| 1. |
- Campanini, Donato, et al.
(författare)
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Superconducting gap evolution in overdoped BaFe2(As1-xPx)(2) single crystals through nanocalorimetry
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:24
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Tidskriftsartikel (refereegranskat)abstract
- We report on specific heat measurements on clean overdoped BaFe2(As1-xPx)(2) single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature gamma(r) = C/T backslash(T -> 0) is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave alpha model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of Delta(0) similar to 5.3 meV, corresponding to Delta(0)/k(B)T(c) similar to 2.2. Increasing the phosphorus concentration x, the main gap reduces till a value of Delta(0) similar to 1.9 meV for x = 0.55 and a second weaker gap becomes evident. From the magnetic field effect on gamma(r), all samples however show similar behavior [gamma(r)(H) -gamma(r)(H = 0) proportional to H-n, with n between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.
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| 2. |
- Chapai, R., et al.
(författare)
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Superconducting properties of the spin Hall candidate Ta3Sb with eightfold degeneracy
- 2022
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 105:18
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Tidskriftsartikel (refereegranskat)abstract
- We report the synthesis and characterization of phase pure Ta3Sb, a material predicted to be topological with eightfold degenerate fermionic states [Bradlyn et al., Science 353, aaf5037 (2016)] and to exhibit a large spin Hall effect [Derunova et al., Sci. Adv. 5, eaav8575 (2019)]. We observe superconductivity in Ta3Sb with Tc∼0.67 K in both electrical resistivity ρ(T) and specific heat c(T) measurements. Field-dependent measurements yield the superconducting phase diagram with an upper critical field of Hc2(0)∼ 0.95 T, corresponding to a superconducting coherence length of ξ≈18.6nm. The gap ratio deduced from specific heat anomaly 2Δ0/kBTc is 3.46, a value close to the Bardeen-Cooper-Schrieffer value of 3.53. From a detailed analysis of both the transport and thermodynamic data within the Ginsburg-Landau (GL) framework, a GL parameter of κ≈90 is obtained, identifying Ta3Sb as an extreme type-II superconductor. The observation of superconductivity in an eightfold degenerate fermionic compound with topological surface states and predicted large spin Hall conductance positions Ta3Sb as an appealing platform to further explore exotic quantum states in multifold degenerate systems.
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| 3. |
- Chaparro, C., et al.
(författare)
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Doping dependence of the specific heat of single crystal bafe2(as1 xpx)(2)
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:18, s. 184525-
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Tidskriftsartikel (refereegranskat)abstract
- We present specific heat measurements on a series of BaFe2(As1-xPx)(2) single crystals with phosphorous doping ranging from x = 0.3 to x = 0.55. Our results reveal that BaFe2(As1-xPx)(2) follows the scaling Delta C/T-c approximate to T-c(2) remarkably well. The clean-limit nature of this material imposes additional 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 approximate to 2.6, independent of doping.
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| 4. |
- Diao, Zhu, et al.
(författare)
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Microscopic parameters from high-resolution specific heat measurements on superoptimally substituted BaFe2(As1-xPx)(2) single crystals
- 2016
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 93:1
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the electronic specific heat of superoptimally substituted BaFe2(As1-x P-x(x))(2) single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the substitution dependence of the condensation energy, superconducting gap Delta, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump Delta C proportional to T-c(3) , found in many iron-based superconductors, in this system originates from a T-c-dependent ratio Delta/k(B)T(c) in combination with a substitution-dependent density of states N(epsilon(F)). A clear enhancement is seen in the effective mass m* as the composition approaches the value that has been associated with a quantum critical point at optimum substitution. However, a simultaneous increase in the superconducting carrier concentration n(s) yields a penetration depth lambda that decreases with increasing T-c without sharp divergence at the quantum critical point. Uemura scaling indicates that T-c is governed by the Fermi temperature T-F for this multiband system.
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| 5. |
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| 6. |
- Lukyanchuk, I., et al.
(författare)
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Rayleigh instability of confined vortex droplets in critical superconductors
- 2015
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Ingår i: Nature Physics. - 1745-2473 .- 1745-2481. ; 11:1, s. 21-25
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Tidskriftsartikel (refereegranskat)abstract
- Depending on the Ginzburg-Landau parameter kappa, superconductors can either be fully diamagnetic if kappa < 1/root 2 (type I superconductors) or allow magnetic flux to penetrate through Abrikosov vortices if kappa > 1/root 2 (type II superconductors; refs 1,2). At the Bogomolny critical point, kappa = kappa(c) = 1/root 2, a state that is infinitely degenerate with respect to vortex spatial configurations arises(3,4). Despite in-depth investigations of conventional type I and type II superconductors, a thorough understanding of the magnetic behaviour in the near-Bogomolny critical regime at kappa similar to kappa(c) remains lacking. Here we report that in confined systems the critical regime expands over a finite interval of kappa forming a critical superconducting state. We show that in this state, in a sample with dimensions comparable to the vortex core size, vortices merge into a multi-quanta droplet, which undergoes Rayleigh instability(5) on increasing kappa and decays by emitting single vortices. Superconducting vortices realize Nielsen-Olesen singular solutions of the Abelian Higgs model, which is pervasive in phenomena ranging from quantum electrodynamics to cosmology(6-9). Our study of the transient dynamics of Abrikosov-Nielsen-Olesen vortices in systems with boundaries promises access to non-trivial effects in quantum field theory by means of bench-top laboratory experiments.
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| 7. |
- Rydh, Andreas, et al.
(författare)
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Emerging measurement techniques for studies of mesoscopic superconductors, in Electron Transport in Nanosystems
- 2008
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Ingår i: Proceedings of the NATO Advanced Research Workshop on Electron Transport in Nanosystems. - Dordrecht : Springer. - 9781402091469 ; , s. 117-126
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Konferensbidrag (refereegranskat)abstract
- Experimental research on mesoscopic systems puts high demands on the measurement infrastructure, including measurement system with associated sample preparation, experimental design, measurement electronics, and data collection. Successful experiments require both the ability to manufacture small samples and to successfully and accurately study their novel properties. Here, we discuss some aspects and recent advancements of general measurement techniques that should benefit several characterization methods such as thermodynamic, magnetic, and transport studies of mesoscopic superconductors.
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| 8. |
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| 9. |
- Smylie, M. P., et al.
(författare)
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Anisotropic superconductivity and magnetism in single-crystal RbEuFe4As4
- 2018
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Ingår i: Physical Review B. - College Park : American Physical Society. - 2469-9950 .- 2469-9969. ; 98:10
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe4As4 using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu moments of Tm=15K and a superconducting transition temperature of Tc=36.8K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of Γ=1.7. Ginzburg-Landau parameters of κc∌67 and κab∌108 indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu magnetism is quasi-two dimensional (2D), as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and <0.04 K. A consequence of the quasi-2D nature of the Eu magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a kinklike anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 and 4 kG. © 2018 American Physical Society.
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| 10. |
- Tagliati, Stella, 1983-, et al.
(författare)
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Membrane-based calorimetry for studies of sub-microgram samples
- 2009
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 150, s. 052256-
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Tidskriftsartikel (refereegranskat)abstract
- We have developed a membrane-based microcalorimeter for general measurements of heat capacity and latent heat using a combination of ac steady-state and relaxation methods. The differential calorimeter is designed for sub-microgram samples studied over a wide range of temperatures and magnetic fields. The device is based on free-standing silicon nitride membranes of 150 nm thickness onto which thin film heaters and temperature sensors are fabricated. While production-line fabrication may benefit from back-etching as a final step, it is often easier to start with pre-etched membranes in a research laboratory. With selected nano-fabrication methods this is possible. Due to a robust heater and very low thermal conductance of the final calorimeter, the sample can be heated to more than 100 K above base temperature. This enables instantaneous calibration, and makes the device capable of being used for ultra-fast temperature control, relaxation studies, and measurements that combine good absolute accuracy, high resolution, and information on latent heat.
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