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The authors summarize the results of selective-area growth of vertical MnAs/InAs heterojunction nanowire (NW) arrays and present a preliminary characterization of the transport properties of a single MnAs/InAs heterojunction NW and a single InAs host NW for MnAs inclusions. During the endotaxy of MnAs after the selective-area growth of host InAs nanowires (NWs) on partially SiO2-masked GaAs(111)B substrates, hexagonal NiAs-type MnAs nanoclusters (NCs), which exhibit spontaneous magnetization at room temperature, are formed with the 〈0001〉 direction oriented parallel to the 〈111〉B direction of the zinc-blende-type InAs host NWs. For InAs host NWs, a large positive ordinary magnetoresistance (MR) effect up to 165% is observed at temperatures between 7 and 280 K. In addition, magnetotransport measurements reveal universal conductance fluctuations and a weak Anderson localization at temperatures up to 20 K due to a charge-accumulation layer formed at the surface. Single MnAs/InAs heterojunction NWs, however, exhibit only a negative MR effect, which is independent of temperature T < 10 K and linearly decreases up to −10% at 10 T with increasing magnetic field. These results reveal the tremendous influence of ferromagnetic NCs on the transport behavior inside the InAs host NWs.

Three different methods of experimental mesoscopic physics, namely, weak antilocalization effects, universal conductance fluctuations, and Aharonov-Bohm oscillations, have been used to extract the electron phase-coherence scattering rate in two-dimensional gas of InGaAs/AlInAs heterostructures. The Aharonov-Bohm oscillations reveal strong beating effects, indicating the existence of two similar periodicities of the flux dependencies. As suggested by certain theoretical models, such a behavior might be expected from the so-called Berry phase acquired by electrons with different spin orientations in the presence of strong spin-orbit coupling and Zeeman splitting. In our paper we deduce the experimental values of the dephasing length and try to compare the observed beating pattern with possible scenarios for the appearance of the Berry phase.

This work experimentally investigated quantum transport characteristics of heavily doped bismuth selenide topological insulator nanoribbons to understand their physical origins. Transport properties of nanoribbons were measured via a suspended micro-device for eliminating the substrate effect. A series of quantum transport behaviors such as weak antilocalization, Shubnikov-de Haas oscillations, universal conductance fluctuation, and linear perpendicular-field magnetoresistance have been systematically studied to achieve a coherent understanding on their origins in topologically protected surface states, band bending, or bulk states. The parallel-field magnetoresistance, however, is found to be diverse, which can exhibit negative or positive values for the whole measurement range of the magnetic field strength or change from positive to negative values with the increase of the magnetic field strength. The tunable behavior of the parallel-field magnetoresistance is suggested to be the collective effects of the positive magnetoresistance from surface transport and the negative magnetoresistance possibly owing to the axial anomaly, resulting from long-range ionic impurity-scattering processes in bulk carriers. The transport properties of nanoribbons of heavily doped bismuth selenide, a topological insulator, have been systematically studied in suspended devices. Hao Tang, Juekuan Yang, Minghui Lu, Dongyan Xu and colleagues from several Chinese institutions characterized nanoribbons with different carrier densities and surface-area-to-volume ratios; they observed quantum signatures such as weak antilocalization, Shubnikov-de Haas oscillations, universal conductance fluctuations, and linear magnetoresistance, discussing whether they originate from the surface states, band bending, or bulk states. In particular, the magnetoresistance is anisotropic, and whereas the perpendicular-field magnetoresistance displays the same behavior for all samples, the parallel-field magnetoresistance varies from sample to sample and can be tuned from positive to negative values. The authors thus identify it as a collective effect, with contributions from both the topologically protected surface states and scattering processes in the bulk.

Much effort has been devoted to the electronic properties of relatively thick $\mathrm{Zr}{\mathrm{Te}}_{5}$ crystals, focusing on their three-dimensional topological effects. Thin $\mathrm{Zr}{\mathrm{Te}}_{5}$ crystals, on the other hand, were much less explored experimentally. Here we present detailed magnetotransport studies of few-layer $\mathrm{Zr}{\mathrm{Te}}_{5}$ devices, in which electron-electron interactions and weak antilocalization are observed. The coexistence of the two effects manifests itself in corroborating evidence presented in the temperature and magnetic field dependence of the resistance. Notably, the temperature-dependent phase coherence length extracted from weak antilocalization agrees with strong electron-electron scattering in the sample. Meanwhile, universal conductance fluctuations have temperature and gate voltage dependence that is similar to that of the phase coherence length. Last, all the transport properties in thin $\mathrm{Zr}{\mathrm{Te}}_{5}$ crystals show strong two-dimensional characteristics. Our results provide insight into the highly intricate properties of topological material $\mathrm{Zr}{\mathrm{Te}}_{5}$.

The two-dimensional layered compound PdCoO$_2$ is one of the best oxide conductors, providing an intriguing research arena opened by the long mean free path and the very high mobility of ~51000 cm2/Vs. These properties turn PdCoO$_2$ into a candidate material for nanoscale quantum devices. By exploring universal conductance fluctuations originating at nanoscale PdCoO$_2$ Hall-bar devices, we determined the phase coherence length of electron transport in c-axis oriented PdCoO$_2$ thin films to equal ~100 nm. The weak temperature dependence of the measured phase coherence length suggests that defect scattering at twin boundaries in the PdCoO$_2$ thin film governs phase breaking. These results suggest that phase coherent devices can be achieved by realizing the devices smaller than the size of twin domains, via refined microfabrication and suppression of twin boundaries., 6 pages, 5 figures

Universal conductance fluctuations are usually observed in the form of aperiodic oscillations in the magnetoresistance of thin wires as a function of the magnetic field B. If such oscillations are completely random at scales exceeding \xi_B, their Fourier analysis should reveal a white noise spectrum at frequencies below \xi_B^{-1}. Comparison with the results for 1D systems suggests another scenario: according to it, such oscillations are due to the superposition of incommensurate harmonics and their spectrum should contain discrete frequencies. An accurate Fourier analysis of the classical experiment by Washburn and Webb reveals a practically discrete spectrum in agreement with the latter scenario. However, this spectrum is close in shape to the discrete white noise spectrum whose properties are similar to a continuous one. More detailed analysis reveals the existence of the continuous component, whose smallness is explained theoretically. A lot of qualitative results are obtained, which confirm the presented picture. The distribution of phases, frequency differences and the growth exponents agree with theoretical predictions. Discrete frequencies depends weakly on the treatment procedure. The discovered shift oscillations confirm the analogy with 1D systems. Microscopical estimates show agreement of the obtained results with geometrical dimensions of the sample., Comment: Latex, 17 pages, 13 figures included. arXiv admin note: text overlap with arXiv:1911.10441

We combine the ideas of scaling theory and universal conductance fluctuations with density-functional theory to analyze the conductance properties of doped silicon nanowires. Specifically, we study the cross-over from ballistic to diffusive transport in boron (B) or phosphorus (P) doped Si-nanowires by computing the mean free path, sample averaged conductance , and sample-to-sample variations std(G) as a function of energy, doping density, wire length, and the radial dopant profile. Our main findings are: (i) the main trends can be predicted quantitatively based on the scattering properties of single dopants; (ii) the sample-to-sample fluctuations depend on energy but not on doping density, thereby displaying a degree of universality, and (iii) in the diffusive regime the analytical predictions of the DMPK theory are in good agreement with our ab initio calculations., Comment: Accepted for PRL

The Aharonov-Casher effect is the analogue of the Aharonov-Bohm effect that applies to neutral particles carrying a magnetic moment. This can be manifested by vortices or fluxons flowing in trajectories that encompass an electric charge. These have been predicted to result in a persistent voltage which fluctuates for different sample realizations. Here we show that disordered superconductors exhibit reproducible voltage fluctuation, antisymmetrical with respect to magnetic field, as a function of various parameters such as magnetic field amplitude, field orientations and gate voltage. These results are interpreted as the vortex equivalent of the universal conductance fluctuations typical of mesoscopic disordered metallic systems. We analyze the data in the framework of random matrix theory and show that the fluctuation correlation functions and curvature distributions exhibit behavior which is the fingerprint of Aronov-Casher physics. The results demonstrate the quantum nature of the vortices in highly disordered superconductors both above and below $T_c$., Comment: 8 pages 9 figures

We investigate the universal properties of quantum transport in graphene nanowires that engender subtle universal conductance fluctuations. We present results for three of the main microscopic models that describe the sublattice of graphene and generate, as we shall show, all the chiral universal symmetries. The results are robust and demonstrate the widely sought sign of chirality even in the regime of many open channels. The fingerprints paves the way to distinguish systems with sublattice symmetry such as topological insulators from ordinary ones by an order of magnitude. The experimental realization requires a single measurement of the chaotic fluctuations of the associated valleytronics conductante. Through the phase coherence length, our theoretical predictions are confirmed with the data from traditional measurements in the literature concerning quantum magnetotransport., Comment: 8 pages, 7 figures, published in Physical Review B

In condensed matter systems, coherent backscattering and quantum interference in the presence of time-reversal symmetry lead to well-known phenomena such as weak localization (WL) and universal conductance fluctuations (UCF). Here we use multi-pass Landau-Zener transitions at the avoided crossing of a highly-coherent superconducting qubit to emulate these phenomena. The average and standard deviation of the qubit transition rate exhibit a dip and peak when the driving waveform is time-reversal symmetric, analogous to WL and UCF, respectively. The higher coherence of this qubit enabled the realization of both effects, in contrast to earlier work arXiv:1204.6428, which successfully emulated UCF, but did not observe WL. This demonstration illustrates the use of non-adiabatic control to implement quantum emulation with superconducting qubits., 14 pages, 7 figures; two reference added; figure 4 changed; 2 more figures added in Suppl. Info

Low-temperature magnetotransport measurements are performed on GaAs/InSb core-shell nanowires. The nanowires were self-catalyzed grown by molecular beam epitaxy. The conductance measurements as a function of back-gate voltage show an ambipolar behavior comprising an insulating range in between the transition from the p-type to the n-type region. Simulations based on a self-consistent Schr\"odinger--Poisson solver revealed that the ambipolar characteristics originate from a Fermi level dependent occupation of hole and electron states within the approximately circular quantum well formed in the InSb shell. By applying a perpendicular magnetic field with respect to the nanowire axis, conductance fluctuations were observed, which are used to extract the phase-coherence length. By averaging the magneto-conductance traces at different back-gate voltages, weak antilocalization features are resolved. Regular flux-periodic conductance oscillations are measured when an axial magnetic field is applied. These oscillations are attributed to closed-loop quantized states located in the InSb shell which shift their energetic position periodically with the magnetic flux. Possible reasons for experimentally observed variations in the oscillation patterns are discussed using simulation results., Comment: 19 pages, 9 figures, 1 table, title changed, additional datasets and plots

Phase-coherent transport manifests itself in various quantum phenomena. Universal conductance fluctuations (UCF) are one of them. Here, we report the detailed study of dephasing of electronic wavefunction by analyzing UCF and weak antilocalization in a device, made by exfoliation from a bulk insulating topological insulator single crystal ( B i 1 S b 1 T e 1.5 S e 1.5). From the parallel magnetic field measurement, we proved that the quantum phenomena in our sample are caused by surface Dirac electrons. Our sample showed very slow dephasing of electronic wavefunction, which has rarely been observed in topological insulator materials. This kind of dephasing indicates the presence of electron–hole charge puddles in our system. The RMS amplitude of the UCF is also surprisingly higher ( 13 e 2 h) than the universal value of the conductance fluctuation ( e 2 h) and the fluctuations persist in our sample until the temperature as high as 20 K and above. We also show that the shape of the flake has a significant impact on the prefactor, α, of the Hikami–Larkin–Nagaoka equation. Moreover, the formation of Aharonov–Bohm loops of different areas is responsible for UCF. By analyzing the fast Fourier transform of the UCF data, we have obtained these Aharonov–Bohm loops which are contributing to the UCF.

We have measured weak antilocalization effects, universal conductance fluctuations, and Aharonov-Bohm oscillations in the two-dimensional electron gas formed in InGaAs/AlInAs heterostructures. This system possesses strong spin-orbit coupling and a high Land\'{e} factor. Phase-coherence lengths of 2$-$4 $\mu$m at 1.5$-$4.2 K are extracted from the magnetoconductance measurements. The analysis of the coherence-sensitive data reveals that the temperature dependence of the decoherence rate complies with the dephasing mechanism originating from electron-electron interactions in all three experiments. Distinct beating patterns superimposed on the Aharonov-Bohm oscillations are observed over a wide range of magnetic fields, up to 0.7 Tesla at the relatively high temperature of 1.5 K. The possibility that these beats are due to the interplay between the Aharonov-Bohm phase and the Berry one, different for electrons of opposite spins in the presence of strong spin-orbit and Zeeman interactions in ring geometries, is carefully investigated. It appears that our data are not explained by this mechanism; rather, a few geometrically-different electronic paths within the ring's width can account for the oscillations' modulations.

We present low-temperature magnetotransport measurements on GaAs/InAs core/shell nanowires contacted by regular source-drain leads as well as laterally attached Hall contacts, which only touch parts of the nanowire sidewalls. Low-temperature measurements between source and drain contacts show typical phase coherent effects, such as universal conductance fluctuations in a magnetic field aligned perpendicularly to the nanowire axis as well as Aharonov-Bohm-type oscillations in a parallel aligned magnetic field. However, the signal between the Hall contacts shows a Hall voltage buildup, when the magnetic field is turned perpendicular to the nanowire axis while current is driven through the wire using the source-drain contacts. At low temperatures, the phase coherent effects measured between source and drain leads are superimposed on the Hall voltage, which can be explained by nonlocal probing of large segments of the nanowire. In addition, the Aharonov-Bohm-type oscillations are also observed in the magnetoconductance at magnetic fields aligned parallel to the nanowire axis, using the laterally contacted leads. This measurement geometry hereby directly corresponds to classical Aharonov-Bohm experiments using planar quantum rings. In addition, the Hall voltage is used to characterize the nanowires in terms of charge carrier concentration and mobility, using temperature- and gate-dependent measurements as well as measurements in tilted magnetic fields. The GaAs/InAs core/shell nanowire used in combination with laterally attached contacts is therefore the ideal system to three-dimensionally combine quantum ring experiments using the cross-sectional plane and Hall experiments using the axial nanowire plane.

We theoretically investigate the conductance fluctuation of two-terminal device in Sierpinski carpets. We find that, for the circular orthogonal ensemble (COE), the conductance fluctuation does not display a universal feature; but for circular unitary ensemble (CUE) without time-reversal symmetry or circular symplectic ensemble (CSE) without spin-rotational symmetry, the conductance fluctuation can reach an identical universal value of 0:74 ± 0:01(e2/h). We further find that the conductance distributions around the critical disorder strength for both CUE and CSE systems share the similar distribution forms. Our findings provide a better understanding of the electronic transport properties of the regular fractal structure.

The so called phase-coherence length ${l}_{\ensuremath{\varphi}}$ and its relation to the geometrical dimensions of a sample determine the electronic transport regime. Different approaches are established for extracting ${l}_{\ensuremath{\varphi}}$ from magnetotransport data of mesoscopic systems and need to be cross-checked by using experimental data on the same model system in order to ensure an overall consistent theoretical description. Suitable model systems for testing this consistency are single GaN:Ge nanowires. Their magnetoconductance at low temperatures exhibits universal conductance fluctuations as well as weak localization effects. We find that the values of ${l}_{\ensuremath{\varphi}}$ obtained from the established analysis of the magnitude of the conductance fluctuations rms($\mathrm{\ensuremath{\Delta}}G$) decrease with increasing measurement current, whereas the corresponding values of ${l}_{\ensuremath{\varphi}}$ determined by the analyses of the correlation field ${B}_{\text{C}}$ and the weak localization effect yield the same value for ${l}_{\ensuremath{\varphi}}$ independent of the measurement current used. We apply and modify the existing theoretical framework for bias-dependent differential conductance fluctuations, in order to explain the decrease of the conductance fluctuations rms($\mathrm{\ensuremath{\Delta}}G$) with increasing current density in our dc measurements. This leads to the same values of ${l}_{\ensuremath{\varphi}}$ independent of the analysis approach applied to the same set of data.

We report on phase-coherent transport studies of a Bi$_2$O$_2$Se nanoplate and on observation of universal conductance fluctuations and spin-orbit interaction induced reduction in fluctuation amplitude in the nanoplate. Thin-layered Bi$_2$O$_2$Se nanoplates are grown by chemical vapor deposition (CVD) and transport measurements are made on a Hall-bar device fabricated from a CVD-grown nanoplate. The measurements show weak antilocalization at low magnetic fields at low temperatures, as a result of spin-orbit interaction, and a crossover toward weak localization with increasing temperature. Temperature dependences of characteristic transport lengths, such as spin relaxation length, phase coherence length, and mean free path, are extracted from the low-field measurement data. Universal conductance fluctuations are visible in the low-temperature magnetoconductance over a large range of magnetic fields and the phase coherence length extracted from the autocorrelation function is in consistence with the result obtained from the weak localization analysis. More importantly, we find a strong reduction in amplitude of the universal conductance fluctuations and show that the results agree with the analysis assuming strong spin-orbit interaction in the Bi$_2$O$_2$Se nanoplate., 11 pages, 4 figures, supplementary materials

I present the results of extensive numerical simulations which reveal the glassy properties of Anderson localization in dimension two at zero temperature: pinning, avalanches and chaos. I first show that strong localization confines quantum transport along paths which are pinned by disorder but can change abruptly and suddenly (avalanches) when the energy is varied. I determine the roughness exponent $\zeta$ characterizing the transverse fluctuations of these paths and find that its value $\zeta=2/3$ is the same as for the directed polymer problem. Finally, I characterize the chaos property, namely the fragility of the conductance with respect to small perturbations in the disorder configuration. It is linked to interference effects and universal conductance fluctuations at weak disorder, and more spin-glass-like behavior at strong disorder., Comment: 6 + 5 (supplemental material) pages, 4 + 6 figures. Corresponds to the published version

In this work, we report on hot carrier diffusion in graphene across large enough length scales that the carriers are not thermalized across the crystal. The carriers are injected into graphene at one site and their thermal transport is studied as a function of applied power and distance from the heating source, up to tens of micrometers away. Superconducting contacts prevent out-diffusion of hot carriers to isolate the electron-phonon coupling as the sole channel for thermal relaxation. As local thermometers, we use the amplitude of the universal conductance fluctuations, which varies monotonically as a function of temperature. By measuring the electron temperature simultaneously along the length we observe a thermal gradient which results from the competition between electron-phonon cooling and lateral heat flow.

We investigate the low-temperature transport in 8-nm-diam Si junctionless nanowire field-effect transistors fabricated by top down techniques with a wraparound gate and two different phosphorus doping concentrations. First we extract the intrinsic gate capacitance of the device geometry from a device that demonstrates Coulomb blockade at 12 mK with over 500 Coulomb peaks across a gate-voltage range of 6 V indicating the formation of an island in the entire 150-nm-long nanowire channel. In two other devices, made from silicon on insulator wafers that were doped to an activated dopant concentration of Si:P $4\ifmmode\times\else\texttimes\fi{}{10}^{19}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}3}$, we observe quantum interference and use the extracted gate coupling to determine the mean free paths from the dominant energy scale on the gate-voltage axis. For the higher doped device, the analysis yields a mean free path of $4\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{4pt}{0ex}}\text{nm}$, which is on the order of the average spacing of phosphorus atoms and suggests scattering on unactivated or activated dopants. For the device with an implanted phosphorus density of $4\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}3}$, the quantum interference effects suggest a mean free path of $10\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{4pt}{0ex}}\text{nm}$, which is comparable to the nanowire width, and thus allows for coherent formation of transversal modes. The results suggest that the low-temperature mobility is limited by scattering on phosphorus dopants rather than the expected surface roughness scattering for nanowires with diameters larger than or comparable to the Fermi wavelength. A temperature-dependent analysis of universal conductance fluctuations indicates a phase-coherence length greater than the nanowire length for temperatures below 1.9 K, and decoherence from one-dimensional electron-electron interactions dominates transport for higher temperatures. Our measurements, therefore, provide insight into scattering and dephasing mechanisms in technologically relevant silicon device geometries, which will help with future design choices with regard to, e.g., doping density.

In this paper, we apply Andreev’s reflection to study the sub-gap coherent transport properties for a quantum dot attached to normal metal and superconductor reservoirs via non-ideal leads. We use the random matrix theory to obtain numerically the conductance and shot noise power probability densities varying the transparencies and the number of open scattering channels in each lead for the three Wigner–Dyson ensembles. We obtain results from the extreme quantum limit (where the conductance is of the order of 2e 2/h, the quantum of conductance) to the semiclassical regime (conductance much larger than 2e 2/h). We observe the appearance of non-analyticities in these probability densities and an interesting invariance break under the operation of exchanging the lead labels when time-reversal symmetry is absent.

Gallium nitride nanowires (GaN NWs) with triangular cross-section exhibit universal conductance fluctuations (UCF) originating from the quantum interference of electron wave functions in the NWs. The amplitude of UCF is inversely proportional to the applied bias current. The bias dependence of UCF, combined with temperature dependence of the resistance suggests that phase coherent transport dominates over normal transport in GaN NWs. A unique temperature dependence of phase-coherent length and fluctuation amplitude is associated with inelastic electron–electron scattering in NWs. The phase-coherence length extracted from the UCF is as large as 400 nm at 1.8 K, and gradually decreases as temperature increases up to 60 K.

The transport properties of Ge-doped single GaN nanowires are investigated, which exhibit a weak localization effect as well as universal conductance fluctuations at low temperatures. By analyzing these quantum interference effects, the electron phase coherence length was determined. Its temperature dependence indicates that in the case of highly doped nanowires electron-electron scattering is the dominant dephasing mechanism, while for the slightly doped nanowires dephasing originates from Nyquist-scattering. The change of the dominant scattering mechanism is attributed to a modification of the carrier confinement caused by the Ge-doping. The results demonstrate that the phase coherence length can be tuned by the donor concentration making Ge-doped GaN nanowires an ideal model system for studying the influence of impurities on quantum-interference effects in mesoscopic and nanoscale systems.

Universal conductance fluctuations and the weak antilocalization effect are defect structure specific fingerprints in the magnetoconductance that are caused by electron interference. Experimental evidence is presented that the conductance fluctuations in the present topological insulator (Bi$_{0.57}$Sb$_{0.43}$)$_2$Te$_3$ nanoribbons which are selectively grown by molecular beam epitaxy are caused by well-defined and sharply resolved phase-coherent loops. From measurements at different magnetic field tilt angles we deduced that these loops are preferentially oriented parallel to the quintuple layers of the topological insulator material. Both from a theoretical analysis of universal conductance fluctuations and from weak antilocalization measured at low temperature the electronic phase-coherence lengths $l_\phi$ are extracted, which is found to be larger in the former case. Possible reasons for this deviation are discussed., Comment: 7 pages, 6 figure

Charge carriers in graphene exhibit distinct characteristics from those in other two-dimensional materials because of their chiral nature. Additionally, multiple Dirac cones that emerge in graphene superlattices have been regarded as an interesting point in condensed-matter physics in recent years. Here, we report an investigation of the magneto-conductance in graphene encapsulated on the top and bottom by aligned h-BN. The bottom h-BN is precisely aligned with graphene, while the top h-BN is rotated a very small angle relative to it. Such a heterostructure could spoil the commensurate state existing in precisely aligned graphene while the giant moiré superlattice remains. A clear signature of weak localization and weak anti-localization is observed at multiple Dirac cones. Both the weak (anti)localization and the universal conductance fluctuations exhibit strong dependencies on the carrier density, temperature and channel length. This artificial heterostructure allows one to explore quantum interference in graphene with a wide spectrum of electronic properties.

The transport properties of topological insulator nanostructures prepared by selective-area molecular beam epitaxy is investigated. For the nanocolumn structures based on Sb 2 Te 3 /Bi 2 Te 3 -heterostructures pronounced universal conductance fluctuations are observed in the magnetoconductance, indicating phase-coherent transport. Furthermore, angle-dependent measurements indicate that the phase coherent loops are mainly oriented parallel to the substrate plane. Measurements on nanoribbons based on (Bi 0.57 Sb 0.43 ) 2 Te 3 revealed a resistance dip due to weak antilocalization as well as universal conductance fluctuations. Here, we also found indications, that the phase-coherent loops are predominantly oriented parallel to the quintuple layers forming the topological insulator.

A key feature of topological insulators (TI) is symplectic symmetry of the Hamiltonian which changes to unitary when time reversal symmetry is lifted and the topological phase transition occurs. However, such a crossover has never been explicitly observed, by directly probing the symmetry class of the Hamiltonian. In this report, we have probed the symmetry class of topological insulators by measuring the mesoscopic conductance fluctuations in the TI Bi$_{1.6}$Sb$_{0:4}$Te$_2$Se, which shows an exact factor of two reduction on application of a magnetic field due to crossover from symplectic to unitary symmetry classes. The reduction provides an unambiguous proof that the fluctuations arise from the universal conductance fluctuations (UCF), due to quantum interference and persists from T = 22 mK to 4.2 K. We have also compared the phase breaking length (l$_\phi$) extracted from both magneto-conductivity and UCF which agree well within a factor of two in the entire temperature and gate voltage range. Our experiment confirms UCF as the major source of fluctuations in mesoscopic disordered topological insulators, and the intrinsic preservation of time reversal symmetry in these systems., Comment: 6 pages, 4 figures

The experimental work developed during this PhD is situated at the interface of two fields of condensed matter physics, namely spin glasses and mesoscopic physics. Spin glasses have been widely studied and are one of the problem that has been the most discussed over the years, both on a theoretical and experimental point of view. This state is characterized by very peculiar properties that come to light as it exhibits a magnetic phase transition at low temperatures that is already unusual. Indeed, this transition is due to a mix of frustration and disorder in the magnetic structure of the system, making it an exceptional model system for glasses and frustrated systems in general. After many efforts, theoreticians managed to described the fundamental state of the system by the mean of two different and apparently incompatible approaches. The first one, called RSB theory, is based on a mean-field approximation and predicts a complex phase space with an unconventional hierarchical organization. The second is based on more phenomenological approach and is named Droplet theory. It points towards a unique ground state and explain all the observation by slow relaxation processes. However, the question of the true nature of the spin glass phase is still heavily debated. Mesoscopic physics, for its part, addresses the question of electronic transport for samples in which the electrons keep their phase coherence. If the electrons remains coherent, it is possible to see interference effects that are quantum signs of what happens at the atomic level. In this work, it is used to probe the magnetic and static disorder in spin glasses. Indeed, it is possible to interpret the change in those interferences as changes in the microscopic disorder configuration and to know exactly how the spin glass state evolves. Some work have already tried to use coherent transport in spin glasses but this remains an open field. This work has then be dedicated to the implementation of transport measurement in spin glasses and mesocopic conductors. The first part will be focused on a the experimental setup that was used to perform very precise transport measurements and on the processing of the data taken out of them. In a second part, we will present some general physical characteristics of our samples such as their resistance dependence to the temperature or magnetic field, before extracting the quantum signature in magnetoresistance measurements. Finally, we will discuss the results obtained. We show that strong changes in the microscopic disorder happen even at low temperatures, in opposition to what is believed. We argue that those observed changes are purely structural and come from systems that are widely distributed in energy.; Le travail expérimental développé pendant cette thèse se situe à l'interface de deux champs en physique de la matière condensée, à savoir les verres de spins et la physique mésoscopique. Les verres de spins ont été largement étudié et font partie des problèmes les plus débattus au cours des années tant d'un point de vue expérimental que théorique. Cet état est caractérisé par des propriétés très particulières qui se font jour lors d'une transition de phase magnétique à très basses températures qui est elle-même inhabituelle. En effet, cette transition est due à un mélange de frustration et de désordre dans la structure magnétique du système. Ce faisant, c'est un système modèle pour les verres et les systèmes frustrés en général. Après bien des efforts, des travaux théoriques ont réussi à décrire l'état fondamental du système au moyen de deux approches différentes et apparemment incompatibles. Cependant, la question de la vraie nature de la phase verre de spin reste grandement débattue.La physique mésoscopique, de son côté, traite du transport électronique dans les échantillons pour lesquels les électrons gardent leur cohérence de phase. Si les électrons restent cohérents, il est possible de voir des effets d'interférences qui sont des signes quantiques de ce qu'il se passe au niveau atomique. Dans cette thèse, il est utilisé pour sonder le désordre aussi bien magnétique que statique dans un verre de spins.Nous montrons que, contrairement à ce qui est cru, de forts changements se déroulent dans le désordre microscopique même à basses températures. Nous prétendons également que ces changements sont purement structuraux et viennent du fait de systèmes dont la distribution en énergie est très large.

International audience; We report on the magneto-transport properties of individual Bi2Te3 nanowires with diameters ranging from 25 nm to 120 nm fabricated by electrodeposition in etched ion-track membranes. The metallic behavior of all nanowires is typical for highly-degenerate semiconductors and the weak temperature dependence of the resistivity suggests a weak electron-phonon coupling, as expected for topological surface states with quantum confinement. The diameter dependence of the resistivity further confirms the quasi-ballistic nature of charge carriers, in agreement with the reduction of the number of surface conductance channels in disordered quantum wires with a stronger quantum confinement. Although the bulk conductance is not negligible, both the transport length of bulk carriers and their electrostatic screening length remain shorter than the nanowire diameter, probably leading to a small contribution to the resistivity change. Quantum transport measurements give further evidence of finite-size effects, as shown by the weak anti-localization correction to the conductance, below 15K, and reveal the coexistence of surface Aharonov-Bohm oscillations and bulk universal conductance fluctuations, below 5K.

We present a complete numerical calculation and an experimental data analysis of the universal conductance fluctuations in quasi-one-dimension nanowires. The conductance peak density model, introduced in nanodevice research on [Phys. Rev. Lett. 107, 176807 (2011)], is applied successfully to obtain the coherence length of InAs nanowire magnetoconductance and we prove its equivalence with correlation methods. We show the efficiency of the method and therefore a prominent alternative to obtain the phase-coherence length. The peak density model can be similarly applied to spintronic setups, graphene and topological isolator where phase-coherence length is a relevant experimental parameter., Comment: Accepted for publication as a Regular Article in Physical Review B

The experimental work developed during this PhD is situated at the interface of two fields of condensed matter physics, namely spin glasses and mesoscopic physics. Spin glasses have been widely studied and are one of the problem that has been the most discussed over the years, both on a theoretical and experimental point of view. This state is characterized by very peculiar properties that come to light as it exhibits a magnetic phase transition at low temperatures that is already unusual. Indeed, this transition is due to a mix of frustration and disorder in the magnetic structure of the system, making it an exceptional model system for glasses and frustrated systems in general. After many efforts, theoreticians managed to described the fundamental state of the system by the mean of two different and apparently incompatible approaches. The first one, called RSB theory, is based on a mean-field approximation and predicts a complex phase space with an unconventional hierarchical organization. The second is based on more phenomenological approach and is named Droplet theory. It points towards a unique ground state and explain all the observation by slow relaxation processes. However, the question of the true nature of the spin glass phase is still heavily debated. Mesoscopic physics, for its part, addresses the question of electronic transport for samples in which the electrons keep their phase coherence. If the electrons remains coherent, it is possible to see interference effects that are quantum signs of what happens at the atomic level. In this work, it is used to probe the magnetic and static disorder in spin glasses. Indeed, it is possible to interpret the change in those interferences as changes in the microscopic disorder configuration and to know exactly how the spin glass state evolves. Some work have already tried to use coherent transport in spin glasses but this remains an open field. This work has then be dedicated to the implementation of transport measurement in spin glasses and mesocopic conductors. The first part will be focused on a the experimental setup that was used to perform very precise transport measurements and on the processing of the data taken out of them. In a second part, we will present some general physical characteristics of our samples such as their resistance dependence to the temperature or magnetic field, before extracting the quantum signature in magnetoresistance measurements. Finally, we will discuss the results obtained. We show that strong changes in the microscopic disorder happen even at low temperatures, in opposition to what is believed. We argue that those observed changes are purely structural and come from systems that are widely distributed in energy., Le travail expérimental développé pendant cette thèse se situe à l'interface de deux champs en physique de la matière condensée, à savoir les verres de spins et la physique mésoscopique. Les verres de spins ont été largement étudié et font partie des problèmes les plus débattus au cours des années tant d'un point de vue expérimental que théorique. Cet état est caractérisé par des propriétés très particulières qui se font jour lors d'une transition de phase magnétique à très basses températures qui est elle-même inhabituelle. En effet, cette transition est due à un mélange de frustration et de désordre dans la structure magnétique du système. Ce faisant, c'est un système modèle pour les verres et les systèmes frustrés en général. Après bien des efforts, des travaux théoriques ont réussi à décrire l'état fondamental du système au moyen de deux approches différentes et apparemment incompatibles. Cependant, la question de la vraie nature de la phase verre de spin reste grandement débattue.La physique mésoscopique, de son côté, traite du transport électronique dans les échantillons pour lesquels les électrons gardent leur cohérence de phase. Si les électrons restent cohérents, il est possible de voir des effets d'interférences qui sont des signes quantiques de ce qu'il se passe au niveau atomique. Dans cette thèse, il est utilisé pour sonder le désordre aussi bien magnétique que statique dans un verre de spins.Nous montrons que, contrairement à ce qui est cru, de forts changements se déroulent dans le désordre microscopique même à basses températures. Nous prétendons également que ces changements sont purement structuraux et viennent du fait de systèmes dont la distribution en énergie est très large.

In quantum systems, signatures of multifractality are rare. They have been found only in the multiscaling of eigenfunctions at critical points. Here we demonstrate multifractality in the magnetic field-induced universal conductance fluctuations of the conductance in a quantum condensed matter system, namely, high-mobility single-layer graphene field-effect transistors. This multifractality decreases as the temperature increases or as doping moves the system away from the Dirac point. Our measurements and analysis present evidence for an incipient Anderson-localization near the Dirac point as the most plausible cause for this multifractality. Our experiments suggest that multifractality in the scaling behavior of local eigenfunctions are reflected in macroscopic transport coefficients. We conjecture that an incipient Anderson-localization transition may be the origin of this multifractality. It is possible that multifractality is ubiquitous in transport properties of low-dimensional systems. Indeed, our work suggests that we should look for multifractality in transport in other low-dimensional quantum condensed-matter systems. Multifractality is ubiquitous in classical systems but rare in quantum ones. Here the authors present observations demonstrating that universal conductance fluctuations in high-mobility single-layer graphene field-effect transistors are multifractal and may arise from Anderson-localization.

We study the conductance fluctuation in topological semimetals. Through statistic distribution of energy levels of topological semimetals, we determine the dominant parameters of universal conductance fluctuation (UCF), i.e., the number of uncorrelated bands $k$, the level degeneracy $s$, and the symmetry parameter $\beta$. These parameters allow us to predict the zero-temperature intrinsic UCF of topological semimetals by the Altshuler-Lee-Stone theory. Then, we obtain numerically the conductance fluctuations for topological semimetals of quasi-1D geometry. We find that for Dirac/Weyl semimetals, the theoretical prediction coincides with the numerical results. However, a non-universal conductance fluctuation behavior is found for topological nodal line semimetals, i.e., the conductance fluctuation amplitude increases with the enlargement of SOC strength. We find that such unexpected parameter-dependent phenomena of conductance fluctuation are related to Fermi surface shape of 3D topological semimetals. These results will help us to understand the existing and future experimental results of UCF in 3D topological semimetals., Comment: 9 pages, 8 figures

Experimental demonstrations of one-dimensional (1D) van der Waals material tellurium (Te) have been presented by Raman spectroscopy under strain and magneto-transport. Raman spectroscopy measurements have been performed under strains along different principle axes. Pronounced strain response along the c-axis is observed due to the strong intrachain covalent bonds, while no strain response is obtained along the a-axis due to the weak interchain van der Waals interaction. Magneto-transport results further verify its anisotropic property, which results in dramatically distinct magneto-resistance behaviors in terms of three different magnetic field directions. Specifically, phase coherence length extracted from weak antilocalization effect, Lϕ ≈ T–0.5, claims its two-dimensional (2D) transport characteristics when an applied magnetic field is perpendicular to the thin film. In contrast, Lϕ ≈ T–0.33 is obtained from universal conductance fluctuations once the magnetic field is along the c-axis of Te, which ind...

We present results of resistance fluctuation spectroscopy on single crystals of the predicted Kondo topological insulator material SmB$_6$. Our measurements show that at low temperatures, transport in this system takes place only through surface states. The measured noise in this temperature range arises due to Universal Conductance Fluctuations whose statistics was found to be consistent with theoretical predictions for that of two-dimensional systems in the Symplectic symmetry class. At higher temperatures, we find signatures of glassy dynamics and establish that the measured noise is caused by mobility fluctuations in the bulk. We find that, unlike the topological insulators of the dichalcogenide family, the noise in surface and bulk conduction channels in SmB$_6$ are completely uncorrelated. Our measurements establish that at sufficiently low temperatures, the bulk has no discernible contribution to electrical transport in SmB$_6$ making it an ideal platform for probing the physics of topological surface states., 9 pages, 11 figures

The standard treatment of quantum corrections to semiclassical electronic conduction assumes that charge carriers propagate many wavelengths between scattering events, and succeeds in explaining multiple phenomena (weak localization magnetoresistance (WLMR), universal conductance fluctuations, Aharonov-Bohm oscillations) observed in polycrystalline metals and doped semiconductors in various dimensionalities. We report apparent WLMR and conductance fluctuations in H$_{x}$VO$_{2}$, a poor metal (in violation of the Mott-Ioffe-Regel limit) stabilized by the suppression of the VO$_{2}$ metal-insulator transition through atomic hydrogen doping. Epitaxial thin films, single-crystal nanobeams, and nanosheets show similar phenomenology, though the details of the apparent WLMR seem to depend on the combined effects of the strain environment and presumed doping level. Self-consistent quantitative analysis of the WLMR is challenging given this and the high resistivity of the material, since the quantitative expressions for WLMR are derived assuming good metallicity. These observations raise the issue of how to assess and analyze mesoscopic quantum effects in poor metals., Comment: 14 pages, 4 figures + 3 supplemental figures

Simulating quantum transport through mesoscopic, ring-shaped graphene structures, we address various quantum coherence and interference phenomena. First, a perpendicular magnetic field, penetrating the graphene ring, gives rise to Aharonov-Bohm oscillations in the conductance as a function of the magnetic flux, on top of the universal conductance fluctuations. At very high fluxes the interference gets suppressed and quantum Hall edge channels develop. Second, applying an electrostatic potential to one of the ring arms, $nn'n$- or $npn$-junctions can be realized with particle transmission due to normal tunneling or Klein tunneling. In the latter case the Aharonov-Bohm oscillations weaken for smooth barriers. Third, if potential disorder comes in to play, both Aharonov-Bohm and Klein tunneling effects rate down, up to the point where particle localization sets in., Comment: 14 pages, 7 figures, proceedings of the International Conference on Mesoscopic Transport and Quantum Coherence (QTC 2017)

We report quantum transport measurements on two dimensional (2D) Si:P and Ge:P δ-layers and compare the inelastic scattering rates relevant for weak localization (WL) and universal conductance fluctuations (UCF) for devices of various doping densities (0.3–2.5 × 1018 m−2) at low temperatures (0.3–4.2 K). The phase breaking rate extracted experimentally from measurements of WL correction to conductivity and UCF agree well with each other within the entire temperature range. This establishes that WL and UCF, being the outcome of quantum interference phenomena, are governed by the same dephasing rate.

Spin dependent electron transport measurements on graphene are of high importance to explore possible spintronic applications. Up to date all spin transport experiments on graphene were done in a semi-classical regime, disregarding quantum transport properties such as phase coherence and interference. Here we show that in a quantum coherent graphene nanostructure the non-local voltage is strongly modulated. Using non-local measurements, we separate the signal in spin dependent and spin independent contributions. We show that the spin dependent contribution is about two orders of magnitude larger than the spin independent one, when corrected for the finite polarization of the electrodes. The non-local spin signal is not only strongly modulated but also changes polarity as a function of the applied gate voltage. By locally tuning the carrier density in the constriction we show that the constriction plays a major role in this effect and indicates that it can act as a spin filter device. Our results show the potential of quantum coherent graphene nanostructures for the use in future spintronic devices.