Yashar Komijani

Evidence for localization and 0.7 anomaly in hole quantum point contacts

Quantum point contacts (QPCs) implemented in p-type GaAs/AlGaAs heterostructures are investigated by low-temperature electrical conductance spectroscopy. Besides one-dimensional conductance quantization, a pronounced extra plateau is found at about 0.7(2e2/h) which possesses the characteristic properties of the so-called 0.7 anomaly known from experiments with n-type samples. The evolution of the 0.7 plateau in high perpendicular magnetic field reveals the existence of a quasi-localized state and is consistent with the explanation of the 0.7 anomaly based on self-consistent charge localization. These observations are robust when lateral electrical fields are applied which shift the relative position of the electron wave function in the QPC, testifying to the intrinsic nature of the underlying physics.

Observation of excited states in a p-type GaAs quantum dot

A quantum dot fabricated by scanning probe oxidation lithography on a p-type, C-doped GaAs/AlGaAs heterostructure is investigated by low-temperature electrical conductance measurements. Clear Coulomb blockade oscillations are observed and analyzed in terms of sequential tunneling through the single-particle levels of the dot at Thole = 185 mK. The charging energies as large as ∼ 2 meV evaluated from Coulomb diamond measurements together with the well-resolved single-hole excited-state lines in the charge stability diagram indicate that the dot is operated with a small number of confined particles close to the ultimate single-hole regime. Copyright c EPLA, 2008

Origins of conductance anomalies in a p-type GaAs quantum point contact

Low-temperature transport measurements on a p-GaAs quantum point contact are presented which reveal the presence of a conductance anomaly that is markedly different from the conventional “0.7 anomaly.” A lateral shift by asymmetric gating of the conducting channel is utilized to identify and separate different conductance anomalies of local and generic origins experimentally. While the more generic 0.7 anomaly is not directly affected by changing the gate configuration, a model is proposed which attributes the additional conductance features to a gate-dependent coupling of the propagating states to localized states emerging due to a nearby potential imperfection. Finite bias conductivity measurements reveal the interplay between the two anomalies consistently with a two-impurity Kondo model.

Counting statistics of hole transfer in a p-type GaAs quantum dot with dense excitation spectrum

Low-temperature transport experiments on a p-type GaAs quantum dot capacitively coupled to a quantum point contact are presented. The time-averaged as well as time-resolved detection of charging events of the dot are demonstrated and they are used to extract the tunneling rates into and out of the quantum dot. The extracted rates exhibit a super-linear enhancement with the bias applied across the dot, which is interpreted in terms of a dense spectrum of excited states contributing to the transport, characteristic for heavy hole systems. The full counting statistics of charge transfer events and the effect of back action is studied. The normal cumulants as well as the recently proposed factorial cumulants are calculated and discussed in view of their importance for interacting systems. DOI: 10.1103/PhysRevB.88.035417 PACS number(s): 73.23.−b, 73.63.−b I. INTRODUCTION Quantum dots (QDs or simply dots) are small conducting islands that confine charge carriers in three dimensions, resultinginadiscretespectrumofexcitedstates.Thisspectrum is often studied in transport experiments by measuring the current, 1 which is allowing carriers to tunnel between the dot and source and drain leads (reservoirs). The capacitive coupling of QD to nearby gates enables tuning the energy of excited states with respect to electrochemical potential of the leads.Itisafascinatingexperimentalobservationthatasimilar capacitive coupling to a nearby electrical current passing through a constriction provides the possibility of measuring the charge of the dot with a precision of a small fraction of an electron’s charge. 2 The conductance of the constriction changes as a function of the average charge population of the

Nanostructures in p-GaAs with improved tunability

A nanofabrication technique is presented which enables the fabrication of highly tunable devices on p-type, C-doped GaAs/AlGaAs heterostructures containing shallow two-dimensional hole systems. The high tunability of these structures is provided by the complementary electrostatic effects of intrinsic in-plane gates and evaporated metallic top-gates. Quantum point contacts fabricated with this technique were tested by electrical conductance spectroscopy.

Anisotropic Zeeman shift in p-type GaAs quantum point contacts

Low-temperature electrical conductance spectroscopy measurements of quantum point contacts implemented in p-type GaAs/AlGaAs heterostructures are used to study the Zeeman splitting of 1D subbands for both in-plane and out-of-plane magnetic field orientations. The resulting in-plane g-factors agree qualitatively with those of previous experiments on quantum wires while the quantitative differences can be understood in terms of the enhanced quasi-1D confinement anisotropy. The influence of confinement potential on the anisotropy is discussed and an estimate for the out-of-plane g-factor is obtained which, in contrast to previous experiments, is close to the theoretical prediction.

Analytical slave-spin mean-field approach to orbital selective Mott insulators

We use the slave-spin mean-field approach to study particle-hole symmetric one- and two-band Hubbard models in presence of Hunds coupling interaction. By analytical analysis of Hamiltonian, we show that the locking of the two orbitals vs.\,orbital-selective Mott transition can be formulated within a Landau-Ginzburg framework. By applying the slave-spin mean-field to impurity problem, we are able to make a correspondence between impurity and lattice. We also consider the stability of the orbital selective Mott phase to the hybridization between the orbitals and study the limitations of the slave-spin method for treating inter-orbital tunnellings in the case of multi-orbital Bethe lattices with particle-hole symmetry.

Luttinger liquid in contact with a Kramers pair of Majorana bound states

We discuss the signatures of a Kramers pair of Majorana modes formed in a Josephson junction on top of a quantum spin Hall system. We show that, while ignoring interactions on the quantum spin Hall edge allows arbitrary Andreev processes in the system, moderate repulsive interactions stabilize Andreev transmission---the hole goes into the opposite lead from where the electron has arrived. We analyze the renormalization group equations and deduce the phase diagram as a function of interaction strength.

Detecting a quantum critical point in topological SN junctions

A spin-orbit coupled quantum wire, with one end proximate to an s-wave superconductor, can become a topological superconductor, with a Majorana mode localized at each end of the superconducting region. It was recently shown that coupling one end of such a topological superconductor to

Interaction effects in Aharonov-Bohm–Kondo rings

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