

Superconductivity at 23 K and Low Anisotropy in Rb-Substituted BaFe_2As_2 Single Crystals

Single crystals of Ba_{1-x}Rb_{x}Fe_2As_2 with x=0.05-0.1 have been grown from Sn flux and are bulk superconductors with T_c up to 23 K. The crystal structure was determined by X-ray diffraction analysis, and Sn is found to be incorporated for 9% Ba, shifted by 1.1 Angstroem away from the Ba site towards the (Fe_2As_2)-layers. The upper critical field deduced from resistance measurements is anisotropic with slopes of 7.1(3) T/K (H || ab-plane) and 4.2(2) T/K (H || c-axis), sufficiently far below T_c. The extracted upper critical field anisotropy of 3 close to T_c, is in good agreement with the estimate from magnetic torque measurements. This indicates that the electronic properties in the doped BaFe_2As_2 compound are significantly more isotropic than those in the LnFeAsO family. The in-plane critical current density at 5 K exceeds 10^6 A/cm^2, making Ba_{1-x}Rb_xFe_2As_2 a promising candidate for technical applications.

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arXiv2009

Evidence for two distinct anisotropies in the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and NdFeAsO_(0.8)F_(0.2)

Single crystals of the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and NdFeAsO_(0.8)F_(0.2) with T_c in the range of 44 K to 48 K were investigated by torque magnetometry. An analysis of the data in terms of a recently proposed model for the anisotropic magnetization in the superconducting state, treating the penetration depth anisotropy differently than the upper critical field anisotropy, provides evidence that in the oxypnictide superconductors two distinct anisotropies are present. As a result the penetration depth anisotropy differs significantly in magnitude and in temperature dependence from the upper critical field anisotropy, analogous to MgB_2 but with a reversed sign of slope. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductors.

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arXiv2009

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).

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arXiv2009

Influence of low-level Pr substitution on the superconducting properties of YBa2Cu3O7-delta single crystals

We report on measurements on Y1-xPrxBa2Cu3O7-delta single crystals, with x varying from 0 to 2.4%. The upper and the lower critical fields, Hc2 and Hc1, the Ginzburg-Landau parameter and the critical current density, Jc(B), were determined from magnetization measurements and the effective media approach scaling method. We present the influence of Pr substitution on the pinning force density as well as on the trapped field profiles analyzed by Hall probe scanning.

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arXiv2009

Anisotropy of superconducting single crystal SmFeAsO_(0.8)F_(0.2) studied by torque magnetometry

Single crystals of the oxypnictide superconductor SmFeAsO_(0.8)F_(0.2) with T_c=45(1) K were investigated by torque magnetometry. The crystals of mass~0.1 mug were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter was found to be field independent, but varies strongly with temperature ranging from 8 at T=T_c to 23 at T=0.4T_c. This unusual behavior signals unconventional superconductivity.

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arXiv2008

Single crystal of superconducting SmFeAsO1-xFy grown at high pressure

Single crystals of SmFeAsO1-xFy of a size up to 120 micrometers have been grown from NaCl/KCl flux at a pressure of 30 kbar and temperature of 1350-1450 C using the cubic anvil high-pressure technique. The superconducting transition temperature of the obtained single crystals varies between 45 and 53 K.Obtained crystals are characterized by a full diamagnetic response in low magnetic fields and by a high critical current density in high magnetic fields. Structural refinement has been performed on single crystal. Differential thermal analysis investigations at 1 bar Ar pressure show decomposition of SmFeAsO1-xFy at 1302 C.

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arXiv2007

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.

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arXiv2006

Metastable diamagnetism in Sm0.1Ca0.84Sr0.06MnO3 manganite

Magnetic properties of polycrystalline Sm0.1Ca0.84Sr0.06MnO3 in pristine and metastable states have been investigated in wide range of temperatures and magnetic fields. It was found that below Curie temperature TC = 105 K the pristine state exhibits phase separation comprising ferromagnetic and antiferromagnetic phases. The metastable states with reduced magnetization were obtained by successive number of quick coolings of the sample placed in container with kerosene-oil mixture. By an increasing number of quick coolings (> 6) the long time relaxation appeared at 10 K and the magnetization reversed its sign and became strongly negative in wide temperature range, even under an applied magnetic field of 15 kOe. The observed field and temperature dependences of the magnetization in this state are reversed in comparison with the ordinary ferromagnetic ones. Above TC, the observed diamagnetic susceptibility of the reversed magnetization state at T = 120 K is ~ - 0.9 x 10-4 emu g-1 Oe-1. Only after some storage time at room temperature, the abnormal magnetic state is erasable. It is suggested that the negative magnetization observed results from a specific coupling of the nano/micro-size ferromagnetic regions with a surrounding diamagnetic matrix formed, in a puzzled way, by the repeating training (quick cooling) cycles.

cond-mat.str-el↗

arXiv2005

Interplay between itinerant and localized states in CaMn1-xRuxO3 (x =0 - 0.5) manganites

Magnetic properties of polycrystalline CaMn1-xRuxO3 (x = 0 - 0.5) samples were investigated in the temperature range 4.2 - 250 K, under external magnetic field up to 15 kOe and under hydrostatic pressure up to 12 kbar. Transport properties of the samples with x = 0.1, 0.2, 0.4, 0.5 were also investigated under pressure up to 10 kbar. For x up to 0.4, the pressure was found to suppress ferromagnetic correlations and to increase the resistivity, while for x = 0.5 to act in the opposite way. While long ferromagnetic order is completely suppressed, in small clusters ferromagnetic correlations probably survive under pressure, as was revealed for CaMn0.9Ru0.1O3. The pressure effect on the magnetic interactions and on the volume of ferromagnetic phase was found to depend strongly on the Ru-content, and absolute value of the pressure coefficient of spontaneous magnetization was found to decrease practically linearly with increasing x in the range 0.1 < x < 0.5. The experimental data are discussed in the frame of proposed energy-level diagram, which includes magneto-impurity states at low and moderate Ru-doping and mixed-valence states of Ru presented by a strongly-correlated t2g-like band at heavy Ru-doping. An impact of disorder introduced by Ru-doping on the energy diagram and on derived magnetic interactions is discussed. Predictions of the model regarding the pressure effects on conductivity and temperature scales characteristic for magnetic interactions are in reasonable agreement with experiment.

cond-mat.str-el↗

arXiv2005

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.

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arXiv2004

Al substitution in MgB2 crystals: influence on superconducting and structural properties

Single crystals of Mg1-xAlxB2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x < 0.1 were obtained as a result of optimization of already developed MgB2 crystal growth procedure. Systematic decrease of the c-axis lattice constant with increasing Al content, when the a-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in Mg1-xAlxB2 crystals was found to be slightly different than that one observed for polycrystalline samples since, even a very small substitution of 1-2% of Al leads to the decrease of Tc by about 2-3 K. X-ray and high resolution transmission electron microscopy investigations indicate on the appearance of second precipitation phase in the crystals with x > 0.1. This is in a form of non-superconducting MgAlB4 domains in the structure of superconducting Mg1-xAlxB2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, Hc2, for H//c for the samples with small x, significant reduction of the Hc2 anisotropy at lower temperatures, and decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with Tc in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing Tc down to 20 K. It may be related to an appearance of the precipitation phase in the Mg1-xAlxB2 structure.

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arXiv2004

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.

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