43 research outputs found
Shot-noise evidence of fractional quasiparticle creation in a local fractional quantum Hall state
We have experimentally identified fractional quasiparticle creation in a
tunneling process through a local fractional quantum Hall (FQH) state. The
local FQH state is prepared in a low-density region near a quantum point
contact (QPC) in an integer quantum Hall (IQH) system. Shot-noise measurements
reveal a clear transition from elementary-charge tunneling at low bias to
fractional-charge tunneling at high bias. The fractional shot noise is
proportional to T1(1 ? T1) over a wide range of T1, where T1 is the
transmission probability of the IQH edge channel. This binomial distribution
indicates that fractional quasiparticles emerge from the IQH state to be
transmitted through the local FQH state. The study of this tunneling process
will enable us to elucidate the dynamics of Laughlin quasiparticles in FQH
systems.Comment: 5 pages, 5 figure
Fractionalized Wave Packets from an Artificial Tomonaga-Luttinger Liquid
The model of interacting fermion systems in one dimension known as
Tomonaga-Luttinger liquid (TLL) provides a simple and exactly solvable
theoretical framework, predicting various intriguing physical properties.
Evidence of TLL has been observed as power-law behavior in the electronic
transport and momentum-resolved spectroscopy on various types of
one-dimensional (1D) conductors. However, these measurements, which rely on dc
transport involving tunneling processes, cannot identify the eigenmodes of the
TLL, i.e., collective excitations characterized by non-trivial effective charge
e* and charge velocity v*. The elementary process of charge fractionalization,
a phenomenon predicted to occur at the junction of a TLL and non-interacting
leads, has not been observed. Here we report time-resolved transport
measurements on an artificial TLL comprised of coupled integer quantum Hall
edge channels, successfully identifying single charge fractionalization
processes. An electron wave packet with charge e incident from a
non-interacting region breaks up into several fractionalized charge wave
packets at the edges of the artificial TLL region, from which e* and v* can be
directly evaluated. These results are informative for elucidating the nature of
TLLs and low-energy excitations in the edge channels.Comment: Submitte
Two-step breakdown of a local v = 1 quantum Hall state
We report quantum Hall effect breakdown of a local filling factor v_local = 1
state formed in a bulk v_bulk = 2 system in an AlGaAs/GaAs heterostructure.
When a finite source-drain bias is applied across the local system, the
breakdown occurs in two steps. At low bias, quantized conductance through the
v_local = 1 system breaks down due to inter-edge electron tunneling. At high
bias, the incompressibility of the v_local = 1 system breaks down because the
spin gap closes. The two steps are resolved by combining measurements of
resistively detected nuclear magnetic resonance and shot noise, which allows
one to evaluate electron spin polarization in the local system and
spin-dependent charge transport through the system, respectively.Comment: 5 pages, 5 figure
Charge equilibration in integer and fractional quantum Hall edge channels in a generalized Hall-bar device
Charge equilibration between quantum-Hall edge states can be studied to
reveal geometric structure of edge channels not only in the integer quantum
Hall (IQH) regime but also in the fractional quantum Hall (FQH) regime
particularly for hole-conjugate states. Here we report on a systematic study of
charge equilibration in both IQH and FQH regimes by using a generalized Hall
bar, in which a quantum Hall state is nested in another quantum Hall state with
different Landau filling factors. This provides a feasible way to evaluate
equilibration in various conditions even in the presence of scattering in the
bulk region. The validity of the analysis is tested in the IQH regime by
confirming consistency with previous works. In the FQH regime, we find that the
equilibration length for counter-propagating = 1 and = -1/3 channels along a hole-conjugate state at Landau filling factor
= 2/3 is much shorter than that for co-propagating = 1 and
= 1/3 channels along a particle state at = 4/3. The
difference can be associated to the distinct geometric structures of the edge
channels. Our analysis with generalized Hall bar devices would be useful in
studying edge equilibration and edge structures.Comment: 10 pages, 6 figure
Distributed-element circuit model of edge magnetoplasmon transport
We report experimental and theoretical studies of edge magnetoplasmon (EMP)
transport in quantum Hall (QH) devices. We develop a model that allows us to
calculate the transport coefficients of EMPs in QH devices with various
geometries. In our model, a QH system is described as a chiral
distributed-element (CDE) circuit, where the effects of Coulomb interaction are
represented by an electrochemical capacitance distributed along unidirectional
transmission lines. We measure the EMP transport coefficients through single-
and coupled-edge channels, a quantum point contact, and single- and
double-cavity structures. These measured transmission spectra can be reproduced
well by simulations using the corresponding CDE circuits. By fitting the
experimental results with the simulations, we deduce the circuit parameters
that characterize the electrostatic environment around the edge channels in a
realistic QH system. The observed gate-voltage dependences of the EMP transport
properties in gate-defined structures are explained in terms of the gate tuning
of the circuit parameters in CDE circuits.Comment: 12 pages, 12 figure
On-chip coherent frequency-domain THz spectroscopy for electrical transport
We developed a coherent frequency-domain THz spectroscopic technique on a
coplanar waveguide in the ultrabroad frequency range from 200 MHz to 1.6 THz
based on continuous wave (CW) laser spectroscopy. Optical beating created by
mixing two frequency-tunable CW lasers is focused on photoconductive switches
to generate and detect high-frequency current in a THz circuit. In contrast to
time-domain spectroscopy, our frequency-domain spectroscopy enables
unprecedented frequency resolution of 10 MHz without using complex building
blocks of femtosecond laser optics. Furthermore, due to the coherent nature of
the photomixing technique, we are able to identify the origin of multiple
reflections in the time domain using the Hilbert analysis and inverse Fourier
transform. These results demonstrate that the advantages of on-chip coherent
frequency-domain spectroscopy, such as its broadband, frequency resolution,
usability, and time-domain accessibility, provide a unique capability for
measuring ultrafast electron transport in integrated THz circuits.Comment: 13 pages, 4 figure
Charge Fractionalization in Artificial Tomonaga-Luttinger Liquids with Controlled Interaction Strength
We investigate charge fractionalizations in artificial Tomonaga-Luttinger
liquids (TLLs) composed of two capacitively coupled quantum Hall edge channels
(ECs) in graphene. The interaction strength of the artificial TLLs can be
controlled through distance W between the ECs. We show that the
fractionalization ratio r and the TLL mode velocity v vary with W. The
experimentally obtained relation between v and r follows a unique function
predicted by the TLL theory. We also show that charged wavepackets are
reflected back and forth multiple times at both ends of the TLL region.Comment: to be published in Phys. Rev. B Rapid Communicatio
Coupling between Quantum Hall Edge Channels on Opposite Sides of a Hall Bar
We investigate the coupling between quantum Hall (QH) edge channels (ECs)
located at opposite sides of a 50-um-wide Hall bar by exciting a charged
wavepacket in one EC and detecting time-dependent current in the other EC. In a
QH state, the current shows a peak followed by a dip, demonstrating the
existence of capacitive coupling across the incompressible two-dimensional
electron system (2DES). The observed magnetic field dependence of the amplitude
and time delay of the current suggests that the capacitance is affected by the
presence of localized states. We also show that the dominant manner of the
coupling changes gradually as the system changes between the QH and non-QH
states
Cryogenic GaAs high-electron-mobility-transistor amplifier for current noise measurements
We show that a cryogenic amplifier composed of a homemade GaAs
high-electron-mobility transistor (HEMT) is suitable for current-noise
measurements in a mesoscopic device at dilution-refrigerator temperatures. The
lower noise characteristics of our homemade HEMT leads to a lower noise floor
in the experimental setup and enables more efficient current-noise measurement
than is available with a commercial HEMT. We present the dc transport
properties of the HEMT and the gain and noise characteristics of the amplifier.
With the amplifier employed for current-noise measurements in a quantum point
contact, we demonstrate the high resolution of the measurement setup by
comparing it with that of the conventional one using a commercial HEMT.Comment: 10 pages, 13 figure
Shot Noise Induced by Electron-nuclear Spin-flip Scattering in a Nonequilibrium Quantum Wire
We study the shot noise (nonequilibrium current fluctuation) associated with
dynamic nuclear polarization in a nonequilibrium quantum wire (QW) fabricated
in a two-dimensional electron gas. We observe that the spin-polarized
conductance quantization of the QW in the integer quantum Hall regime collapses
when the QW is voltage biased to be driven to nonequilibrium. By measuring the
shot noise, we prove that the spin polarization of electrons in the QW is
reduced to instead of unity as a result of electron-nuclear
spin-flip scattering. The result is supported by Knight shift measurements of
the QW using resistively detected NMR.Comment: 5 pages, 4 figure