Engineering topics

Panagopoulos, C.

Publications and source records attributed to Panagopoulos, C..

12 records

DOE OSTI2018

Dimensional crossover in the quasi-one-dimensional superconductor Tl 2 Mo 6 Se 6

Here, we present magnetic penetration depth and electrical transport data in single crystals of quasi-one-dimensional (q1D) Tl 2 Mo 6 Se 6 , which reveal a 1D→3D superconducting dimensional crossover. The c-axis penetration depth shows the onset of superconducting fluctuations below T ons 1D = 6.7 K, whereas signatures of superconductivity in the ab-plane penetration depth (a uniquely sensitive probe of the transverse phase stiffness) only emerge below T ons 3D = 4.9 K. An anomalously low superfluid density persists down to ~3 K before rising steeply, in agreement with a theoretical model for crossovers in q1D superconductors. Our data analysis suggests that a sequence of pairing and phase fluctuation regimes controls the unusually broad superconducting transition. In particular, the electrical resistivity below T ons 3D is quantitatively consistent with the establishment of phase coherence through gradual binding of Josephson vortex strings to form 3D loops. This dimensional crossover within the superconducting state occurs despite the relatively large transverse hopping predicted from the band structure. Furthermore, our results have important consequences for the low-temperature normal state in Tl 2 Mo 6 Se 6 and similar q1D metals, which may retain one-dimensional behavior to lower temperatures than expected from theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Pressure-tuned superconductivity and normal-state behavior in Ba ( Fe 0.943 Co 0.057 ) 2 As 2 near the antiferromagnetic boundary

Superconductivity in iron pnictides is unconventional and pairing may be mediated by magnetic fluctuations in the Fe sublattice. Pressure is a clean method to explore superconductivity in iron based superconductors by tuning the ground state continuously without introducing disorder. Here we present a systematic high pressure transport study in Ba (Fe 1 - x Co x ) 2 As 2 single crystals with x = 0.057, which is near the antiferromagnetic instability. Resistivity ρ = ρ 0 + AT n was studied under applied pressure up to 7.90 GPa. The parameter n approaches a minimum value of n ≈ 1 at a critical pressure P c = 3.65 GPa. Near P c , the superconducting transition temperature T c reaches a maximum value of 25.8 K. In addition, the superconducting diamagnetism at 2 K shows a sudden change around the same critical pressure. Finally, these results may be associated with a possible quantum critical point hidden inside the superconducting dome, near optimum T c .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Insulator to metal transition in WO 3 induced by electrolyte gating

Tungsten oxide and its associated bronzes (compounds of tungsten oxide and an alkali metal) are well known for their interesting optical and electrical characteristics. We have modified the transport properties of thin WO 3 films by electrolyte gating using both ionic liquids and polymer electrolytes. We are able to tune the resistivity of the gated film by more than five orders of magnitude, and a clear insulator-to-metal transition is observed. To clarify the doping mechanism, we have performed a series of incisive operando experiments, ruling out both a purely electronic effect (charge accumulation near the interface) and oxygen-related mechanisms. We propose instead that hydrogen intercalation is responsible for doping WO 3 into a highly conductive ground state and provide evidence that it can be described as a dense polaronic gas.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Two superconducting transitions in single-crystal La 2 - x Ba x CuO 4

Here, we use spatially-resolved transport techniques to investigate the superconducting properties of single crystals La 2-x Ba x CuO 4 . We also found a superconducting transition temperature T cs associated with the ab-plane surface region which is considerably higher than the bulk T c . This effect is pronounced in the region of charge carrier doping x with strong spin-charge stripe correlations, reaching T cs = 36 K or 1.64T c .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Ultrafast relaxation dynamics in BiFeO 3 /YBa 2 Cu 3 O 7 bilayers

The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic BiFeO 3 (BFO) and superconducting YBa 2 Cu 3 O 7 (YBCO) grown on (001) SrTiO 3 substrate is studied by time-resolved pump-probe technique, and compared with that of pure YBCO thin film grown under the same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe a speeding up of the YBCO recombination dynamics in the superconducting state of the heterostructure, and attribute it to the presence of weak ferromagnetism at the BFO/YBCOinterface as observed inmagnetization data. An extension of the Rothwarf-Taylor model is used to fit the ultrafast dynamics of BFO/YBCO, that models an increased quasiparticle occupation of the ferromagnetic interfacial layer in the superconducting state of YBCO.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Induced Ferromagnetism at BiFeO 3 /YBa 2 Cu 3 O 7 Interfaces

We report that transition metal oxides (TMOs) exhibit many emergent phenomena ranging from high-temperature superconductivity and giant magnetoresistance to magnetism and ferroelectricity. In addition, when TMOs are interfaced with each other, new functionalities can arise, which are absent in individual components. Here, we report results from first-principles calculations on the magnetism at the BiFeO 3 /YBa 2 Cu 3 O 7 interfaces. By comparing the total energy for various magnetic spin configurations inside BiFeO 3 , we are able to show that a metallic ferromagnetism is induced near the interface. We further develop an interface exchange-coupling model and place the extracted exchange coupling interaction strengths, from the first-principles calculations, into a resultant generic phase diagram. Our conclusion of interfacial ferromagnetism is confirmed by the presence of a hysteresis loop in field-dependent magnetization data. Lastly, the emergence of interfacial ferromagnetism should have implications to electronic and transport properties.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Long-range electronic reconstruction to a d xz,yz -dominated Fermi surface below the LaAlO 3 /SrTiO 3 interface

Low dimensionality, broken symmetry and easily-modulated carrier concentrations provoke novel electronic phase emergence at oxide interfaces. However, the spatial extent of such reconstructions - i.e. the interfacial “depth” - remains unclear. Examining LaAlO 3 /SrTiO 3 heterostructures at previously unexplored carrier densities $n_{2D}$ ≥ 6.9 × 10 14 cm –2 , we observe a Shubnikov-de Haas effect for small in-plane fields, characteristic of an anisotropic 3D Fermi surface with preferential d xz,yz orbital occupancy extending over at least 100 nm perpendicular to the interface. Quantum oscillations from the 3D Fermi surface of bulk doped SrTiO 3 emerge simultaneously at higher $n_{2D}$. We distinguish three areas in doped perovskite heterostructures: narrow (<20 nm) 2D interfaces housing superconductivity and/or other emergent phases, electronically isotropic regions far (>120 nm) from the interface and new intermediate zones where interfacial proximity renormalises the electronic structure relative to the bulk.

36 MATERIALS SCIENCE↗

DOE OSTI2013