Low-temperature Quantum Transport Research Articles

Tuning magnetoresistance of chromium chloride tunnel junction through the interface and multi-field effect

<sec>Magnetic tunnel junctions (MTJs) serve as essential platforms for investigating spin transport properties, magnetic phase transitions, and anisotropy in magnetic materials. Recently two-dimensional van der Waals antiferromagnetic insulators like chromium chloride (CrCl<sub>3</sub>) or chromium iodide (CrI<sub>3</sub>) have been used to develop spin-filtering magnetic tunnel junctions (sf-MTJs), improving the device performance for material property exploration and spintronic applications. However, it is crucial to recognize that the spin-filtering effect is not the sole determining factor of tunneling magnetoresistance (TMR) in these junctions; the interface magnetic exchange interactions and adjustable electrode density of states (DOS) fluctuations, response to applied electric or magnetic fields, can also influence the tunneling current.</sec><sec>In this study, we fabricate MTJ devices by using mechanically-exfoliated few-layer CrCl<sub>3</sub> as the tunnel barrier and few-layer graphene (FLG) as electrodes through dry transfer technique. Conducting low-temperature quantum transport measurements, we observe unconventional TMR behaviors, including bias-voltage-dependent TMR, oscillatory tunneling current under high magnetic fields, and tunable tunneling current via gate voltage.</sec><sec>A qualitative model of elastic tunneling current is employed to analyze the spin and band characteristics of the MTJ device. The observed bias-voltage-dependent TMR is attributed to the changes in the tunneling mechanism due to magnetic proximity effect, which induces magnetization in the FLG electrode near the FLG/CrCl<sub>3</sub> interface. The antiparallel alignment of polarized spin to CrCl<sub>3</sub>’s magnetization results in injected charge carriers facing a higher tunnel barrier, leading to negative TMR at lower bias voltages. As the bias voltage increases, the magnetic proximity effect lessens, and the device reverts to its conventional spin-filtering functionality. The oscillatory tunneling current is explained by the graphene electrode’s quantum oscillatory density of states behavior under vertical magnetic fields, which can be controlled by the applied gate voltage.</sec><sec>This study contributes to the understanding of previously unexplored TMR phenomena in two-dimensional MTJs, deepening our insights into carrier transport properties in these heterostructures and broadening avenues for investigating the physical properties of two-dimensional magnetic materials and their spintronic applications.</sec>

Band-Gap-Dependent Electronic Compressibility of Carbon Nanotubes in the Wigner Crystal Regime.

Electronic compressibility, the second derivative of ground-state energy with respect to total electron number, is a measurable quantity that reveals the interaction strength of a system and can be used to characterize the orderly crystalline lattice of electrons known as the Wigner crystal. Here, we measure the electronic compressibility of individual suspended ultraclean carbon nanotubes in the low-density Wigner crystal regime. Using low-temperature quantum transport measurements, we determine the compressibility as a function of carrier number in nanotubes with varying band gaps. We observe two qualitatively different trends in compressibility versus carrier number, both of which can be explained using a theoretical model of a Wigner crystal that accounts for both the band gap and the confining potential experienced by charge carriers. We extract the interaction strength as a function of carrier number for individual nanotubes and show that the compressibility can be used to distinguish between strongly and weakly interacting regimes.

From Diffusive to Ballistic Transport in Etched Graphene Constrictions and Nanoribbons

AbstractGraphene nanoribbons and constrictions are envisaged as fundamental components of future carbon‐based nanoelectronic and spintronic devices. At nanoscale, electronic effects in these devices depend heavily on the dimensions of the active channel and the nature of edges. Hence, controlling both these parameters is crucial to understand the physics in such systems. This review is about the recent progress in the fabrication of graphene nanoribbons and constrictions in terms of low temperature quantum transport. In particular, recent advancements using encapsulated graphene allowing for quantized conductance and future experiments towards exploring spin effects in these devices are presented. The influence of charge carrier inhomogeneity and the important length scales which play a crucial role for transport in high quality samples are also discussed.

Influence of environment and geometry on measured quantum coherence in InGaAs mesoscopic wires and stadia

Lithographically fabricated mesoscopic wires and stadia, coupled to a decohering classical environment by two side-wires, were studied using low-temperature quantum transport to obtain quantum phase coherence lengths. Longer phase coherence lengths were observed in longer wires, an effect due to environmental coupling decoherence being mitigated by averaging over longer wire lengths. In stadia the dominant decoherence mechanism appears as device-device, here stadium-wire, coupling decoherence rather than environmental coupling decoherence. Consistency between the experimental results and a decoherence mechanism due to stadium-wire coupling is found in the relation between lower decoherence rate and relative geometrical similarity between the stadia and their connecting wires.

Low-temperature quantum transport in CVD-grown single crystal graphene

Chemical vapor deposition (CVD) is typically used for large-scale graphene synthesis for practical applications. However, the inferior electronic properties of CVD graphene are one of the key problems to be solved. Therefore, we present a detailed study on the electronic properties of high-quality single-crystal monolayer graphene. The graphene is grown via CVD on copper, by using a cold-wall reactor, and then transferred to Si/SiO2. Our low-temperature magneto-transport data demonstrate that the characteristics of the single-crystal CVD graphene samples are superior to those of polycrystalline graphene and have a quality which is comparable to that of exfoliated graphene on Si/SiO2. The Dirac point in our best samples occurs at back-gate voltages lower than 10 V, and a maximum mobility of 11,000 cm2/(V·s) is attained. More than 12 flat and discernible half-integer quantum Hall plateaus occur under a high magnetic field on both the electron and hole sides of the Dirac point. At a low magnetic field, the magnetoresistance exhibits a weak localization peak. Using the theory of McCann et al., we obtain inelastic scattering lengths of >1 µm, even at the charge neutrality point of the samples.

Interaction effects on a Majorana zero mode leaking into a quantum dot

We have recently shown [Phys. Rev. B 89, 165314 (2014)] that a noninteracting quantum dot coupled to a one-dimensional topological superconductor and to normal leads can sustain a Majorana mode even when the dot is expected to be empty, i.e., when the dot energy level is far above the Fermi level of the leads. This is due to the Majorana bound state of the wire leaking into the quantum dot. Here we extend this previous work by investigating the low-temperature quantum transport through an interacting quantum dot connected to source and drain leads and side coupled to a topological wire. We explore the signatures of a Majorana zero mode leaking into the quantum dot for a wide range of dot parameters, using a recursive Green's function approach. We then study the Kondo regime using numerical renormalization group calculations. We observe the interplay between the Majorana mode and the Kondo effect for different dot-wire coupling strengths, gate voltages, and Zeeman fields. Our results show that a ``0.5'' conductance signature appears in the dot despite the interplay between the leaked Majorana mode and the Kondo effect. This robust feature persists for a wide range of dot parameters, even when the Kondo correlations are suppressed by Zeeman fields and/or gate voltages. The Kondo effect, on the other hand, is suppressed by both Zeeman fields and gate voltages. We show that the zero-bias conductance as a function of the magnetic field follows a well-known universality curve. This can be measured experimentally, and we propose that the universal conductance drop followed by a persistent conductance of $0.5{e}^{2}/h$ is evidence of the presence of Majorana-Kondo physics. These results confirm that this ``0.5'' Majorana signature in the dot remains even in the presence of the Kondo effect.

Silver-epoxy microwave filters and thermalizers for millikelvin experiments

We present silver-epoxy filters combining excellent microwave attenuation with efficient wire thermalization, suitable for low temperature quantum transport experiments. Upon minimizing parasitic capacitances, the attenuation reaches ≥100 dB above ≈150 MHz and—when capacitors are added—already above ≈30 MHz. We measure the device electron temperature with a GaAs quantum dot and demonstrate excellent filter performance. Upon improving the sample holder and adding a second filtering stage, we obtain electron temperatures as low as 7.5 ± 0.2 mK in metallic Coulomb blockade thermometers.

Low-Temperature Quantum Transport Characteristics in Single n-Channel Junctionless Nanowire Transistors

A single n-channel junctionless nanowire transistor is fabricated and characterized for low-temperature quantum transport behavior. Transfer characteristics exhibit current oscillations below flat-band voltage $(V_{\rm FB})$ up to temperature 75 K, possibly due to cotunneling through unintentional quantum dots. Furthermore, regular current steps are observed above $V_{\rm FB}$ , that is, each current plateau corresponds to a fully populated subband. Experimental result of transconductance peaks indicates that the subband energy spacing in the 1-D channel agrees well with theoretical prediction.

Inelastic low-temperature transport through a quantum dot with a Mn ion

Using the nonequilibrium Hubbard operator Green's function technique, we study the inelastic low-temperature quantum transport through an artificial single-molecule magnet coupled to a single phonon mode. For a minimal model based on CdTe quantum dot doped with a single Mn2+ ion (S=5/2), the calculated results show that in the presence of hole–phonon coupling, in addition to main Kondo-like peaks associated with (2S+1) sublevels of spin pair states, satellite Kondo-like peaks originating from emitting phonons appear in the local density of states and differential conductance. Moreover, the number of these phonon-induced Kondo-like peaks depends on the parity of the local large spin, i.e., S=integer or half-integer. It is expected that the intrinsic properties of artificial single-molecule magnets can be obtained by detecting these transport characteristics.

Comparison of nickel silicide and aluminium ohmic contact metallizations for low-temperature quantum transport measurements

We examine nickel silicide as a viable ohmic contact metallization for low-temperature, low-magnetic-field transport measurements of atomic-scale devices in silicon. In particular, we compare a nickel silicide metallization with aluminium, a common ohmic contact for silicon devices. Nickel silicide can be formed at the low temperatures (<400°C) required for maintaining atomic precision placement in donor-based devices, and it avoids the complications found with aluminium contacts which become superconducting at cryogenic measurement temperatures. Importantly, we show that the use of nickel silicide as an ohmic contact at low temperatures does not affect the thermal equilibration of carriers nor contribute to hysteresis in a magnetic field.

Towards Graphene GHz/THz Nanosensor

A quantum dot (QD) sensing device, fabricated from nanoscaled carbon material has been studied using of a bilayer graphene field effect transistor in order to enable its application to the detection of microwave (GHz) and/or terahertz (THz) radiation. Recently, it has been found that there exist several common features in low temperature quantum transport, found in experimental results of conductance quantization in a semiconductor QDs and the magnetoresistance of a graphene QDs. The applicability of a graphene field effect transistor at the GHz/THz range is discussed in terms of the microwave transconductance characteristics up to 40 GHz.

Towards Graphene GHz/THz Nanosensor

A quantum dot (QD) sensing device, fabricated from nanoscaled carbon material has been studied using of a bilayer graphene field effect transistor in order to enable its application to the detection of microwave (GHz) and/or terahertz (THz) radiation. Recently, it has been found that there exist several common features in low temperature quantum transport, found in experimental results of conductance quantization in a semiconductor QDs and the magnetoresistance of a graphene QDs. The applicability of a graphene field effect transistor at the GHz/THz range is discussed in terms of the microwave transconductance characteristics up to 40 GHz.

Universal conductance fluctuations and localization effects in InN nanowires connected in parallel

The low-temperature quantum transport of InN nanowires grown by plasma-assisted molecular beam epitaxy is investigated. Two sets of nanowires with diameters of 100 and 45 nm originating from two different growth runs are studied. Magnetic-field-dependent as well as gate-dependent measurements of universal conductance fluctuations are performed to gain information on the phase-coherence in the electron transport. By analyzing the correlation field and the average fluctuation amplitude a phase-coherence length of several hundred nanometers is extracted for both sets of nanowires at temperatures below 1 K. Conductance fluctuations are also observed when the Fermi wavelength is varied by applying a bias voltage to a back-gate. The results on the electron phase-coherence obtained from the gate-dependent measurements are consistent with the findings from the magnetic field dependent measurements. A considerable damping of the fluctuation amplitude by ensemble averaging is achieved by connecting nanowires in parallel. The suppression of the fluctuation amplitude is studied systematically by measuring samples with different numbers of nanowires. By utilizing the damping of the conductance fluctuations by connecting nanowires in parallel in combination with an averaging over the gate voltage, weak localization effects are resolved. For both sets of nanowires a clear evidence of the weak antilocalization is found, which indicates the presence of spin-orbit coupling. For the spin-orbit scattering length lso values in the order of 100 nm are extracted.

Spin-orbit coupling and phase-coherent transport in InN nanowires

The low-temperature quantum transport properties of gated InN nanowires were investigated. Magnetic-field-dependent as well as gate-dependent measurements of universal conductance fluctuations were performed to gain information on the phase coherence in the electron transport. We found a pronounced decrease in the variance of the conductance by about a factor of 2 in gate-dependent fluctuation measurements if a magnetic field is applied. This effect is explained by the suppression of the Cooperon channel of the electron correlation contributing to the conductance fluctuations. Despite the fact that the diameter of the nanowire is less than 100 nm a clear weak antilocalization effect is found in the averaged magnetoconductance being in strong contrast to the suppression of weak antilocalization for narrow quantum wires based on planar two-dimensional electron gases. The unexpected robustness of the weak antilocalization effect observed here is attributed to the tubular topology of the surface electron gas in InN nanowires.

Quantum transport in neutron-irradiated modulation-doped heterojunctions. I. Fast neutrons.

We have investigated the characteristics of low-temperature quantum transport in Al/sub x/Ga/sub 1-//sub x/As/GaAs modulation-doped heterojunctions irradiated by fast neutrons of about 14 MeV energy. The concentration and the mobility of the two-dimensional electron gas (2D EG) under low magnetic fields decrease with increase in the concentrations of scatterers, such as ionized impurities, lattice defects, and interface roughness. On the other hand, under strong magnetic fields, the Hall plateau broadening associated with the Landau localized states, and the Shubnikov--de Hass (SdH) oscillation enhancement associated with the Landau extended states, increase markedly after fast-neutron irradiation.

Quantum transport in neutron-irradiated modulation-doped heterojunctions. II. Thermal neutrons.

We have investigated the characteristics of low-temperature quantum transport in Al/sub x/Ga/sub 1-//sub x/As/GaAs modulation-doped heterojunctions irradiated by fast neutrons of about 14 MeV energy. The concentration and the mobility of the two-dimensional electron gas (2D EG) under low magnetic fields decrease with increase in the concentrations of scatterers, such as ionized impurities, lattice defects, and interface roughness. On the other hand, under strong magnetic fields, the Hall plateau broadening associated with the Landau localized states, and the Shubnikov--de Hass (SdH) oscillation enhancement associated with the Landau extended states, increase markedly after fast-neutron irradiation.

Conductance Fluctuations in Disordered Sub-Micron Wires and Rings

A review of the many of the unexpected experimental results obtained over the last three years on the low temperature quantum transport properties of sub-micron normal metal samples is presented. Measurements can now be made on devices where the lithographically patterned distances between measurements probes is much smaller the the characteristic distances over which the conduction electrons maintain their quantum mechanical phase coherence. This in turn has permitted the discoveries of many new quantum interference effects such as the Aharonov-Bohm effect in highly disordered conductors, universal conductance fluctuations, and non-local quantum effects. The ability to vary both the temperature and sample size has yielded information on the effects of averaging these quantum effects over uncorrelated regions and has shown us to some extent how the transition from quantum to a classical system is achieved.