Joshua Folk
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Vanderwaals heterostructures; Entropy of quantum systems; Thermodynamic probes of 2D materials; Fractional quantum Hall effect
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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
Topological insulators (TI) have been the subject of intense theoretical and experimental investigation due to their distinct electronic properties compared to conventional electronic systems. This thesis investigates electronic properties of two topological insulators, InAs/GaSb double quantum wells and monolayer WTe₂, through transport measurements at ultra-low temperatures. Using double gate geometry, InAs/GaSb quantum wells can be tuned between topological and trivial states. Previous works have reported the existence of robust helical edge conduction in the inverted regime. Here, we found an enhanced edge conduction in the trivial state with superficial similarity to the observed edge conduction in those reports. However, using various transport techniques and sample geometries, the edge conduction in our samples was found to have a non-helical origin.Another topological insulator that is studied in this thesis is monolayer WTe₂. Here, we report that monolayer WTe₂, already known to be a 2D TI, becomes a superconductor by mild electrostatic doping, at temperatures below 1K. The 2D TI-superconductor transition can be easily driven by applying a small gate voltage. Furthermore, we observed peculiar features such as enhancement of parallel critical magnetic field above the Pauli limit possibly, from spin orbit scattering.
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The properties of several two-dimensional (2d) materials are studied. Themain content is the study of magnetic impurities in graphene through phasecoherent transport phenomena, including weak localization (WL) and universalconductance fluctuations (UCF). Magnetic impurities manifest themselvesin the in-plane magnetic field and temperature dependence of the dephasing (phase breaking) rate. Our experiments unambiguously revealthe existence of magnetic impurities through the in-plane magnetic field dependence of WL andUCF. The properties of the magnetic impurities are further studied throughthe dephasing rate as a function of magnetic field and temperature. TheWL dephasing rate as a function of in-plane magnetic field shows a non-monotonic behaviour,which is rooted in the existence of magnetic impurities with a Landé g factordifferent from that of the free electron. The collapse of the dephasing rate asa function of temperature is a sign of the quenching of magnetic impurities,which could come from Kondo coupling between magnetic impurities andconduction electrons or Ruderman-Kittel-Kasuya-Yosida (RKKY) couplingbetween magnetic impurities. The implications of our graphene experimentsare two-fold: on the one hand, they provide new knowledge about the interplayamong magnetic moments and electrons in graphene; on the otherhand, they also established an effective tool to reveal the existence of magneticimpurities in 2d systems.In addition to graphene, we also studied the exfoliation and stabilityof thin sheets of FeSe -- a member of the Fe-based superconductorsfamily. Our Raman Spectroscopy, Atomic Force Microscopy, Optical Microscopyand Time-of-Flight-Secondary-Ion-Mass-Spectroscopy experimentsshow that FeSe nanosheets decay in air, precipitation of Se and oxidationlikely occurring during the decay process. Our transport measurements show that FeSe nanosheets exposed briefly to air can still retain superconductivity.
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The discovery of the integer quantum Hall effect (IQHE) and the fractional quantum Hall effect (FQHE) in a 2-dimensional electron gas (2 DEG ) have created a new and rich field in condensed matter physics in low dimensions. Almost 35 years after these discoveries, there are still several unanswered questions regarding the nature of various electronic phases formed in such systems. The 2 DEG in ultra-high mobility quantum well (QW) samples in large magnetic fields and millikelvin temperatures are studied in this thesis.We developed a reproducible recipe for enhancing the quality of the very fragile FQHE states reliably, which can be used to reset an electrically shocked sample in-situ at low temperatures. We then developed a protocol for measuring the local electronic density on QW samples using a single-electron transistor (SET). We also developed a technique for modulating the temperature of the sample at about 10Hz by about 10mK. We used the electrometer and the fast temperature modulator to obtain a measure of changes in chemical potential as a function of temperature oscillations. This quantity can reveal the existence of an enhanced entropy in the state of the electrons.We investigated theories that predict the non-Abelian state of matter, that follows neither Fermionic nor Bosonic statistics. Non-Abelian quasi-particles are expected to form as collective excitations in the fractional quantum Hall regime at filling factor ν=5/2. The experimental results were incompatible with the non-Abelian theory under investigation.We also studied the nature of localization of electrons in the bulk of the sample when the system is in one of the incompressible states near an IQHE plateau. The noise characteristics detected by the ultra-sensitive charge sensor implanted on the surface of the sample revealed new behaviours not observed in the past. The results can be explained by telegraph noise arising from charge carriers jumping from one localized potential pocket to another, evolving into 1/f noise, as the filling factor shifts away from the centre of the integer state.
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Electron-electron interactions inside of two dimensional electron gases (2DEG) in out-of-plane magnetic field and at very low temperatures under certain conditions can lead to electron localization in Wigner crystals or even more complex periodic structures. These states are usually referred to as electron solid phases and result in Reentrant Integer Quantum Hall Effect (RIQHE) in transport measurements. However, their microscopic description remains unclear, as insulating phases with different microscopic structure demonstrate indistinguishable macroscopic transport properties. In this work the transport of the electron solids is investigated away from equilibrium conditions. This approach allows to break an insulating state by application of significant current bias to the 2DEG. As bias current increases, longitudinal and Hall resistivities measured for these states show multiple sharp breakdown transitions. Whereas the high bias breakdown of fractional quantum Hall states is consistent with simple heating, the nature of RIQH breakdown remains to be a subject of a considerable debate. A comparison of RIQH breakdown characteristics at multiple voltage probes indicates that these signatures can be ascribed to a phase boundary between broken-down and unbroken regions, spreading chirally from source and drain contacts as a function of bias current and passing voltage probes one by one. It is shown, that the chiral sense of the spreading is not set by the chirality of the edge state itself, instead depending on electron- or hole-like character of the RIQH state. Although at high current bias the electron temperature is unmeasurable with standard techniques, the data shows that electron solid states appear to stay temperature sensitive even after the RIQH effect is destroyed. A comparison of temperature dependence and the spatial distribution of the Hall potential along the edge provides an evidence, that the bulk 2DEG remains insulating up to surprisingly high biases. Finally a metastable stripe phase around $\nu=9/2$ is investigated under non-equilibrium conditions in the sample with electron density, which is close to the stripe reorientation critical point. The anisotropy of non-equilibrium stripe phase under high current biases shows a strong dependence of the natural orientation of stripes on exact filling factor.
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This thesis investigates the effect of adatom deposition, especially alkali and heavyadatoms, on graphene’s electronic and transport properties. While there are manytheoretical predictions for tuning graphene’s properties via adatom deposition, onlya few of them have been observed. Solving this enigma of inconsistency betweentheory and experiment raises the need for deeper experimental investigation of thismatter. To achieve this goal, an experimental set up was built which enables us toevaporate different metal adatoms on graphene samples while they are at cryogenictemperatures and ultra-high vacuum (UHV) conditions.The critical role of in situ high-temperature annealing in creating reliable interactionsbetween adatoms and graphene is observed. This contradicts the commonlyaccepted assumption in the transport community that placing a graphene sample inUHV and performing in situ 400-500 K annealing is enough to provide a reliableadatom-graphene interaction. Even charge doping by alkali atoms (Li), which isarguably the simplest of all adatom effects, cannot be achieved completely withoutin situ 900 K annealing. This observation may explain the difficulty manygroups have faced in inducing superconductivity, spin-orbit interaction, or similarelectronic modifications to graphene by adatom deposition, and it points toward astraightforward, if experimentally challenging, solution.The first experimental evidence of short-range scattering due to alkali adatomsin graphene is presented in this thesis, a result that contradicts the naive expectationthat alkali adatoms on graphene only cause long-range Coulomb scattering.The induced short-range scattering by Li caused decline of intervalley time andlength (i.e., enhancement of intervalley scattering). No signatures of theoreticallypredicted superconductivity of Li doped graphene were observed down to 3 K. Cryogenic deposition of copper increased the dephasing rate of graphene. Thisincrease in dephasing rate is either a sign of inducing spin-orbit interaction or magneticmoments by copper. No similar effect was observed for indium.
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The phase coherent properties of electrons in low temperature graphene are measured and analyzed. I demonstrate that graphene is able to coherently transport spin-polarized electrons over micrometer distances, and prove that magnetic defects in the graphene sheet are responsible for limiting spin transport over longer distances. It is shown that these magnetic defects are also partly responsible for the high decoherence (phase loss) observed at low temperature, and that another (as yet unknown) non-magnetic mechanism is required to explain the remainder.Similar measurements are used to probe and characterize the size scales of the roughness of the graphene sheet. The effects of an in-plane magnetic field threading through the rough graphene sheet are analogous to the effects of the built-in strain; I argue that the observed large valley-dependent scattering rates are a consequence of this built-in strain.I also describe an original, robust technique for extracting coherence information from conductance fluctuations. The technique is demonstrated in experiments on graphene, used to efficiently detect the presence of magnetic defects. This new approach to studying phase coherence can be easily carried over to other mesoscopic semiconducting systems.
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Quantum point contacts (QPCs) are narrow constrictions between large reservoirs of two-dimensional electron gas, with conductance quantized in units of G=2e²/h at zero magnetic field. Despite decades of investigation, some conductance features of QPCs remain mysterious, such as an extra conductance plateau at 0.7(2e²/h) (0.7 structure) and a zero-bias peak (ZBP) in nonlinear conductance. In this thesis, we present experimental studies of transport anomalies in QPCs, aiming at shedding more light on these features. Conductance measurements are performed for ZBPs in a much wider range than in most previous work, focused especially on the low- and high-conductance regimes. The Kondo model and a model of subband motion are compared with experimental results, but both of them fall short of explaining the data. The subband-motion model is not spin-dependent, so it conflicts with the spin-related nature of ZBPs as confirmed by measurements of nuclear spin polarization in QPCs in an in-plane magnetic field. However, the motion of subbands and the spin dependence of these motions are clearly shown by thermopower spectroscopy. These results may help understand the origin of ZBPs and 0.7 structure.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Two dimensional (2D) van der Waals materials with a lattice mismatch or a relative twist angle, stacked atop each other forms a moiré superlattice. The electronic properties of such a system can be modified by controlling the relative twist angle between the layers, the most famous example being magic-angle twisted bilayer graphene. Since the observation of correlated insulator states and superconductivity in this flat band moiré system, investigations on other 2D moiré systems has gathered pace. Two Bernal stacked bilayer graphene sheets twisted relative to each other, i.e., twisted double bilayer graphene, gives the additional opportunity of tuning the electronic structure by a displacement electric field. This thesis presents the fabrication and electrical transport measurements of twisted double bilayer graphene devices. In order to compare the efficacy of various fabrication techniques, we fabricated the devices using two different modified dry transfer techniques: “tear-and-stack” and “cut-and-stack”. By calculating the twist angles from the electrical transport data, we find the “cut-and-stack” technique to give a better control of the twist angle of the fabricated devices. Subsequently, we studied the transport properties of a device with a twist angle of 1.39 degree, in order to replicate the electronic phase diagram of twisted double bilayer graphene devices. Tuning the displacement electric field and carrier density independently in a double gated geometry, we were able to observe correlated insulating states at quarter, half and three-quarter filling of the moiré band for a range of electric field values. The evolution of these insulating states in an in-plane magnetic field suggests spin/valley polarization. Additionally, the metallic states surrounding the correlated insulating state at half-filling displays a strong temperature dependence and nonlinear current-voltage characteristics, the nature of which remains ambiguous in our measurements.
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Graphene was expected to prove useful in the field of spintronics because a long spin relaxation time (few micro second) was theoretically expected. However, experimental results using exfoliated graphene have shown that the spin relaxation time is a few orders of magnitude less than the theoretical prediction. It was discovered that the reason for this unexpected shorter spin relaxation time is the presence of magnetic moments on graphene and magnetic moments exist on most forms of graphene. Many theoretical articles expected these magnetic moments to arise due to graphene defects. However, it is not experimentally clear where and how they arise. To answer where and how, we investigates it with dephasing rate (phase relaxation rate) monitored via weak localization on graphene, grown by chemical vapour deposition (CVD graphene). The experiments are performed on field-effect devices made from CVD graphene on various substrates under perpendicularly applied magnetic fields at 4.2 K. The samples are thermally annealed under various conditions, which is a commonplace technique used to clean the surface of graphene. Only the gas annealing induces the additional source of dephasing rate on CVD graphene. However, this could not be seen in before-annealed samples and vacuum annealed samples. Additional experiment confirms that this additional source on gas annealed sample has the magnetic property. The result on this thesis can help answer the origin of magnetic moments on graphene.
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