Recent quantum Hall experiments conducted on disordered graphene pn junction provide evidence that the junction resistance could be described by a simple Ohmic sum of the n and p mediums' resistances. However in the ballistic limit, theory predicts the existence of chirality-dependent quantum Hall plateaus in a pn junction. We show that two distinctively separate processes are required for this ballistic-Ohmic plateau transition, namely (i) hole/electron Landau states equilibration and (ii) valley iso-spin dilution of the incident Landau edge state. These conclusions are obtained by a simple scattering theory argument, and confirmed numerically by performing ensembles of quantum magneto-transport calculations on a 0.1um-wide disordered graphene pn junction within the tight-binding model. The former process is achieved by pn interface roughness, where a pn interface disorder with a root-mean-square roughness of 10nm was found to suffice under typical experimental conditions. The latter process is mediated by extrinsic edge roughness for an armchair edge ribbon and by intrinsic localized intervalley scattering centers at the edge of the pn interface for a zigzag ribbon. In light of these results, we also examine why higher Ohmic type plateaus are less likely to be observable in experiments.
We study ballistic hole transport through Ge/Si core/shell nanowires at low temperatures. We observe Fabry-P$\acute{e}$rot interference patterns as well as conductance plateaus at integer multiples of 2e$^2$/h at zero magnetic field. Magnetic field evolution of these plateaus reveals large effective Land$\acute{e}$ g-factors. Ballistic effects are observed in nanowires with silicon shell thicknesses of 1 - 3 nm, but not in bare germanium wires. These findings inform the future development of spin and topological quantum devices which rely on ballistic subband-resolved transport.
Ballistic-to-Localized Dynamics as Signature of Quantum Phase Transition in Josephson Junction
Università di Napoli Federico II（那不勒斯费德里科二世大学）；INFN, Sezione di Napoli（意大利国家核物理研究所那不勒斯分部）；SPIN-CNR（凝聚态物理特殊相互作用中心研究所）；The Abdus Salam International Center for Theoretical Physics (ICTP)（阿卜杜斯·萨拉姆国际理论物理中心）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所涌现物质科学中心）；Fundamental Quantum Science Program (FQSP), TRIP Headquarters, RIKEN（理化学研究所量子科学计划总部）
We report experimental evidence of ballistic hole transport in one-dimensional quantum wires gate-defined in a strained SiGe/Ge/SiGe quantum well. At zero magnetic field, we observe conductance plateaus at integer multiples of 2e^2/h. At finite magnetic field, the splitting of these plateaus by Zeeman effect reveals largely anisotropic g-factors, with absolute values below 1 in the quantum-well plane, and exceeding 10 out of plane. This g-factor anisotropy is consistent with a heavy-hole character of the propagating valence-band states, in line with a predominant confinement in the growth dire…
In graphene superlattices, bulk topological currents can lead to long-range charge-neutral flow and non-local resistance near Dirac points. A ballistic version of these phenomena has never been explored. Here, we report transport properties of ballistic graphene superlattices. This allows us to study and exploit giant non-local resistances with a large valley Hall angle without a magnetic field. In the low-temperature regime, a crossover occurs toward a new state of matter, referred to as a quantum valley Hall state (qVHS), which is an analog of the quantum Hall state without a magnetic field.…
Electron transport in clean 2D systems with weak electron-phonon (e-ph) coupling can transition from an Ohmic to a ballistic or a hydrodynamic regime. The ballistic regime occurs when electron-electron (e-e) scattering is weak whereas the hydrodynamic regime arises when this scattering is strong. Despite this difference, we find that vortices and a negative nonlocal resistance believed to be quintessentially hydrodynamic are equally characteristic of the ballistic regime. These non-Ohmic regimes cannot be distinguished in DC transport without changing experimental conditions. Further, as our k…
Quantum point contact with large localized spin: fractional quantization of the ballistic conductance
我们分析了包含大的局域自旋J的量子点接触的电导。如果只有一个传播通道呈现导电，则在弹道电导阶梯上形成额外的平台。该平台处的电导值强烈依赖于J，当J从1/2增加到无穷大时，电导值从3e^2/2 h减小到e^2/h，这与实验观察结果非常一致[D. J. Reilly等人，Phys.Rev.B63，121311（2001）]。
Electronic transport in Fermi liquids is usually Ohmic, because of momentum-relaxing scattering due to defects and phonons. These processes can become sufficiently weak in two-dimensional materials, giving rise to either ballistic or hydrodynamic transport, depending on the strength of electron-electron scattering. We show that the ballistic regime is a quantum critical point (QCP) on the regime boundary separating Ohmic and hydrodynamic transport. The QCP corresponds to a \emph{free} conformal field theory (CFT) with a dynamical scaling exponent $z = 1$. Its nontrivial aspects emerge in devic…
Two dimensional electron gases based on SrTiO$_3$ are an intriguing platform for exploring mesoscopic superconductivity combined with spin-orbit coupling, offering electrostatic tunability from insulator to metal to superconductor within a single material. So far, however, quantum effects in SrTiO$_3$ nanostructures have been complicated by disorder. Here we introduce a facile approach to achieving high mobility and patterning gate-tunable structures in SrTiO$_3$, and use it to demonstrate ballistic constrictions with clean normal state conductance quantization. Conductance plateaus show two-f…
Quantum ballistic transport in in-plane-gate transistors showing onset of a novel ferromagnetic phase transition
我们研究了聚焦离子束写入III-V族异质结平面栅晶体管中的一维输运，温度适中，零偏置，无任何外部磁场施加。根据A.黄金和L。准一维电子气中的谷位和自旋占据不稳定性。Mag. Lett. 74，33-42（1996）和早期的实验数据，我们观察到源-漏电导率的平台被认为是栅极电压的函数，不仅在2e^2/h的倍数处，而且在e^2/h处也很明显，就在沟道接近零电导率之前。这可以解释为一种多电子效应，即一种新的弹道铁磁基态，避开了标准的描述和定理。
In high-quality solid-state systems at low temperatures, the hydrodynamic or the ballistic regimes of heat and charge transport are realized in the electron and the phonon systems. In these regimes, the thermal and the electric conductance of the sample can reach abnormally large magnitudes. In this paper, we study the Hall effect in a system of interacting two-dimensional charged particles in a ballistic regime. We demonstrated that the Hall electric field is caused by a change in the densities of particles due to the effect of external fields on their free motions between the sample edges. I…
Graphene nanoribbons (GNRs) are natural waveguides for electrons in graphene. Nevertheless, unlike micron-sized samples, conductance is nearly suppressed in these narrow graphene stripes, mainly due to scattering with edge disorder generated during synthesis or cut. A possible way to circumvent this effect is to define an internal waveguide that isolates specific modes from the edge disorder and allows ballistic conductance. There are several proposals for defining waveguides in graphene; in this manuscript, we consider strain folds and scalar potentials and numerically evaluate these proposal…
Anamolous conductance plateau in an asymmetrically biased InAs/InAlAs quantum point contact
在横向自旋-轨道耦合存在下，研究了In0.52Al0.48As/InAs量子点接触（QPC）在0.4（单位为2 e ~ 2/h）处的反常电导平台的出现和演化.异常平台，在通道中的自旋极化的签名，只出现在偏置不对称的中间范围（约3 V）。它是相当稳健的，在两个平面内栅极共用的扫描电压的最大范围内观察到。我们的电导测量结果表明表面粗糙度散射的QPC的侧壁的证据。我们发现，一个强大的垂直磁场导致磁约束在通道中，这降低了从侧壁散射的重要性，有利于近弹道输运通过QPC的发病。
More than a decade after the discovery of graphene, ballistic transport in nanostructures based on this intriguing material still represents a challenging field of research in two-dimensional electronics. The presence of rough edges in nanostructures based on this material prevents the appearance of truly ballistic electron transport as theo\-re\-tically predicted and, therefore, not well-developed plateaus of conductance have been revealed to date. In this work we report on a novel implementation of the cryo-etching method, which enabled us to fabricate graphene nanoconstrictions encapsulated…
We study the quantization of Dirac fermions in lithographically defined graphene nanoconstrictions. We observe quantized conductance in single nanoconstrictions fabricated on top of a thin hexamethyldisilazane layer over a Si/SiO_2 wafer. This nanofabrication method allows us to obtain well defined edges in the nanoconstrictions, thus reducing the effects of edge roughness on the conductance. We prove the occurrence of ballistic transport and identify several size quantization plateaus in the conductance at low temperature. Experimental data and numerical simulations show good agreement, demon…
A thermoelectric voltage is induced in a junction, constituted of two dissimilar materials under a temperature gradient. Similarly, a thermosize voltage is expected to be induced in a junction made by the same material but having different sizes, so-called thermosize junction. This is a consequence of dissimilarity in Seebeck coefficients due to differences in classical and/or quantum size effects in the same materials with different sizes. The studies on thermosize effects in literature are mainly based on semi-classical models under relaxation time approximation or even simpler local equilib…
The flow of charge carriers in materials can, under some circumstances, mimic the flow of viscous fluids. In order to visualize the consequences of such effects, new methodologies must be developed that can probe the quasiparticle flow profile with nm-scale resolution as the geometric parameters of the system are continuously evolved. In this work, scanning tunneling potentiometry (STP) is used to image quasiparticle flow around engineered electrostatic barriers in graphene/hBN heterostructures. Measurements are performed as electrostatic dams - defined by lateral pn-junction barriers - are br…
我们研究了含有PN结的单层石墨烯霍尔条结构在弹道机制下的霍尔和弯曲电阻的磁场依赖性。当两个区域都是n型时，霍尔电阻占主导地位，并形成霍尔型平台。这些平台的出现是由于斯涅耳定律对角度的限制，只允许具有一定初始角度的电子通过电势阶跃。平台的大小及其位置由电势界面的位置以及所施加电势的值确定。当第二区域是p型时，弯曲电阻占主导地位，由于蛇态的存在，弯曲电阻在场中是不对称的。改变霍尔条中pn界面的位置会强烈影响这些状态，因此弯曲电阻也会改变。改变施加的电势，我们观察到弯曲电阻在电荷中性点（CNP）附近表现出与pn界面的位置无关的峰值，而霍尔电阻在CNP交叉时表现出符号反转，这与最近的实验非常一致[J. R. Williams等人，物理修订版信函107，046602（2011）]。
Electrical and thermal conductances of a quantum Hall bar reflect the topological structure of the incompressible bulk phase. Here we show that noise of electrical current carried through the edge evidences the interplay between these two topological observables. Transport through a structured edge is modeled by a voltage-biased line junction made up of two counter-propagating modes associated with respective filling factors. Specifically, we focus on the edge of a $ν=2/3$ fractional quantum Hall state. Noise is generated at a point distinctly separated from the hot spot (where most of the Ohm…
Linear and non-linear transport properties through an atomic-size point contact based on oxides two-dimensional electron gas is examined using the tight-binding method and the $\mathbf{k\cdot p}$ approach. The ballistic transport is analyzed in contacts realized at the (001) interface between band insulators $LaAlO_3$ and $SrTiO_3$ by using the Landauer-Büttiker method for many sub-bands derived from three Ti 3d orbitals ($d_{yz}$, $d_{zx}$ and $d_{xy}$) in the presence of an out-of-plane magnetic field. We focus especially on the role played by the atomic spin-orbit coupling and the inversion…
Electron transport in a graphene quantum well can be analogous to photon transmission in an optical fiber. In this work, we present a detailed theoretical analysis to study the transport characteristics of graphene waveguides under the influence of different edge orientations. Non-equilibrium Green's function approach in combination with tight-binding Hamiltonian has been utilized to investigate the conductance properties of straight armchair and zigzag oriented graphene waveguides. Conductance plateaus at integer steps of $4e^2/h$ have been observed in both orientations while the zigzag orien…
Quantization through dissipation: impurity states and the optical quantum Hall effect
The Johns Hopkins University（约翰斯·霍普金斯大学）；Massachusetts Institute of Technology（麻省理工学院）；Princeton University（普林斯顿大学）；Canadian Institute for Advanced Research（加拿大高级研究所）
Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS（巴黎萨克雷大学固体物理实验室，法国国家科学研究中心）；O. Ya. Usikov Institute for Radiophysics and Electronics, National Academy of Sciences of Ukraine（乌克兰国家科学院O.Ya.尤西科夫无线电物理与电子研究所）；Research Center for Electronic and Optical Materials, National Institute for Materials Science（国立材料研究所电子与光学材料研究中心）；International Center for Materials Nanoarchitectonics, National Institute for Materials Science（国立材料研究所国际材料纳米架构中心）；Centre de nanosciences et de nanotechnologies, Université Paris-Saclay, CNRS（巴黎萨克雷大学纳米科学与纳米技术中心，法国国家科学研究中心）
