| dc.description.abstract | Since the discovery of graphene in the early 2000s, two-dimensional (2D) layered quantum materials have revolutionised the field of solid-state and condensed matter physics by enabling the exploration of strongly interacting and highly tunable quantum systems. Graphene, in particular, hosts massless Dirac fermions and displays unconventional electrical and thermal transport properties, especially near charge neutrality—i.e., the Dirac point. Close to this point, transport properties are expected to be universal in nature, mediated by strongly correlated electron-hole interactions and governed by quantum-critical fluctuations. However, in low-mobility graphene samples, these effects are suppressed due to significant charge inhomogeneity near the Dirac point. Accessing this regime experimentally requires exceptionally clean samples with high electron mobility. This thesis is motivated by the need to understand both electrical and thermal transport in ultra-clean, high-quality graphene samples where conventional quasiparticle-based theories fail and transport is instead dictated by collective, hydrodynamic-like phenomena. For the convenience of the readers, the thesis is organised into ten chapters, providing a structured progression from fundamental concepts to cutting-edge experimental results.
Chapter 1 provides the foundational context for this work and introduces the key two-dimensional materials. In addition to this, it also establishes the central scientific questions under investigation and motivates the inquiry into the rich physics of disorder-free pristine electron systems.
Chapter 2 details the experimental methodology. This chapter offers a comprehensive overview of our optimised device fabrication protocol and the bespoke measurement instrumentation developed for this research. A central focus is placed on electron noise thermometry, establishing the theoretical and experimental framework for how this technique uniquely probes momentum-conserving scattering processes.
Chapter 3 serves as a critical validation of our experimental approach. In this section, we present electron noise thermometry as a sensitive probe of thermal transport in van der Waals heterostructures of 2D materials. As a proof of concept, we apply a variant of this technique —- Johnson-Nyquist noise thermometry (JNT) -— to a key problem in the field of optoelectronics: out-of-plane energy transfer in vdW heterostructures. Using an optimised device fabrication technique, we prepare graphene-TMD heterostructures with varying degrees of interfacial electronic coupling and extract the out-of-plane thermal conductance, highlighting the roles of interlayer charge and energy transfer. Through simultaneous photocurrent and electronic temperature measurements, we demonstrate the power of JNT as a direct probe of energy relaxation, showcasing that its sensitivity is not restricted by the presence of disorder. This firmly establishes JNT as a powerful tool for probing disorder-independent energy relaxation pathways in 2D systems.
Having established our methods, Chapter 4 addresses a central theme of this thesis: charge transport in state-of-the-art high-mobility graphene. We begin by rigorously quantifying the ultra-high quality of our devices using a variety of parameters estimated using electrical transport, followed by an in-depth study of the hydrodynamic electron flow in graphene, mapping its behaviour across a wide phase space of carrier density and temperature. Further, this chapter also presents multiple experimental signatures of hydrodynamic electron flow, thereby allowing for a direct evaluation of the shear viscosity of the Dirac fluid near charge neutrality.
Chapter 5 delves into the device-dependent variability in electrical properties of the electron fluid in graphene away from quantum criticality. A key experimental finding presented here is the observation of negative local resistance, which is widely considered a signature of non-diffusive electron flow. In contrast to previous experimental studies, we argue that the origin of this experimental feature is a direct consequence of the interplay between momentum-conserving and momentum-relaxing scattering in our ultra-clean devices. Further, using a phenomenological expression, inspired by the Landauer-Büttiker formalism, we investigate the shear viscosity in high-density electron fluids, which exhibits a peculiar temperature and density dependence, hinting towards the onset of tomographic electron fluidity.
Chapter 6 transitions from charge transport studies to the exploration of quantum heat transport in pristine graphene. Using JNT, we measure the electronic temperature under low electric fields and probe deviations from the Wiedemann–Franz law—a hallmark of strongly interacting electron fluids. The observed giant violations of the Wiedemann–Franz law align with the relativistic hydrodynamic description of a non-Galilean-invariant fluid and offer critical insights into the thermal enthalpy and entropy densities of quantum-critical graphene. By combining our electrical and thermal transport data, we provide compelling evidence for the quantisation of conductivity at the Dirac point, which can be interpreted as a direct manifestation of quantum criticality. We also evaluate the viscosity-to-entropy density ratio for the Dirac fluid in graphene, which approaches the holographic lower bound in the quantum-critical regime, in line with theoretical predictions for the Dirac fluid in quark-gluon plasma.
Chapters 7 and 8 explore the non-equilibrium regime in ultra-clean graphene by simultaneously monitoring charge and heat flow under the application of large current biases. In Chapter 7, we observe non-linear current-voltage characteristics and differential resistance measurements, which exhibit peculiar features like negative differential resistance, zero-voltage offsets, etc. We discuss the possible physical mechanisms, drawing parallels from theoretical results attributing such features to a mesoscopic description of Schwinger pair production. These findings are further corroborated in Chapter 8, which details electrical noise measurements under strong bias, revealing a distinct anomaly near the Dirac point that signals a departure from equilibrium dynamics. This unusual behaviour of the Dirac fluid under strong driving conditions may serve as a signature of current-driven instability or the onset of turbulent flow in graphene-based electron fluids.
Chapter 9 offers a forward-looking perspective, outlining promising future research directions and the broader implications of this work for quantum condensed matter physics.
Finally, Chapter 10 summarises all the key results and findings from the preceding chapters. In summary, this thesis investigates the quantum-critical transport properties of ultraclean graphene through a combination of DC electrical transport and electron noise thermometry. The results presented here establish ultra-clean graphene as a compelling platform for the simultaneous realisation of quantum-critical phenomena and exotic electron flow patterns. | en_US |
