arXiv subjects

H. W. Jiang

Publications and source records attributed to H. W. Jiang.

At least 19 recordsLinked to original sources

arXiv2020

High Frequency Microwave Emission of a Tri-layer Magnetic Tunnel Junction in the Absence of External Bias-Magnetic Field

We perform an experimental study of DC current induced microwave emission in a magnetic tunnel junction (MTJ) consisting of three active magnetic layers. For this tri-layer structure, in addition to a conventional bilayer orthogonal MTJ containing a perpendicular free layer and an in-plane fixed layer, a second perpendicular layer has been introduced. We found that the microwave emission frequency induced by spin-transfer torque (STT) can reach as high as 6 GHz in the absence of any applied magnetic field. Moreover, microwave emission is observed for both current polarizations where a redshift is seen with increase in magnitude of current. We discuss spin-dynamics of the observed bi-directional high-frequency emission and the physical origin of the red-shift. The distinct microwave emission properties exhibited in this tri-layer MTJ structure could potentially be useful for future applications in nanoscale spintronics devices such as microwave communication and neuromorphic computing.

cond-mat.mes-hall↗

arXiv2010

Germanium electrostatic quantum dot with integrated charge detector in an MOS structure

We report the fabrication and characterization of an electrostatic quantum dot in pure Germanium with an integrated charge measurement transistor. The device uses the Al2O3/Germanium interface for the confinement of carriers in the Germanium and an hybrid design with an electron quantum dot and hole transistor for the charge detection. The hole transistor, using with NiGe source and drain contacts, despite the modest low temperature carrier mobility of 450 cm2/Vs, has shown a sensitivity to the dot electric potential sufficient to detect single charges tunneling in and out of the quantum dot. The device is realized with a two level gate stack, with the top level used to attract electrons and the lower one to define the electron confinement potential and accumulate the hole transistor. The possibility to improve the device operation using a Al2O3/SiGe/Ge multilayer for the confinement of electrons at a smoother interface is discussed.

cond-mat.mes-hall↗

arXiv2009

Measurement of the Spin Relaxation Time of Single Electrons in a Silicon MOS-Based Quantum Dot

We report on measurements of the spin relaxation time T1 of individual electron spins in the few electron regime of a Si/SiO2-based quantum dot (QD). Energy-spectroscopy of the QD has been performed using a charge sensing technique. The spin relaxation times are subsequently measured in the time-domain by a pump-and-probe method. For the QD that contains an unpaired spin, likely only a single electron, we find that T1 depends strongly on the applied magnetic field. Possible mechanisms leading to the observed spin relaxation are discussed.

cond-mat.mes-hall↗

arXiv2009

Probing Quantum Hall Pseudospin Ferromagnet by Resistively Detected NMR

Resistively Detected Nuclear Magnetic Resonance (RD-NMR) has been used to investigate a two-subband electron system in a regime where quantum Hall pseudo-spin ferromagnetic (QHPF) states are prominently developed. It reveals that the easy-axis QHPF state around the total filling factor $ν=4 $ can be detected by the RD-NMR measurement. Approaching one of the Landau level (LL) crossing points, the RD-NMR signal strength and the nuclear spin relaxation rate $1/T_{1}$ enhance significantly, a signature of low energy spin excitations. However, the RD-NMR signal at another identical LL crossing point is surprisingly missing which presents a puzzle.

cond-mat.mes-hall↗

arXiv2008

Numerical Studies of Quantum Hall Ferromagnetism in Two-Subband Systems

We carry out a numerical study of the quantum Hall ferromagnetism in a two-subband system using a set of experimental parameters in a recently experiment [X. C. Zhang, I. Martin, and H. W. Jiang, Phys. Rev. B \textbf{74}, 073301 (2006)]. Employing the self-consistence local density approximation for growth direction wave function and the Hartree-Fock theory for the pseudospin anisotropy energy, we are able to account for the easy-axis and easy-plane quantum Hall ferromagnetism observed at total filling factor $ν= 3$ and $ν= 4$, respectively. Our study provides some insight of how the anisotropy energy, which highly depends upon the distribution of growth direction wave functions, determines the symmetry of the quantum Hall ferromagnets.

cond-mat.mes-hall↗

arXiv2008

Experimental Studies of Scaling Behavior of a Quantum Hall System with a Tunable Landau Level Mixing

Temperature dependence of the longitudinal and Hall resistance is studied in the regime of localization-delocalization transition. We carry out measurements of a scaling exponent $κ$ in the Landau level mixing region at several filling factors. The localization exponent $γ$ is extracted using an approach based on the variable range hopping theory. The values of $γ$ and $κ$ are found to be universal, independent of filling factor in our sample. We can conclude that although Landau level mixing can change the degeneracy of a quantum Hall state, the value of the scaling exponent remains the same for a given sample that contains a fixed disorder profile.

cond-mat.mes-hall↗

arXiv2007

Observation of an in-plane magnetic-field-driven phase transition in a quantum Hall system with SU(4) symmetry

In condensed matter physics, the study of electronic states with SU(N) symmetry has attracted considerable and growing attention in recent years, as systems with such a symmetry can often have a spontaneous symmetry-breaking effect giving rise to a novel ground state. For example, pseudospin quantum Hall ferromagnet of broken SU(2) symmetry has been realized by bringing two Landau levels close to degeneracy in a bilayer quantum Hall system. In the past several years, the exploration of collective states in other multi-component quantum Hall systems has emerged. Here we show the conventional pseudospin quantum Hall ferromagnetic states with broken SU(2) symmetry collapsed rapidly into an unexpected state with broken SU(4) symmetry, by in-plane magnetic field in a two-subband GaAs/AlGaAs two-dimensional electron system at filling factor around $ν=4$. Within a narrow tilting range angle of 0.5 degrees, the activation energy increases as much as 12 K. While the origin of this puzzling observation remains to be exploited, we discuss the possibility of a long-sought pairing state of electrons with a four-fold degeneracy.

cond-mat.mes-hall↗

arXiv2006

Sputtered Gold as an Effective Schottky Gate for Strained Si/SiGe Nanostructures

Metallization of Schottky surface gates by sputtering Au on strained Si/SiGe heterojunctions enables the depletion of the two dimensional electron gas (2DEG) at a relatively small voltage while maintaining an extremely low level of leakage current. A fabrication process has been developed to enable the formation of sub-micron Au electrodes sputtered onto Si/SiGe without the need of a wetting layer.

cond-mat.mes-hall↗

arXiv2006

Mechanism of Enhanced Rectification in Unimolecular Borromean Ring Devices

We have studied charge transport through individual Borromean Ring complexes, both with and without anchor groups, in gated double barrier tunneling junctions (DBTJs) formed using the electrical breakjunction technique on gold nanowires. While common single molecule device characteristics can be observed with either form of the Borromean Rings, the complexes with anchor groups show strong rectification of conduction in a relatively high percentage of samples. We present our data along with a simple model underlining the mechanism by which the arrangement and composition of the weakly bonding anchor groups attached to the electroactive element may promote a device configuration resulting in rectification.

cond-mat.mes-hall↗

arXiv2006

NMR Probing Spin Excitations in the Ring-Like Structure of a Two-Subband System

Resistively detected nuclear magnetic resonance (NMR) is observed inside the ring-like structure, with a quantized Hall conductance of 6e^2/h, in the phase diagram of a two subband electron system. The NMR signal persists up to 400 mK and is absent in other states with the same quantized Hall conductance. The nuclear spin-lattice relaxation time, T1, is found to decrease rapidly towards the ring center. These observations are consistent with the assertion of the ring-like region being a ferromagnetic state that is accompanied by collective spin excitations.

cond-mat.dis-nn↗

arXiv2006

Landau Level Anticrossing Manifestations in the Phase Diagram Topology of a Two Subband System

In a two-subband GaAs/AlGaAs two-dimensional electron system, the phase diagram of longitudinal resistivity ρ_xx in density and magnetic field plane exhibits an intriguing structure centered at filling factor ν= 4 which is strikingly different from the ring-like structures at lower magnetic fields. Thermal activation measurements reveal an anticrossing gap on each boundary of the structure where intersubband Landau Levels with parallel or antiparallel spin are brought into degeneracy. While the physics of the anticrossing can be ascribed to the pseudospin quantum Hall ferromagnetism, as reported earlier by Muraki et al, the mapping and modeling of the phase diagram topology allow us to establish a more complete picture of the consequences of real spin / pseudo-spin interactions for the two subband system.

cond-mat.mes-hall↗

arXiv2005

Nonequilibrium Magnetization of a Two-Dimensional Electron Gas in a Static Magnetic Field

Using a sensitive DC torque magnetometer we measure the orbital magnetization by sweeping the density at fixed magnetic fields on GaAs-AlGaAs heterostructures. At low temperatures strong nonequilibrium magnetization signals dominate the data. In literature the observation of nonequilibrium signals are often associated with eddy currents generated by sweeping the magnetic field. The elimination of a changing magnetic field then poses a question regarding the origin of these signals. Our data suggests another source of nonequilibrium magnetization potentially due to the change of area occupied by compressible and incompressible states as one sweeps the Fermi energy from one Landau level to the next.

cond-mat.mes-hall↗

arXiv2005

Multiple Phases with the Same Quantized Hall Conductance in a Two-Subband System

In a GaAs/AlGaAs two-dimensional electron system with two occupied subbands, the experimentally determined phase diagram in the density-magnetic field plane exhibits rich topological features. Ring-like structures are observed at even integer filling factors, in the phase diagram. Even with the identical quantized Hall resistance values as those given rise by the ordinary integer quantum Hall effect due to the Landau level quantization; the activation energies of these states within the rings are much smaller. These ring structures cannot account for by the simple single particle picture. We argue that ferromagnetic quantum Hall states, due to the interaction of two energy levels with opposite spin and different subband indices, are responsible for these unusual structures.

cond-mat.mes-hall↗

arXiv2005

Differential Conductance Peak Exchange in Borromean Ring Single Molecule Transistors

While Single Molecule Transistors have been employed in laboratory research for a number of years, there remain many details related to charge transport that have yet to be delineated. We have used the technique of electromigration to create a nanometer-size gap for hosting a single molecule in the narrowest portion of a gold wire defined by electron beam lithography. A recently synthesized Borromean Ring complex was used as the active molecular element of the transistor devices. It is well established in Single Molecule Transistors that, for sweeps of source-drain voltage, a region of zero conductance commonly appears - known as a conductance gap - which is centered about V_sd = 0. We have observed differential conductance peaks on opposing sides of these conductance gaps exchange positions as a result of changing gate bias. We associate this property with a change in sign of the majority charge carriers flowing through the molecule. That is, a measured change in differential conductance occurs when current through a molecular energy level shifts between the flow of holes and the flow of electrons.

cond-mat.mes-hall↗

arXiv2005

Fabrication and Characterization of Electrostatic Quantum Dots in a Si/SiGe 2D Electron Gas, Including an Integrated Read-out Channel

A new fabrication technique is used to produce quantum dots with read-out channels in silicon/silicon-germanium two-dimensional electron gases. The technique utilizes Schottky gates, placed on the sides of a shallow etched quantum dot, to control the electronic transport process. An adjacent quantum point contact gate is integrated to the side gates to define a read-out channel and thus allow for noninvasive detection of the electronic occupation of the quantum dot. Reproducible and stable Coulomb oscillations and the corresponding jumps in the read-out channel resistance are observed at low temperatures. The fabricated dot combined with the read-out channel represent a step towards the spin-based quantum bit in Si/SiGe heterostructures.

cond-mat.mes-hall↗

arXiv2003

Orbital Magnetization Measurement of the Quantum Hall to Insulator Transition

We present a magnetization measurement probing the transition from a quantum Hall to insulating (QH-I) state for a two-dimensional electron gas in a disordered GaAs/AlGaAs heterostructure. Using a highly sensitive DC torque magnetometer, we discover an abrupt change in the orbital magnetization precisely at the critical point for the QH-I transition. Since this transition is predicted to be a second order quantum phase transition, a thermodynamic signature in magnetization is totally unexpected. The observed feature is reminiscent of the well-known de Haas-van Alphen oscillations arising from discontinuous jumps of the chemical potential as Landau Levels are successively populated.

cond-mat.mes-hall↗

arXiv2002

Thermodynamic Compressibility of a Two-Dimensional Electron System: Signature of a Droplet State

We have used a field-penetration method to measure thermodynamic compressibility of a moderately interacting two-dimensional electron system ($r_{s}$ $\approx$ 0.5-3) in a three terminal GaAs/AlGaAs device, fabricated with an epitaxial lift-off technique. We found that the density and temperature dependencies of the compressibility are qualitatively different from that observed in earlier studies of the 2D hole system, where interaction energies are considerably stronger. We show that the observed characteristics can be described by the recently developed formalism for compressibility of the droplet state.

cond-mat.str-el↗

arXiv2002

A scheme for electrical detection of spin resonance signal from a single electron trap

We study a scheme for electrical detection of the spin resonance (ESR) of a single electron trapped near a Field Effect Transistor (FET) conduction channel. In this scheme, the resonant Rabi oscillations of the trapped electron spin cause a modification of the average occupancy of a shallow trap, which can be detected through the change in the FET channel resistivity. We show that the dependence of the channel resistivity on the frequency of the rf field can have either peak or dip at the Larmor frequency of the electron spin in the trap.

cond-mat.mtrl-sci↗