We have measured highly visible Aharonov-Bohm (AB) oscillations in a ring structure defined by local anodic oxidation on a p-type GaAs heterostructure with strong spin-orbit interactions. Clear beating patterns observed in the raw data can be interpreted in terms of a spin geometric phase. Besides h/e oscillations, we resolve the contributions from the second harmonic of AB oscillations and also find a beating in these h/2e oscillations. A resistance minimum at B = 0 T, present in all gate configurations, is the signature of destructive interference of the spins propagating along time-reversed paths.Interference phenomena with particles have challenged physicists since the foundation of quantum mechanics. A charged particle traversing a ring-like mesoscopic structure in the presence of an external magnetic flux Φ acquires a quantum mechanical phase. The interference phenomenon based on this phase is known as the Aharonov-Bohm (AB) effect [1], and manifests itself in oscillations of the resistance of the mesoscopic ring with a period of Φ 0 = h/e, where Φ 0 is the flux quantum. The Aharonov-Bohm phase was later recognized as a special case of the geometric phase [2,3] acquired by the orbital wave function of a charged particle encircling a magnetic flux line.The particle's spin can acquire an additional geometric phase in systems with spin-orbit interactions (SOI) [4,5,6]. The investigation of this spin-orbit (SO) induced phase in a solid-state environment is currently the subject of intensive experimental work [7,8,9,10,11,12]. The common point of these experiments is the investigation of electronic transport in ring-like structures defined on two-dimensional (2D) semiconducting systems with strong SOI. Electrons in InAs were investigated in a ring sample with time dependent fluctuations [7], as well as in a ring side coupled to a wire [9]. An experiment on holes in GaAs [8] showed B-periodic oscillations with a relative amplitude ∆R/R < 10 −3 . These observations [7,8] were analyzed with Fourier transforms and interpreted as a manifestation of Berry's phase. Further studies on electrons in a HgTe ring [10] and in an InGaAs ring network [11] were discussed in the framework of the AharonovCasher effect.In systems with strong SOI, an inhomogeneous, momentum dependent intrinsic magnetic field B int , perpendicular to the particle's momentum, is present in the reference frame of the moving carrier [13]. The total magnetic field seen by the carrier is therefore B tot = B ext + B int , where B ext is the external magnetic field perpendicular to the 2D system and B int is the intrinsic magnetic field in the plane of the 2D system present in the moving reference frame (right inset Fig. 1(a)). The particle's spin precesses around B tot and accumulates an additional geometric phase upon cyclic evolution.Effects of the geometric phases are most prominently expressed in the adiabatic limit, when the precession frequency of the spin around the local field B tot is much faster than the orbital frequency of the charged particl...

The two-terminal magnetoconductance of a hole gas in C-doped AlGaAs/GaAs heterostructures with Ohmic contacts consisting of alloyed In/Zn/Au displays a pronounced hysteresis of the conductance around zero magnetic field. The hysteresis disappears above magnetic fields of around 0.5 T and temperatures above 300 mK. For magnetic fields below 10 mT, we observe a pronounced dip in the magnetoconductance. We tentatively discuss these experimental observations in the light of superconductivity of the Ohmic contacts.Transport measurements in semiconductors require Ohmic contacts between the external metallic leads and the carriers in the semiconductor. At the interface between a semiconductor and a metal, usually a Schottky contact forms giving rise to nonlinear current-voltage ͑I-V͒ characteristics. In order to obtain linear I-V traces, this Schottky barrier has to be overcome, which is usually achieved by highly doping the semiconductor in the contact region. For n-type AlGaAs heterostructures, this is routinely achieved by using Au/ Ge/Ni alloys. For p-type AlGaAs heterostructures, one usually uses layers of AuZn, AuBe, or InZn. 1 Especially at liquid He temperatures and below, the quality of the contacts to hole systems is inferior to the Ohmic contacts on electron gases. As long as the I-V characteristics are linear, high contact resistances can be experimentally overcome by using four-terminal measurements. However, for high impedance devices such as quantum dots, one has to perform voltagebiased measurements, and the contact quality becomes crucial.Here we report on a peculiar hysteresis effect observed in two-terminal magnetoconductance measurements on a twodimensional hole gas contacted by In/Zn/Au alloys. For voltage biased devices, the current signal shows a pronounced dip-like feature around zero magnetic field and hysteretic behavior up to magnetic fields, where Shubnikov-de Haas ͑SdH͒ oscillations become relevant. The feature disappears for high magnetic fields and high temperatures. We tentatively ascribe these observations to the contacts becoming superconducting. This way, we extract an estimate for T c , the superconducting transition temperature, as well as for critical magnetic fields of the In/Zn/Au alloy, which are in tune with values reported in the literature. The purpose of this paper is to show experimental details of unusual Ohmic contacts possibly related to a superconducting phase transition.Our experiments have been performed on AlGaAs heterostructures doped with carbon ͑C͒ acting as an acceptor on ͑100͒ substrates. 2,3 The integer, as well as the fractional quantum Hall effects, have been observed in such samples 4,5 testifying to the electronic quality of these structures. The material quality has strongly improved over the last couple of years 6,7 with mobilities now exceeding 10 6 cm 2 / Vs. 8 Also, single electron transistors, 9 as well as high-quality Aharonov-Bohm rings, 10 have been realized. The effects, which we describe in the following, have been observed in two-terminal measure...

A single-hole transistor is patterned in a p-type, C-doped GaAs∕AlGaAs heterostructure by scanning probe oxidation lithography. Clear Coulomb blockade resonances have been observed at Thole=300mK. A charging energy of ∼1.5meV is extracted from Coulomb diamond measurements, in agreement with the lithographic dimensions of the dot. The absence of excited states in Coulomb diamond measurements, as well as the temperature dependence of Coulomb peak heights indicate that the dot is in the multilevel transport regime. Fluctuations in peak spacings larger than the estimated mean single-particle level spacing are observed.

High-quality C-doped p-type AlGaAs heterostructures with mobilities exceeding 150 000 cm$^2$/Vs are investigated by low-temperature magnetotransport experiments. We find features of the fractional quantum Hall effect as well as a highly resolved Shubnikov-de Haas oscillations at low magnetic fields. This allows us to determine the densities, effective masses and mobilities of the holes populating the spin-split subbands arising from the lack of inversion symmetry in these structures.Comment: 3 pages, 4 figure

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We have explored phase coherent transport of holes in two p-type GaAs quantum rings with orbital radii 420 nm and 160 nm fabricated with AFM oxidation lithography. Highly visible Aharonov-Bohm (AB) oscillations are measured in both rings, with an amplitude of the oscillations exceeding 10% of the total resistance in the case of the ring with a radius of 160 nm. Beside the h/e oscillations, we resolve the contributions from higher harmonics of the AB oscillations. The observation of a local resistance minimum at B=0 T in both rings is a signature of the destructive interference of the holes' spins. We show that this minimum is related to the minimum in the h/2e oscillations.The Aharonov-Bohm (AB) phase [1], represents the geometric phase acquired by the orbital wave function of the charged particle encircling a magnetic flux line. This phase is experimentally well established and manifests itself through oscillations in the resistance of mesoscopic rings as a function of an external magnetic field. The spin part of the particle's wave-function can acquire an additional geometric phase in the systems with strong spinorbit (SO) interactions [2,3]. The SO induced phase additionally modulates the resistance oscillations in mesoscopic rings. This SO induced phase in solid-state systems has been recently the subject of a number of experimental investigations [4,5,6,7,8,9,10].SO interactions are particularly strong in p-type GaAs heterostructures, due to the p-like symmetry of the states at the top of the valence band and the high effective mass of holes. The presence of exceptionally strong SO interactions in carbon doped GaAs heterostructure used for fabrication of rings investigated in this work, is demonstrated by the simultaneous observation of the beating in Shubnikov-de Haas (SdH) oscillations and weak antilocalization in the measured magnetoresistance. The hole density in an unpatterned sample is 3.8×10 11 cm −2 and the mobility is 200 000 cm 2 /Vs at a temperature of 60 mK. The strength of the Rashba spin-orbit interaction is estimated to be ∆ SO ≈ 0.8 meV, while the Fermi energy in the system is E F = 2.5 meV.Here we study AB oscillations in two quantum rings with radii 420 nm and 160 nm realized in this 2DHG with strong SO interactions. In contrast to previous experiments on p-type GaAs rings, where the signature of the phase acquired by the hole's spin was attributed to the splitting of the h/e peak in the Fourier spectrum [4, 5], we have recently reported the direct observation of beating in the measured resistance of the quantum ring with an orbital radius of 420 nm [10]. An example of the observed beating in the gate configuration V pg1 = −172 mV and V pg2 = −188 mV of the large ring is shown in Fig. 1(a), while the corresponding splitting of the h/e Fourier peak is shown in Fig. 1(c). In addition, we observe a resistance minimum at B = 0 T in all gate configurations, and attribute its origin to the destructive interference of the hole spins propagating along time reversed paths [10].We now focus on the ma...

Strong spin-orbit interaction characteristic for p-type GaAs systems, makes such systems promising for the realization of spintronic devices. Here we report on transport measurements in nanostructures fabricated on p-type, C-doped GaAs heterostructures by scanning probe oxidation lithography. We observe conductance quantization in a quantum point contact, as well as pronounced Coulomb resonances in two quantum dots with different geometries. Charging energies for both dots, extracted from Coulomb diamond measurements are in agreement with the lithographic dimensions of the dots. The absence of excited states in Coulomb diamond measurements indicates that the dots are in the multi-level transport regime.Comment: Proceedings of ICPS-28, Vienna 2006; 2 pages, 2 figure