M. J. Zhang

Charge Number Dependence of the Dephasing Rates of a Graphene Double Quantum Dot in a Circuit QED Architecture

We use an on-chip superconducting resonator as a sensitive meter to probe the properties of graphene double quantum dots at microwave frequencies. Specifically, we investigate the charge dephasing rates in a circuit quantum electrodynamics architecture. The dephasing rates strongly depend on the number of charges in the dots, and the variation has a period of four charges, over an extended range of charge numbers. Although the exact mechanism of this fourfold periodicity in dephasing rates is an open problem, our observations hint at the fourfold degeneracy expected in graphene from its spin and valley degrees of freedom.

Measuring the complex admittance of a nearly isolated graphene quantum dot

We measured the radio-frequency reflection spectrum of an on-chip reflection line resonator coupled to a graphene double quantum dot (DQD), which was etched almost isolated from the reservoir and reached the low tunnel rate region. The charge stability diagram of DQD was investigated via dispersive phase and magnitude shift of the resonator with a high quality factor. Its complex admittance and low tunnel rate to the reservoir was also determined from the reflected signal of the on-chip resonator. Our method may provide a non-invasive and sensitive way of charge state readout in isolated quantum dots.

Symmetric reflection line resonator and its quality factor modulation by a two-dimensional electron gas

We have designed and fabricated a half-wavelength reflection line resonator (RLR) that consists of a pair of two coupled microstrip lines on a GaAs/AlGaAs heterostructure. By changing the top gate voltage on a square of two dimensional electron gas under the resonator, a large range of the quality factors can be obtained. Energy loss in the two-dimensional electron gas can be minimized, thus realizing a versatile resonator suitable for integration with semiconductor quantum circuits.

Angle-resolved vortex glass transition and pinning properties in BaFe1.8Co0.2As2 single crystals

The vortex phase transition in BaFe1.8Co0.2As2 single crystals was studied systematically via measuring the current–voltage (I-V) characteristics by rotating the crystals in magnetic fields from H // c to H // ab. The angle-resolved I-V curves clearly show a three-dimensional vortex glass (VG) scaling collapse. The angular dependent VG transition field Hg and critical current density Jc can be approximately described by the anisotropic Ginzburg-Landau model, and no peaks representing c-axis correlated pinning were recorded for H // c, which reveals that the vortex pinning is mainly caused by randomly distributed uncorrelated-defects. While in the vicinity of H // ab, Jc deviates from the model due to the in-plane intrinsic pinning. Based on the above results, the angle-resolved vortex phase diagram is constructed, indicating the anisotropic superconducting properties of the system.

Effect of carrier density and valence states on superconductivity of oxygen annealed Fe1.06Te0.6Se0.4 single crystals

The variations of carrier density and valence states in oxygen annealed Fe1.06Te0.6Se0.4 single crystals were studied systematically. It was found that the carrier density nH increases after oxygen annealing by Hall coefficient measurements. The X-ray photoelectron spectroscopy experiments reveal that the oxygen annealing changes Fe0 and Te0 states to Fe2+/3+ and Te4+, respectively, while the valence variation of Se is negligible. Our results indicate that the improvement of superconductivity, such as the zero resistance transition temperature Tczero, shielding and Meissner fraction value 4πχ and upper critical field Hc2, could be closely related to the proper manipulation of nH and the valence states by oxygen annealing in the system.

Effect of injected spins with different polarized orientations on the superconductivity of La0.7Sr0.3MnO3/La1.85Sr0.15CuO4 thin films

The influence of spin-polarized quasi-particle injection on the superconductivity of epitaxial La0.7Sr0.3MnO3/La1.85Sr0.15CuO4 composite thin films is systematically investigated. Different electrode configurations on the same specimen are designed to study the role of spin injection. It is found that the upper critical field and flux pinning potential are reduced by the spin injection, especially for the field parallel to ab-plane. The manipulation of superconductivity by the spin polarization orientations of injected quasi-particles possibly originates from the different suppressions on the superconducting pairing strength, which is considered to be related to the local antiferromagnetic spin fluctuations in La1.85Sr0.15CuO4.

Anisotropic transport property anomaly in K0.8Fe1.65Se2 crystal

The anisotropic transport properties of K0.8Fe1.65Se2 crystal have been investigated with the angle θ (between the magnetic field H and c-axis) varied from 0° to 180°. It is found that the angle dependencies of the mixed state resistivities near Tc0 show a small anomalous peak on the resistive valley for H//ab, probably due to the kinked vortex structure with a lower pinning energy in the phase-separated crystal. In the normal state, the resistivities increase with increasing angles and take a maximum at H//ab, which may be related to the larger resistivity when carriers move along the c-axis driven by Lorentz force.

Anisotropic rectifying characteristics induced by the superconducting gap of YBa2Cu3O7−δ/Nb-doped SrTiO3 heterojunctions

In this paper, we investigated the anisotropic rectifying characteristics of a YBa2Cu3O7−δ (YBCO)/Nb-doped SrTiO3 heterojunction in magnetic fields of up to 9 T by rotating the junction from H//c to H//ab of the YBCO film. From the temperature and field dependencies of the diffusion potential Vd, we found that the angle-resolved reductions of Vd from its original value, δVd, were induced by the anisotropic superconducting gap Δ of the YBCO. The anisotropic parameter obtained from Δ was close to that obtained from the angular-dependent upper critical fields of the YBCO. This heterojunction is helpful both in investigating the superconducting gap and in designing sensitive superconducting devices.

Angular dependent vortex glass phase transition in BaFe 1.8 Co 0.2 As 2 single crystal

By measuring the current-voltage (I-V) characteristics through rotating the crystals in magnetic fields from HIIc to HIIab-plane (θ=0° is defined as HIIc-axis), we systematically studied the angle-resolved VG transition in BaFe_1.8Co_0.2As₂ single crystal. It is found a good three-dimensional (3D) vortex glass (VG) scaling collapse through the angular dependent I-V curves, according to the vortex-glass phase theory. On the other hand, the angular dependent vortex glass transition field H_g and critical current density J_c can be approximately described by the anisotropic Ginzburg-Landau(G-L) model.