arXiv subjects

Amir Yacoby

Publications and source records attributed to Amir Yacoby.

122 records · Page 7Linked to original sources

arXiv2010

Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene

The non-interacting energy spectrum of graphene and its bilayer counterpart consists of multiple degeneracies owing to the inherent spin, valley and layer symmetries. Interactions among charge carriers are expected to spontaneously break these symmetries, leading to gapped ordered states. In the quantum Hall regime these states are predicted to be ferromagnetic in nature whereby the system becomes spin polarized, layer polarized or both. In bilayer graphene, due to its parabolic dispersion, interaction-induced symmetry breaking is already expected at zero magnetic field. In this work, the underlying order of the various broken-symmetry states is investigated in bilayer graphene that is suspended between top and bottom gate electrodes. By controllably breaking the spin and sublattice symmetries we are able to deduce the order parameter of the various quantum Hall ferromagnetic states. At small carrier densities, we identify for the first time three distinct broken symmetry states, one of which is consistent with either spontaneously broken time-reversal symmetry or spontaneously broken rotational symmetry.

cond-mat.mes-hall↗

arXiv2010

Universal quantum control of two-electron spin quantum bits using dynamic nuclear polarization

One fundamental requirement for quantum computation is to perform universal manipulations of quantum bits at rates much faster than the qubit's rate of decoherence. Recently, fast gate operations have been demonstrated in logical spin qubits composed of two electron spins where the rapid exchange of the two electrons permits electrically controllable rotations around one axis of the qubit. However, universal control of the qubit requires arbitrary rotations around at least two axes. Here we show that by subjecting each electron spin to a magnetic field of different magnitude we achieve full quantum control of the two-electron logical spin qubit with nanosecond operation times. Using a single device, a magnetic field gradient of several hundred milliTesla is generated and sustained using dynamic nuclear polarization of the underlying Ga and As nuclei. Universal control of the two-electron qubit is then demonstrated using quantum state tomography. The presented technique provides the basis for single and potentially multiple qubit operations with gate times that approach the threshold required for quantum error correction.

cond-mat.mes-hall↗

arXiv2010

Local Compressibility Measurements of Correlated States in Suspended Bilayer Graphene

Bilayer graphene has attracted considerable interest due to the important role played by many-body effects, particularly at low energies. Here we report local compressibility measurements of a suspended graphene bilayer. We find that the energy gaps at filling factors v = 4 do not vanish at low fields, but instead merge into an incompressible region near the charge neutrality point at zero electric and magnetic field. These results indicate the existence of a zero-field ordered state and are consistent with the formation of either an anomalous quantum Hall state or a nematic phase with broken rotational symmetry. At higher fields, we measure the intrinsic energy gaps of broken-symmetry states at v = 0, 1 and 2, and find that they scale linearly with magnetic field, yet another manifestation of the strong Coulomb interactions in bilayer graphene.

cond-mat.mes-hall↗

arXiv2010

Long coherence of electron spins coupled to a nuclear spin bath

Qubits, the quantum mechanical bits required for quantum computing, must retain their fragile quantum states over long periods of time. In many types of electron spin qubits, the primary source of decoherence is the interaction between the electron spins and nuclear spins of the host lattice. For electrons in gate defined GaAs quantum dots, previous spin echo measurements have revealed coherence times of about 1 $μ$s at low magnetic fields below 100 mT. Here, we show that coherence in such devices can actually survive to much longer times, and provide a detailed understanding of the measured nuclear spin induced decoherence. At fields above a few hundred millitesla, the coherence time measured using a single-pulse spin echo extends to 30 $μ$s. At lower magnetic fields, the echo first collapses, but then revives at later times given by the period of the relative Larmor precession of different nuclear species. This behavior was recently predicted, and as we show can be quantitatively accounted for by a semi-classical model for the electron spin dynamics in the presence of a nuclear spin bath. Using a multiple-pulse Carr-Purcell-Meiboom-Gill echo sequence, the decoherence time can be extended to more than 200 $μ$s, which represents an improvement by two orders of magnitude compared to previous measurements. This demonstration of effective methods to mitigate nuclear spin induced decoherence puts the quantum error correction threshold within reach.

cond-mat.mes-hall↗

arXiv2010

Self-aligned nanoscale SQUID on a tip

A nanometer-sized superconducting quantum interference device (nanoSQUID) is fabricated on the apex of a sharp quartz tip and integrated into a scanning SQUID microscope. A simple self-aligned fabrication method results in nanoSQUIDs with diameters down to 100 nm with no lithographic processing. An aluminum nanoSQUID with an effective area of 0.034 $μ$m$^2$ displays flux sensitivity of 1.8$\cdot 10^{-6}$ $Φ_0/\mathrm{Hz}^{1/2} and operates in fields as high as 0.6 T. With projected spin sensitivity of 65 $μ_B/\mathrm{Hz}^{1/2}$ and high bandwidth, the SQUID on a tip is a highly promising probe for nanoscale magnetic imaging and spectroscopy.

cond-mat.supr-con↗

arXiv2009

Broken symmetry states and divergent resistance in suspended bilayer graphene

Graphene [1] and its bilayer have generated tremendous excitement in the physics community due to their unique electronic properties [2]. The intrinsic physics of these materials, however, is partially masked by disorder, which can arise from various sources such as ripples [3] or charged impurities [4]. Recent improvements in quality have been achieved by suspending graphene flakes [5,6], yielding samples with very high mobilities and little charge inhomogeneity. Here we report the fabrication of suspended bilayer graphene devices with very little disorder. We observe fully developed quantized Hall states at magnetic fields of 0.2 T, as well as broken symmetry states at intermediate filling factors $ν= 0$, $\pm 1$, $\pm 2$ and $\pm 3$. The devices exhibit extremely high resistance in the $ν= 0$ state that grows with magnetic field and scales as magnetic field divided by temperature. This resistance is predominantly affected by the perpendicular component of the applied field, indicating that the broken symmetry states arise from many-body interactions.

cond-mat.mes-hall↗

arXiv2008

Charge Fractionalization in nonchiral Luttinger systems

One-dimensional metals, such as quantum wires or carbon nanotubes, can carry charge in arbitrary units, smaller or larger than a single electron charge. However, according to Luttinger theory, which describes the low-energy excitations of such systems, when a single electron is injected by tunneling into the middle of such a wire, it will tend to break up into separate charge pulses, moving in opposite directions, which carry definite fractions $f$ and $(1-f)$ of the electron charge, determined by a parameter $g$ that measures the strength of charge interactions in the wire. (The injected electron will also produce a spin excitation, which will travel at a different velocity than the charge excitations.) Observing charge fractionalization physics in an experiment is a challenge in those (nonchiral) low-dimensional systems which are adiabatically coupled to Fermi liquid leads. We theoretically discuss a first important step towards the observation of charge fractionalization in quantum wires based on momentum-resolved tunneling and multi-terminal geometries, and explain the recent experimental results of H. Steinberg {\it et al.}, Nature Physics {\bf 4}, 116 (2008).

cond-mat.str-el↗

arXiv2005

Domain Patterns in the Microwave-Induced Zero-Resistance State

It has been proposed that the microwave-induced ``zero-resistance'' phenomenon, observed in a GaAs two-dimensional electron system at low temperatures in moderate magnetic fields, results from a state with multiple domains, in which a large local electric field $\bE(\br)$ is oriented in different directions. We explore here the questions of what may determine the domain arrangement in a given sample, what do the domains look like in representative cases, and what may be the consequences of domain-wall localization on the macroscopic dc conductance. We consider both effects of sample boundaries and effects of disorder, in a simple model, which has a constant Hall conductivity, and is characterized by a Lyapunov functional.

cond-mat.mes-hall↗

arXiv2005

Steady States of a Microwave Irradiated Quantum Hall Gas

We consider effects of a long-wavelength disorder potential on the Zero Conductance State (ZCS) of the microwave-irradiated 2D electron gas. Assuming a uniform Hall conductivity, we construct a Lyapunov functional and derive stability conditions on the domain structure of the photo-generated fields. We solve the resulting equations for a general one-dimensional and certain two-dimensional disorder potentials, and find non-zero conductances, photo-voltages, and circulating dissipative currents. In contrast, weak white noise disorder does not destroy the ZCS, but induces mesoscopic current fluctuations.

cond-mat.mes-hall↗

arXiv2004

Many-body dispersions in interacting ballistic quantum wires

We have measured the collective excitation spectrum of interacting electrons in one-dimension. The experiment consists of controlling the energy and momentum of electrons tunneling between two clean and closely situated, parallel quantum wires in a GaAs/AlGaAs heterostructure while measuring the resulting conductance. We measure excitation spectra that clearly deviate from the non-interacting spectrum, attesting to the importance of Coulomb interactions. Notable is an observed 30% enhancement of the velocity of the main excitation branch relative to non-interacting electrons with the same density. In short wires, finite size effects resulting from broken translational invariance are observed. Spin - charge separation is manifested through moire patterns, reflecting different spin and charge excitation velocities.

cond-mat.mes-hall↗

arXiv2003

Signatures of spin-charge separation in double--quantum-wire tunneling

We present evidence for spin-charge separation in the tunneling spectrum of a system consisting of two quantum wires connected by a long narrow tunnel junction at the edge of a GaAs/AlGaAs bilayer heterostructure. Multiple excitation velocities are detected in the system by tracing out electron spectral peaks in the conductance dependence on the applied voltage, governing the energy of tunneled electrons, and the magnetic field, governing the momentum shift along the wires. The boundaries of the wires are important and lead to a characteristic interference pattern in measurements on short junctions. We show that the experimentally observed modulation of the conductance oscillation amplitude as a function of the voltage bias can also be accounted for by spin-charge separation of the elementary excitations in the interacting wires.

cond-mat.mes-hall↗

arXiv2003

Interference and zero-bias anomaly in tunneling between Luttinger-liquid wires

We present theoretical calculations and experimental measurements which reveal the Luttinger-liquid (LL) nature of elementary excitations in a system consisting of two quantum wires connected by a long narrow tunnel junction at the edge of a GaAs/AlGaAs bilayer heterostructure. The boundaries of the wires are important and lead to a characteristic interference pattern in measurements on short junctions. We show that the experimentally observed modulation of the conductance oscillation amplitude as a function of the voltage bias can be accounted for by spin-charge separation of the elementary excitations in the interacting wires. Furthermore, boundaries affect the LL exponents of the voltage and temperature dependence of the tunneling conductance at low energies. We show that the measured temperature dependence of the conductance zero-bias dip as well as the voltage modulation of the conductance oscillation pattern can be used to extract the electron interaction parameters in the wires.

cond-mat.mes-hall↗

arXiv2002

Finite-size effects in tunneling between parallel quantum wires

We present theoretical calculations and experimental measurements which reveal finite-size effects in the tunneling between two parallel quantum wires, fabricated at the cleaved edge of a GaAs/AlGaAs bilayer heterostructure. Observed oscillations in the differential conductance, as a function of bias voltage and applied magnetic field, provide direct information on the shape of the confining potential. Superimposed modulations indicate the existence of two distinct excitation velocities, as expected from spin-charge separation.

cond-mat.mes-hall↗

arXiv2002

Fabrication of Nano-Scale Gaps in Integrated Circuits

Nano-size objects like metal clusters present an ideal system for the study of quantum phenomena and for constructing practical quantum devices. Integrating these small objects in a macroscopic circuit is, however, a difficult task. So far the nanoparticles have been contacted and addressed by highly sophisticated techniques which are not suitable for large scale integration in macroscopic circuits. We present an optical lithography method that allows for the fabrication of a network of electrodes which are separated by gaps of controlled nanometer size. The main idea is to control the gap size with subnanometer precision using a structure grown by molecular beam epitaxy.

cond-mat.mes-hall↗