arXiv subjects

U. Welp

Publications and source records attributed to U. Welp.

At least 55 records · Page 3Linked to original sources

arXiv2013

Effect of Heavy-Ion Irradiation on Superconductivity in Single Crystals of Ba0.6K0.4Fe2As2 Pnictide Superconductor

The London penetration depth was measured in optimally doped Ba0.6K0.4Fe2As2 crystals, with and without columnar defects produced by 1.4 GeV 208Pb irradiation. The low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap delta_min/(k_B T_C) = 1. Similar gap values were observed for irradiation levels corresponding to mean column-column separations of 32 nm and 22 nm. At very high irradiation levels (column-column separation of 10 nm) a T^2 power law was observed below Tc/3, most likely due to elevated scattering. Neither the location nor the sharpness of the superconducting transition was affected by irradiation. The data provides evidence for an s+/- pairing state.

cond-mat.supr-con↗

arXiv2013

Direct imaging of hot spots in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} mesa terahertz sources

Stacks of intrinsic Josephson junctions (IJJs) made from high-temperature superconductors such as Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} (Bi-2212) are a promising source of coherent continuous-wave terahertz radiation. It is thought that at electrical bias conditions under which THz-emission occurs hot spots may form due to resistive self-heating, and that these spots may be highly beneficial for the generation of high levels of THz power. Here we perform an imaging study of the temperature distribution at the surface of BSCCO stacks utilizing the temperature-dependent 612nm fluorescence line of Eu^{3+} in a europium chelate. The images directly reveal a highly non-uniform temperature distribution in which the temperature in the middle of the stack can exceed the superconducting transition temperature by tens of Kelvin under biasing conditions typical for THz-emission.

cond-mat.supr-con↗

arXiv2013

Upper critical field of isoelectron substituted SrFe$_2$(As$_{1-x}$P$_x$)$_2$

The upper critical field $H_{c2}$ of optimally doped iron-based superconductor SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ ($x$ = 0.35, $T_c$ = 25 K) was measured as a function of temperature down to 1.6 K for two principal directions of magnetic field $H \parallel c$ and $H \parallel a$. Measurements were performed in pulsed magnetic fields up to 65 T using a tunnel-diode resonator technique on as-grown and heavy-ion irradiated single crystals, with columnar defect density corresponding to a matching field $Bϕ$ = 25 T. The $H_{c2,c}(T)$ is close to $T$-linear, while clear saturation is observed for $H_{c2,a}(T)$, leading to a strongly temperature dependent anisotropy parameter $γ$. The linear shape of $H_{c2,c}(T)$ is very similar to that observed in nodal KFe$_2$As$_2$ but very different from full-gap LiFeAs. Irradiation does not introduce any additional features on $H_{c2}(T)$ line corresponding to the matching field. Instead, it suppresses uniformly both $T_c$ and $H_{c2}$, keeping their ratio constant.

cond-mat.supr-con↗

arXiv2012

Catalyst-Free Growth of Millimeter-Long Topological Insulator Bi2Se3 Nanoribbons and the Observation of pi Berry Phase

We report the growth of single-crystalline Bi2Se3 nanoribbons with lengths up to several millimeters via a catalyst-free physical vapor deposition method. Scanning transmission electron microscopy analysis reveals that the nanoribbons grow along the (1120) direction. We obtain a detailed characterization of the electronic structure of the Bi2Se3 nanoribbons from measurements of Shubnikov-de Haas (SdH) quantum oscillations. Angular dependent magneto-transport measurements reveal a dominant two-dimensional contribution originating from surface states and weak contribution from the bulk states. The catalyst-free synthesis yields high-purity nanocrystals enabling the observation of a large number of SdH oscillation periods and allowing for an accurate determination of the pi-Berry phase, one of the key features of Dirac fermions in topological insulators. The long-length nanoribbons can empower the potential for fabricating multiple nanoelectronic devices on a single nanoribbon.

cond-mat.mes-hall↗

arXiv2011

Doping dependence of the specific heat of single crystal BaFe2(As1-xPx)2

We present specific heat measurements on a series of BaFe2(As1-xPx)2 single crystals with phosphorous doping ranging from x = 0.3 to 0.55. Our results reveal that BaFe2(As1-xPx)2 follows the scaling Delta_C/Tc ~ Tc^2 remarkably well. The clean-limit nature of this material imposes new restraints on theories aimed at explaining the scaling. Furthermore, we find that the Ginzburg-Landau parameter decreases significantly with doping whereas the superconducting anisotropy is gamma~2.6, independent of doping.

cond-mat.supr-con↗

arXiv2011

Doping and Irradiation Controlled Vortex Pinning Behavior in BaFe2(As1-xPx)2 Single Crystals

We report on the systematic evolution of vortex pinning behavior in isovalent doped single crystals of BaFe2(As1-xPx)2. Proceeding from optimal doped to ovedoped samples, we find a clear transfor- mation of the magnetization hysteresis from a fishtail behavior to a distinct peak effect followed by a reversible magnetization and Bean Livingston surface barriers. Strong point pinning dominates the vortex behavior at low fields whereas weak collective pinning determines the behavior at higher fields. In addition to doping effects, we show that particle irradiation by energetic protons can tune vortex pinning in these materials.

cond-mat.supr-con↗

arXiv2011

Tunable terahertz emission from Bi2Sr2CaCu2O8+δ mesa devices

We have measured coherent terahertz emission spectra from Bi2Sr2CaCu2O8+δ mesa devices, as a function of temperature and mesa bias voltage. The emission frequency is found to be tunable by up to 12% by varying the temperature and bias voltage. We attribute the appearance of tunability to asymmetric boundaries at the top and bottom, and the non-rectangular cross-section of the mesas. This interpretation is consistent with numerical simulations of the dynamics of intrinsic Josephson junctions in the mesa. Easily tunable emission frequency may have important implications for the design of terahertz devices based on stacked intrinsic Josephson junctions.

cond-mat.supr-con↗

arXiv2011

Optical Excitation and Decay Dynamics of Ytterbium Atoms Embedded in a Solid Neon Matrix

Neutral ytterbium atoms embedded in solid neon qualitatively retain the structure of free atoms. Despite the atom-solid interaction, the 6s6p $^3$P$_0$ level is found to remain metastable with its lifetimes determined to be in the range of ten to hundreds of seconds. The atomic population can be almost completely transferred between the ground level and the metastable level via optical excitation and spontaneous decay. The dynamics of this process is examined, and is used to explicitly demonstrate that the transition broadening mechanism is homogeneous.

physics.atom-ph↗

arXiv2011

Anisotropic Phase Diagram and Superconducting Fluctuations in SmFeAsO0.85F0.15

We report on the specific heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO1-xFx. In zero-field, the optimally doped compound displays a clear cusp-like anomaly in C/T with ΔC/Tc = 24 mJ/molK2 at Tc = 49.5 K. In magnetic fields applied along the c-axis, we find pronounced superconducting fluctuations induced broadening and suppression of the specific heat anomaly which can be described using three-dimensional lowest-Landau-level scaling with an upper critical field slope of -3.5 T/K and an anisotropy of Γ = 8. The small value of ΔC/Tc yields a Sommerfeld coefficient γ ~ 8 mJ/molK2 indicating that SmFeAsO1-xFx is characterized by a modest density of states and strong coupling.

cond-mat.supr-con↗

arXiv2010

Muon spin rotation study of the magnetic penetration depth in the intercalated graphite superconductor CaC6

We report temperature- and magnetic field-dependent bulk muon spin rotation measurements in a c-axis oriented superconductor CaC6 in the mixed state. Using both a simple second moment analysis and the more precise analytical Ginzburg-Landau model, we obtained a field independent in-plane magnetic penetration depth λab (0) = 72(3) nm. The temperature dependencies of the normalized muon spin relaxation rate and of the normalized superfluid density result to be identical, and both are well represented by the clean limit BCS model with 2Δ/kB Tc = 3.6(1), suggesting that CaC6 is a fully gapped BCS superconductor in the clean limit regime.

cond-mat.supr-con↗

arXiv2010

London penetration depth in Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ (T=Co, Ni) superconductors irradiated with heavy ions

Irradiation with Pb ions was used to study the effect of disorder on the in-plane London penetration depth, $λ(T)$, in single crystals of Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ (T=Co, Ni). An increase of the irradiation dose results in a monotonic decrease of the superconducting transition temperature, $T_c$, without affecting much the transition width. In both Co and Ni doped systems we find a power-law behavior, $Δλ(T) \propto T^n$, with the exponent $n$ systematically decreasing with the increase of disorder. This observation, supported by the theoretical analysis, conclusively points to a nodeless $s^\pm$ state with pairbreaking impurity scattering (interband) with strength being intermediate between Born and unitary limits.

cond-mat.supr-con↗

arXiv2010

Magneto-optical study of Ba(Fe$_{1-x}$T$_{x}$)$_2$As$_2$ (T=Co, Ni) single crystals irradiated with heavy-ions

Optimally doped single crystals of Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ (T=Co, Ni) were irradiated with 1.4 GeV $^{208}$Pb$^{56+}$ ions at fluences corresponding to matching fields of $B_ϕ=0.1, 0.5, 1$ and 2 T. Magneto-optical imaging has been used to map the distribution of the magnetic induction in the irradiated samples. The imaging is complemented by the magnetization measurements. The results show a substantial enhancement of the apparent critical current densities as revealed by the much larger Bean penetration fields and an increase of the hysteretic magnetization. However, the effect depends on the compound, temperature and applied magnetic field. In \FeCo crystals, at 15 K and low fields, the enhancement appears to scale with the irradiation dose at a rate of about 0.27 MA$\cdot$cm$^{-2}$T$^{-1}$, whereas in \FeNi crystals, higher irradiation doses are less effective. Our results suggest that moderate irradiation with heavy ions is a an effective way to \emph{homogeneously} enhance the current-currying capabilities of pnictide superconductors.

cond-mat.supr-con↗

arXiv2009

Magnetoresistance Oscillations in Granular Superconducting Niobium Nitride Nanowires

We report on magnetoresistance oscillations in superconducting NbNx nanowires synthesized through ammonia gas annealing of NbSe3 precursor nanostructures. Even though the transverse dimensions of the nanowires are much larger than the superconducting coherence length, the voltage-current characteristics of these nanowires at low temperatures are reminiscent of one-dimensional superconductors where quantum phase slips are associated with the origin of dissipation. We show that both the magnetoresistance oscillations and voltage-current characteristics observed in this work result from the granular structure of our nanowires.

cond-mat.supr-con↗

arXiv2009

Growth and Superconductivity of FeSex Crystals

Iron selenide (FeSex) crystals with lateral dimensions up to millimeters were grown via a vapor self-transport method. The crystals consist of the dominant alpha - phase with trace amounts of beta- phase as identified by powder x-ray diffraction. With four-probe resistance measurements we obtained a zero-resistance critical temperature of 7.5 K and a superconducting onset transition temperature of up to 11.8 K in zero magnetic field as well as an anisotropy of 1.5 +- 0.1 for the critical field. Magnetization measurements on individual crystals reveal the co-existence of superconductivity and ferromagnetism.

cond-mat.supr-con↗

arXiv2009

Emission of Terahertz Waves from Stacks of Intrinsic Josephson Junctions

By patterning mesoscopic crystals of Bi2Sr2CaCu2O8 (BSCCO) into electromagnetic resonators the oscillations of a large number of intrinsic Josephson junctions can be synchronized into a macroscopic coherent state accompanied by the emission of strong continuous wave THz-radiation. The temperature dependence of the emission is governed by the interplay of self-heating in the resonator and by re-trapping of intrinsic Josephson junctions which can yield a strongly nonmonotonic temperature dependence of the emission power. Furthermore, proper shaping of the resonators yields THzsources with voltage-tunable emission frequencies.

cond-mat.supr-con↗

arXiv2009

Thermal Management in Large Bi2212 Mesas used for Terahertz Sources

We present a thermal analysis of a patterned mesa on a Bi2Sr2CaCu2O8 (Bi2212) single crystal that is based on tunneling characteristics of the c-axis stack of ~800 intrinsic Josephson junctions in the mesa. Despite the large mesa volume (e.g., 40x300x1.2 micron3) and power dissipation that result in selfheating and backbending of the current-voltage curve (I-V), there are accessible bias conditions for which significant polarized THz-wave emission can be observed. We estimate the mesa temperature by equating the quasiparticle resistance, Rqp(T), to the ratio V/I over the entire I-V including the backbending region. These temperatures are used to predict the unpolarized black-body radiation reaching our bolometer and there is substantial agreement over the entire I-V. As such, backbending results from the particular Rqp(T) for Bi2212, as first discussed by Fenton, rather than a significant suppression of the energy gap. This model also correctly predicts the observed disappearance of backbending above ~60 K.

cond-mat.supr-con↗

arXiv2008

Anisotropic phase diagram and strong coupling effects in Ba1-xKxFe2As2

We present a thermodynamic study of the phase diagram of single-crystal Ba1-xKxFe2As2 using specific heat measurements. In zero-magnetic field a clear step in the heat capacity of deltaC/Tc = 0.1 J/f.u.K2 is observed at Tc = 34.6K for a sample with x = 0.4. This material is characterized by extraordinarily high slopes of the upper critical field of dHc2,c/dT= -6.5 T/K and dHc2,ab/dT= -17.4 T/K and a surprisingly low anisotropy of gamma ~ 2.6 near Tc. A consequence of the large field scale is the effective suppression of superconducting fluctuations. Using thermodynamic relations we determine Ginzburg-Landau parameters of kappac ~ 100 and kappaab ~ 260 identifying Ba1-xKxFe2As2 as extreme type-II. The large value of the normalized discontinuity of the slopes of the specific heat at Tc, (Tc/deltaC)times delta(dC/dT)~ 6 indicates strong coupling effects in Ba1-xKxFe2As2.

cond-mat.supr-con↗

arXiv2008

Caloric Determination of the Upper Critical Fields and Anisotropy of NdFeAsO1-xFx Single Crystals

We present heat capacity measurements of the upper critical fields of single-crystal NdFeAsO1-xFx. In zero-magnetic field a clear step in the heat capacity is observed at Tc = 47K . In fields applied perpendicular to the FeAs-layers the step broadens significantly whereas for the in-plane orientation the field effects are small. This behavior is reminiscent of the CuO2-high-Tc superconductors and is a manifestation of pronounced fluctuation effects. Using an entropy conserving construction we determine the transition temperatures in applied fields and the upper critical field slopes of dHc2,a = -0.72 T/K and dHc2,ab = -3.1 T/K. Zero-temperature coherence lengths of xiab = 3.7 nm and xic = 0.9 nm and a modest superconducting anisotropy of gamma ~ 4 can be deduced in a single-band model.

cond-mat.supr-con↗