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 $\delta \nu$ = 1 and $\delta \nu$ = -1/3 channels along a hole-conjugate state at Landau filling factor $\nu$ = 2/3 is much shorter than that for co-propagating $\delta \nu$ = 1 and $\delta \nu$ = 1/3 channels along a particle state at $\nu$ = 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

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

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

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

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

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

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 $\sim 0.7$ 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