The quantum transport of electrons in an artificial atom, such as a quantum dot (QD), is governed... more The quantum transport of electrons in an artificial atom, such as a quantum dot (QD), is governed by the Coulomb blockade (CB) effects, revealing the ground-state charge configuration of the electronic system under interplays of the on-site strong Coulomb interactions. In a coherently-coupled QD array, i.e. artificial molecules, the phenomenon of collective CB (CCB) was predicted by theoretical studies circa two decades ago but its evidence remains controversial. Here, we present direct evidence for the observation of CCB in a six-quantum-dot array (QDA) under high magnetic fields at 20 mK. The coherent inter-dot coupling is enhanced and mediated via the Quantum Hall edge states of the GaAs sample substrate. Two continuously fine-tuned gate voltages enable the quantum dot conductance spectrum to undergo a localization to delocalization transition process which manifests as an emergence and a collapse of CCB. The transition between these two distinct quantum phases is analogous to th...
The nanogap electrode is a good experimental tool to study low-dimensional materials, such as sin... more The nanogap electrode is a good experimental tool to study low-dimensional materials, such as single molecules (0D), carbon nanotubes (1D), and graphene (2D). By using nanogap electrode as an antenna to focus terahertz (THz) radiation onto the sample, we have observed ultrafast dynamic processes of molecules/atoms (e.g., molecular vibration) and electrons (e.g., photon-assisted tunneling, intersublevel transitions) at a sub-ps time scale. The magnitude of the THz electric fields in the nanogap determines what kind of mechanism takes place in the THz-induced single electron transport in the nanogap electrodes. However, the exact magnitude of the THz electric field is difficult to measure. Therefore, the purpose of this work is to develop a technique to exactly determine the THz electric fields within the nm-scale gap of the electrodes. In this work, we have successfully obtained the THz voltage by measuring THz induced photocurrent in a single metal nanoparticle transistor. We used a...
We demonstrate a controllable p−n junction in a three-dimensional Dirac semimetal (DSM) Cd3As2 na... more We demonstrate a controllable p−n junction in a three-dimensional Dirac semimetal (DSM) Cd3As2 nanowire with two recessed bottom gates. The device exhibits four different conductance regimes with gate voltages, the unipolar (n−n and p−p) regime and the bipolar (n−p and n−p) one, where p−n junctions are formed. The conductance in the p−n junction regime decreases drastically when a magnetic field is applied perpendicular to the nanowire, which is due to the suppression of Klein tunneling. In this regime, the device shows quantum dot behavior. On the other hand, clear conductance plateaus are observed in the n−n regime likely owing to the cyclotron motion of carriers at high magnetic fields. Our experiment shows that the ambipolar tunability of DSM nanowires can enable the realization of quantum devices based on quantum dots and electron optics.
We implement a logic switch by using a graphene acoustoelectric transducer at room temperature. W... more We implement a logic switch by using a graphene acoustoelectric transducer at room temperature. We operate two pairs of inter-digital transducers (IDTs) to launch surface acoustic waves (SAWs) on a LiNbO3 substrate and utilize graphene as a channel material to sustain acoustoelectric current Iae induced by SAWs. By cooperatively tuning the input power on the IDTs, we can manipulate the propagation direction of Iae such that the measured Iae can be deliberately controlled to be positive, negative, or even zero. We define the zero-crossing Iae as $${I}_{ae}^{off}$$ I a e o f f , and then demonstrate that Iae can be switched with a ratio $${I}_{ae}^{on}/{I}_{ae}^{off}\, \sim \,{10}^{4}$$ I a e o n / I a e o f f ~ 10 4 at a rate up to few tens kHz. Our device with an accessible operation scheme provides a means to convert incoming acoustic waves modulated by digitized data sequence onto electric signals with frequency band suitable for digital audio modulation. Consequently, it could po...
ABSTRACT We have developed a highly tunable, narrow band far-infrared (FIR) photodetector which u... more ABSTRACT We have developed a highly tunable, narrow band far-infrared (FIR) photodetector which utilizes the characteristic merits of graphene and two-dimensional electron gas (2DEG) in GaAs/AlxGa1xAs heterostructure in the Quantum Hall states (QHS). The heterostructure surface is covered with chemical vapor-deposited graphene, which functions as a transparent top-gate to vary the electron density of the 2DEG. FIR response observed in the vicinity of integer QH regime can be effectively tuned in a wide range of 27 - 102 cm-1 with a bias voltage less than -1 V. In addition, we have found the presence of graphene can genuinely modulate the photoresponse. Our results demonstrate a promising direction for realizing a tunable long-wavelength FIR detector using QHS in GaAs 2DEG/graphene composite material.
Quarter-wave operation or a phase shift of more than =2, which is approximately ten times greater... more Quarter-wave operation or a phase shift of more than =2, which is approximately ten times greater than that reported in previous works using liquid crystals (LCs) and graphene electrodes, was demonstrated. The device is transparent to the terahertz (THz) wave, and the driving voltage required was as low as approximately 2.2 V (rms), which is also unprecedented. Experimental results supported a theoretical formalism adapted for LC cells with THz wavelength-scale thickness. The scattering rate, DC mobility, and carrier mean free path of bilayer graphene were also determined using THz spectroscopic techniques; the parameters were inferior to those of monolayer graphene. This observation can be attributed to the higher density of charged impurities in the bilayer graphene. The device performances of LC phase shifters using monolayer and bilayer graphene as electrodes were essentially identical.
We investigate the acoustoelectric properties of graphene and extract its acoustoelectric attenua... more We investigate the acoustoelectric properties of graphene and extract its acoustoelectric attenuation C as a function of the carrier density n, tuned via ionic liquid gating. Acoustoelectric effects in graphene are induced by launching surface acoustic waves (SAWs) on a piezoelectric LiNbO 3 substrate. We measure the acoustoelectric current I ae through graphene and extract the SAW attenuation factor C as a function of n. The magnitude of I ae increases with decreasing n when the n is far from the charge neutral point (CNP). When n is tuned across the CNP, I ae first exhibits a local maximum, vanishes at the CNP, and then changes sign in accordance with the associated change in the carrier polarity. By contrast, C monotonically increases with decreasing n and reaches a maximum at the CNP. The extracted values of C, calibrated at the central frequency of 189 MHz, vary from $0.4 m À1 to 6.8 m À1 , much smaller than the values for known twodimensional systems. Data analysis suggests that the evolution of I ae and C with n manifests the electronic states of graphene. Our experimental findings provide insightful information for developing innovative graphene-based devices. Published by AIP Publishing.
We have constructed a scanning probe microscope for magnetic imaging, which can function as a sca... more We have constructed a scanning probe microscope for magnetic imaging, which can function as a scanning Hall probe microscope (SHPM) and as a scanning SQUID microscope (SSM). The scanning scheme, applicable to SHPM and SSM, consists of a mechanical positioning (sub) micron-XY stage and a flexible direct contact to the sample without a feedback control system for the Z-axis. With the interchangeable capability of operating two distinct scanning modes, our microscope can incorporate the advantageous functionalities of the SHPM and SSM with large scan range up to millimeter, high spatial resolution (⩽4 μm), and high field sensitivity in a wide range of temperature (4.2 K-300 K) and magnetic field (10(-7) T-1 T). To demonstrate the capabilities of the system, we present magnetic images scanned with SHPM and SSM, including a RbFeB magnet and a nickel grid pattern at room temperature, surface magnetic domain structures of a La(2/3)Ca(1/3)MnO3 thin film at 77 K, and superconducting vortices...
We investigate the effects of defect scatterings on the electric transport properties of chemical... more We investigate the effects of defect scatterings on the electric transport properties of chemical vapor deposited (CVD) graphene by measuring the carrier density dependence of the magneto-conductivity. To clarify the dominant scattering mechanism, we perform extensive measurements on large-area samples with different mobility to exclude the edge effect. We analyze our data with the major scattering mechanisms such as short-range static scatters, short-range screened Coulomb disorders, and weak-localization (WL). We establish that the charged impurities are the predominant scatters because there is a strong correlation between the mobility and the charge impurity density. Near the charge neutral point (CNP), the electron-hole puddles that are induced by the charged impurities enhance the inter-valley scattering, which is favorable for WL observations. Away from the CNP, the charged-impurity-induced scattering is weak because of the effective screening by the charge carriers. As a result, the local static structural defects govern the charge transport. Our findings provide compelling evidence for understanding the scattering mechanisms in graphene and pave the way for the improvement of fabrication techniques to achieve high-quality CVD graphene.
We have investigated the transport and noise properties of a micron-sized Hall probe, fabricated ... more We have investigated the transport and noise properties of a micron-sized Hall probe, fabricated on chemical vapor deposited (CVD) graphene, from 300 K to 4.2 K. The field sensitivity of the Hall probe was tunable within ~0.031-0.12 Omega/G, while the field resolution could reach ~0.43-0.09 G/Hz1/2 at room temperature. The characteristics of graphene Hall probes (GHPs) were found to be comparable to present Hall sensors. Our results indicate that the fundamental limitation of the field sensitivity and the field resolution are respectively restricted by intrinsic and extrinsic defects. Our study paves the way for the use of CVD GHPs for scanning Hall probe with high field sensitivity and submicron spatial resolution at room temperature.
We have developed a hybrid quantum Hall midinfrared (QHMIR)-quantum Hall far-infrared (QHFIR) pho... more We have developed a hybrid quantum Hall midinfrared (QHMIR)-quantum Hall far-infrared (QHFIR) photodetector by the use of graphene-GaAs=ðAl; GaÞAs-layered composite material. Both MIR and FIR photoresistance are observed in a single chip by utilizing cyclotron resonance in the quantum Hall regimes of graphene and two-dimensional electron gas (2DEG) in GaAs=ðAl; GaÞAs heterostructure, respectively. By cooperatively operating 2DEG as a back-gate electrode to change the carrier density of graphene or graphene as a top-gate electrode to modulate the carrier density of 2DEG with an applied gate voltage less than 1 V and applying the magnetic field to tune cyclotron resonance, we achieve a wide frequency selectivity, covering 640-790 cm −1 for the graphene-QHMIR detector and 24-89 cm −1 for the 2DEG-QHFIR detector. Moreover, our design integrates a log-periodic antenna with the detector to minimize the device size, while preserving high sensitivity. Our results pave the way for implementing a highly tunable MIR-to-FIR photodetector and a dual-band (MIR-FIR) imaging array.
The quantum transport of electrons in an artificial atom, such as a quantum dot (QD), is governed... more The quantum transport of electrons in an artificial atom, such as a quantum dot (QD), is governed by the Coulomb blockade (CB) effects, revealing the ground-state charge configuration of the electronic system under interplays of the on-site strong Coulomb interactions. In a coherently-coupled QD array, i.e. artificial molecules, the phenomenon of collective CB (CCB) was predicted by theoretical studies circa two decades ago but its evidence remains controversial. Here, we present direct evidence for the observation of CCB in a six-quantum-dot array (QDA) under high magnetic fields at 20 mK. The coherent inter-dot coupling is enhanced and mediated via the Quantum Hall edge states of the GaAs sample substrate. Two continuously fine-tuned gate voltages enable the quantum dot conductance spectrum to undergo a localization to delocalization transition process which manifests as an emergence and a collapse of CCB. The transition between these two distinct quantum phases is analogous to th...
The nanogap electrode is a good experimental tool to study low-dimensional materials, such as sin... more The nanogap electrode is a good experimental tool to study low-dimensional materials, such as single molecules (0D), carbon nanotubes (1D), and graphene (2D). By using nanogap electrode as an antenna to focus terahertz (THz) radiation onto the sample, we have observed ultrafast dynamic processes of molecules/atoms (e.g., molecular vibration) and electrons (e.g., photon-assisted tunneling, intersublevel transitions) at a sub-ps time scale. The magnitude of the THz electric fields in the nanogap determines what kind of mechanism takes place in the THz-induced single electron transport in the nanogap electrodes. However, the exact magnitude of the THz electric field is difficult to measure. Therefore, the purpose of this work is to develop a technique to exactly determine the THz electric fields within the nm-scale gap of the electrodes. In this work, we have successfully obtained the THz voltage by measuring THz induced photocurrent in a single metal nanoparticle transistor. We used a...
We demonstrate a controllable p−n junction in a three-dimensional Dirac semimetal (DSM) Cd3As2 na... more We demonstrate a controllable p−n junction in a three-dimensional Dirac semimetal (DSM) Cd3As2 nanowire with two recessed bottom gates. The device exhibits four different conductance regimes with gate voltages, the unipolar (n−n and p−p) regime and the bipolar (n−p and n−p) one, where p−n junctions are formed. The conductance in the p−n junction regime decreases drastically when a magnetic field is applied perpendicular to the nanowire, which is due to the suppression of Klein tunneling. In this regime, the device shows quantum dot behavior. On the other hand, clear conductance plateaus are observed in the n−n regime likely owing to the cyclotron motion of carriers at high magnetic fields. Our experiment shows that the ambipolar tunability of DSM nanowires can enable the realization of quantum devices based on quantum dots and electron optics.
We implement a logic switch by using a graphene acoustoelectric transducer at room temperature. W... more We implement a logic switch by using a graphene acoustoelectric transducer at room temperature. We operate two pairs of inter-digital transducers (IDTs) to launch surface acoustic waves (SAWs) on a LiNbO3 substrate and utilize graphene as a channel material to sustain acoustoelectric current Iae induced by SAWs. By cooperatively tuning the input power on the IDTs, we can manipulate the propagation direction of Iae such that the measured Iae can be deliberately controlled to be positive, negative, or even zero. We define the zero-crossing Iae as $${I}_{ae}^{off}$$ I a e o f f , and then demonstrate that Iae can be switched with a ratio $${I}_{ae}^{on}/{I}_{ae}^{off}\, \sim \,{10}^{4}$$ I a e o n / I a e o f f ~ 10 4 at a rate up to few tens kHz. Our device with an accessible operation scheme provides a means to convert incoming acoustic waves modulated by digitized data sequence onto electric signals with frequency band suitable for digital audio modulation. Consequently, it could po...
ABSTRACT We have developed a highly tunable, narrow band far-infrared (FIR) photodetector which u... more ABSTRACT We have developed a highly tunable, narrow band far-infrared (FIR) photodetector which utilizes the characteristic merits of graphene and two-dimensional electron gas (2DEG) in GaAs/AlxGa1xAs heterostructure in the Quantum Hall states (QHS). The heterostructure surface is covered with chemical vapor-deposited graphene, which functions as a transparent top-gate to vary the electron density of the 2DEG. FIR response observed in the vicinity of integer QH regime can be effectively tuned in a wide range of 27 - 102 cm-1 with a bias voltage less than -1 V. In addition, we have found the presence of graphene can genuinely modulate the photoresponse. Our results demonstrate a promising direction for realizing a tunable long-wavelength FIR detector using QHS in GaAs 2DEG/graphene composite material.
Quarter-wave operation or a phase shift of more than =2, which is approximately ten times greater... more Quarter-wave operation or a phase shift of more than =2, which is approximately ten times greater than that reported in previous works using liquid crystals (LCs) and graphene electrodes, was demonstrated. The device is transparent to the terahertz (THz) wave, and the driving voltage required was as low as approximately 2.2 V (rms), which is also unprecedented. Experimental results supported a theoretical formalism adapted for LC cells with THz wavelength-scale thickness. The scattering rate, DC mobility, and carrier mean free path of bilayer graphene were also determined using THz spectroscopic techniques; the parameters were inferior to those of monolayer graphene. This observation can be attributed to the higher density of charged impurities in the bilayer graphene. The device performances of LC phase shifters using monolayer and bilayer graphene as electrodes were essentially identical.
We investigate the acoustoelectric properties of graphene and extract its acoustoelectric attenua... more We investigate the acoustoelectric properties of graphene and extract its acoustoelectric attenuation C as a function of the carrier density n, tuned via ionic liquid gating. Acoustoelectric effects in graphene are induced by launching surface acoustic waves (SAWs) on a piezoelectric LiNbO 3 substrate. We measure the acoustoelectric current I ae through graphene and extract the SAW attenuation factor C as a function of n. The magnitude of I ae increases with decreasing n when the n is far from the charge neutral point (CNP). When n is tuned across the CNP, I ae first exhibits a local maximum, vanishes at the CNP, and then changes sign in accordance with the associated change in the carrier polarity. By contrast, C monotonically increases with decreasing n and reaches a maximum at the CNP. The extracted values of C, calibrated at the central frequency of 189 MHz, vary from $0.4 m À1 to 6.8 m À1 , much smaller than the values for known twodimensional systems. Data analysis suggests that the evolution of I ae and C with n manifests the electronic states of graphene. Our experimental findings provide insightful information for developing innovative graphene-based devices. Published by AIP Publishing.
We have constructed a scanning probe microscope for magnetic imaging, which can function as a sca... more We have constructed a scanning probe microscope for magnetic imaging, which can function as a scanning Hall probe microscope (SHPM) and as a scanning SQUID microscope (SSM). The scanning scheme, applicable to SHPM and SSM, consists of a mechanical positioning (sub) micron-XY stage and a flexible direct contact to the sample without a feedback control system for the Z-axis. With the interchangeable capability of operating two distinct scanning modes, our microscope can incorporate the advantageous functionalities of the SHPM and SSM with large scan range up to millimeter, high spatial resolution (⩽4 μm), and high field sensitivity in a wide range of temperature (4.2 K-300 K) and magnetic field (10(-7) T-1 T). To demonstrate the capabilities of the system, we present magnetic images scanned with SHPM and SSM, including a RbFeB magnet and a nickel grid pattern at room temperature, surface magnetic domain structures of a La(2/3)Ca(1/3)MnO3 thin film at 77 K, and superconducting vortices...
We investigate the effects of defect scatterings on the electric transport properties of chemical... more We investigate the effects of defect scatterings on the electric transport properties of chemical vapor deposited (CVD) graphene by measuring the carrier density dependence of the magneto-conductivity. To clarify the dominant scattering mechanism, we perform extensive measurements on large-area samples with different mobility to exclude the edge effect. We analyze our data with the major scattering mechanisms such as short-range static scatters, short-range screened Coulomb disorders, and weak-localization (WL). We establish that the charged impurities are the predominant scatters because there is a strong correlation between the mobility and the charge impurity density. Near the charge neutral point (CNP), the electron-hole puddles that are induced by the charged impurities enhance the inter-valley scattering, which is favorable for WL observations. Away from the CNP, the charged-impurity-induced scattering is weak because of the effective screening by the charge carriers. As a result, the local static structural defects govern the charge transport. Our findings provide compelling evidence for understanding the scattering mechanisms in graphene and pave the way for the improvement of fabrication techniques to achieve high-quality CVD graphene.
We have investigated the transport and noise properties of a micron-sized Hall probe, fabricated ... more We have investigated the transport and noise properties of a micron-sized Hall probe, fabricated on chemical vapor deposited (CVD) graphene, from 300 K to 4.2 K. The field sensitivity of the Hall probe was tunable within ~0.031-0.12 Omega/G, while the field resolution could reach ~0.43-0.09 G/Hz1/2 at room temperature. The characteristics of graphene Hall probes (GHPs) were found to be comparable to present Hall sensors. Our results indicate that the fundamental limitation of the field sensitivity and the field resolution are respectively restricted by intrinsic and extrinsic defects. Our study paves the way for the use of CVD GHPs for scanning Hall probe with high field sensitivity and submicron spatial resolution at room temperature.
We have developed a hybrid quantum Hall midinfrared (QHMIR)-quantum Hall far-infrared (QHFIR) pho... more We have developed a hybrid quantum Hall midinfrared (QHMIR)-quantum Hall far-infrared (QHFIR) photodetector by the use of graphene-GaAs=ðAl; GaÞAs-layered composite material. Both MIR and FIR photoresistance are observed in a single chip by utilizing cyclotron resonance in the quantum Hall regimes of graphene and two-dimensional electron gas (2DEG) in GaAs=ðAl; GaÞAs heterostructure, respectively. By cooperatively operating 2DEG as a back-gate electrode to change the carrier density of graphene or graphene as a top-gate electrode to modulate the carrier density of 2DEG with an applied gate voltage less than 1 V and applying the magnetic field to tune cyclotron resonance, we achieve a wide frequency selectivity, covering 640-790 cm −1 for the graphene-QHMIR detector and 24-89 cm −1 for the 2DEG-QHFIR detector. Moreover, our design integrates a log-periodic antenna with the detector to minimize the device size, while preserving high sensitivity. Our results pave the way for implementing a highly tunable MIR-to-FIR photodetector and a dual-band (MIR-FIR) imaging array.
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Papers by Chiu-Chun Tang