Research interest

We are interested in the fundamental physics of correlated electron systems, quantum nature of emergent phenomena in these systems, and possible applications of such systems in material sciences and engineering. These systems include superconductors, topological materials and two-dimensional electron gas in semiconductors, which represent a large and active portion of contemporary condensed matter physics and material sciences.

Facilities


Cryogenic setups: We have a cryogen free dilution refrigerator (with a 12 Tesla superconducting magnet) which provides temperature down to about 10 miliKelvin. We also have access to the instruments in the UTD Cryogenic Center, such as Physical Property Measurement System (PPMS, 1.5 Kelvin and above), Magnetic Property Measurement System (MPMS), liquid helium dip station, etc. In addition, the Variable Temperature Insert (VTI) in Prof. Mark Lee's group is also available (3 Kelvin and above). Therefore, we have a full coverage of temperature from 10 miliKelvin to 300 Kelvin.

Device fabrication: We have an evaporator in the lab for metal films deposition. In addition, we have access to the UTD Cleanroom Research Laboratory, which includes facilities for device fabrication and characterization with a range of equipment, such as photo-lithography, e-beam lithography, plasma etch, PECVD, LPCVD, metallization (sputter, e-gun, thermal evap), wet chemical stations, thermal (oxidation/diffusion, RTP) Lithography, and surface analysis tools, etc.

Previous research projects

Charge glassiness and emergence of superconductivity in cuprates


From Shi et al., Nature Materials, 12, 47-51 (2013)
A central issue for copper oxides is the nature of the insulating ground state at low carrier densities and the emergence of high-temperature superconductivity from that state with doping. Even though this superconductor-insulator transition (SIT) is a zero-temperature transition, measurements are not usually carried out at low temperatures. Here we use magnetoresistance to probe both the insulating state at very low temperatures and the presence of superconducting fluctuations in La2-xSrxCuO4 films, for doping levels that range from the insulator to the superconductor (x=0.03-0.08). We observe that the charge glass behaviour, characteristic of the insulating state, is suppressed with doping, but it coexists with superconducting fluctuations that emerge already on the insulating side of the SIT. The unexpected quenching of the superconducting fluctuations by the competing charge order at low temperatures provides a new perspective on the mechanism for the SIT.
Related News:


Superconductor-insulator transition in cuprates


From Shi et al., Nature Physics, 10, 437-443 (2014).
In the underdoped pseudogap regime of cuprate superconductors, the normal state is commonly probed by applying a magnetic field (H). However, the nature of the H-induced resistive state has been the subject of a long-term debate, and clear evidence for a zero-temperature H-tuned superconductor-insulator transition has proved elusive. Here we report magnetoresistance measurements on underdoped La2-xSrxCuO4, providing striking evidence for quantum-critical behaviour of the resistivity - the signature of a H-driven superconductor-insulator transition. The transition is not direct, being accompanied by the emergence of an intermediate state, which is a superconductor only at temperature T = 0. Our finding of a two-stage H-driven superconductor-insulator transition goes beyond the conventional scenario in which a single quantum critical point separates the superconductor and the insulator in the presence of a perpendicular magnetic field. Similar two-stage H-driven superconductor-insulator transitions, in which both disorder and quantum phase fluctuations play an important role, may also be expected in other copper-oxide high-temperature superconductors.
Related News:


Vortex dynamics in iron based superconductors


From Nikolo, et al., Journal of Superconductivity and Novel Magnetism, 27, 2231-2239 (2014)
Understanding the flux dynamics in superconductors is essential. Unlike cuprates, iron based superconductors usually exhibit coexistence of superconductivity and antiferromagnetism in the underdoped region, which could dramatically change the flux dynamics. Our transport and magnetization experiments on Co, Ni and K doped Ba(FeAs)2 do show complicated and novel behaviors. For example, the thermally assisted flux flow does not obey the wellk nown Anderson-Kim Arrhenius law. The magnetic field dependence of the superconducting transition width indicates a linear relationship, instead of the Tinkhams theoretical prediction. These observations indicate a new class of flux dynamics in the recently discovered superconductor family.


Giant supercurrent in superconductor-topological insulator junctions


From Shi et al., arXiv:1410.7342
Superconductivity in topological materials has attracted a great deal of interest in both electron physics and material sciences since the theoretical predictions that Majorana fermions can be realized in topological superconductors. Topological superconductivity could be realized in a type II, band-inverted, InAs/GaSb quantum well if it is in proximity to a conventional superconductor. Here we report observations of the proximity effect induced giant supercurrent states in an InAs/GaSb bilayer system that is sandwiched between two superconducting tantalum electrodes to form a superconductor-InAs/GaSb-superconductor junction. Electron transport results show that the supercurrent states can be preserved in a surprisingly large temperature-magnetic field (T-H) parameter space. In addition, the evolution of differential resistance in T and H reveals an interesting superconducting gap structure.


McMillan-Rowell like oscillations in superconductor-topological insulator junctions


From Shi et al., Applied Physics Letters, 107, 052601 (2015)
We have fabricated a superconductor (Ta)-InAs/GaSb bilayer-superconductor (Ta) junction device that has a long mean free path and can preserve the wavelike properties of particles (electrons and holes) inside the junction. Differential conductance measurements were carried out at low temperatures in this device, and McMillan-Rowell like oscillations (MROs) were observed. Surprisingly, a much larger Fermi velocity, compared to that from Shubnikov-de Haas oscillations, was obtained from the frequency of MROs. Possible mechanisms are discussed for this discrepancy.


Electron glassiness in Si-MOSFET from noise measurements


From Lin et al., Physical Review B, 86, 155135 (2012)
A study of the conductance noise in a two-dimensional electron system (2DES) in Si at low temperatures (T) reveals the onset of large, non-Gaussian noise after cooling from an equilibrium state at a high T with a fixed carrier density ns. This behavior, which signifies the falling out of equilibrium of the 2DES as T approaches 0, is observed for ns less than ng (ng denotes the glass transition density). A protocol where density is changed by a small value D_ns at low T produces the same results for the noise power spectra. However, a detailed analysis of the non-Gaussian probability density functions (PDFs) of the fluctuations reveals that D_ns has a qualitatively different and more dramatic effect than D_T, suggesting that D_ns induces strong changes in the free energy landscape of the system as a result of Coulomb interactions. The results from a third, waiting-time (tw) protocol, where ns is changed temporarily during tw by a large amount, demonstrate that non-Gaussian PDFs exhibit history dependence and an evolution toward a Gaussian distribution as the system ages and slowly approaches equilibrium. By calculating the power spectra and higher-order statistics for the noise measured over a wide range of the applied voltage bias, it is established that the non-Gaussian noise is observed in the regime of Ohmic or linear response, i.e., that it is not caused by the applied bias.


Impact of modulation doping layer on v =5/2 fractional quantum hall state (FQHE) anisotropy


From Shi et al., Physical Review B, 91, 125308 (2015)
We have carried out a systematic study of the tilted magnetic field induced anisotropy at the Landau level filling factor v=5/2 in a series of high quality GaAs quantum wells, where the setback distance (d) between the modulation doping layer and the GaAs quantum well is varied from 33 to 164 nm. We have observed that in the sample of the smallest d, electronic transport is anisotropic when the in-plane magnetic field (Bip) is parallel to the [1-10] crystallographic direction, but remains more or less isotropic when Bip||[110]. In contrast, in the sample of largest d, electronic transport is anisotropic in both crystallographic directions. Our results clearly show that the modulation doping layer plays an important role in the tilted field induced v=5/2 anisotropy.


Back to home page