arXiv subjects

R. Thomas Weitz

Publications and source records attributed to R. Thomas Weitz.

At least 19 recordsLinked to original sources

arXiv2025

The role of non-equilibrium populations in dark exciton formation

In two-dimensional transition metal dichalcogenide structures, the optical excitation of a bright exciton may be followed by the formation of a plethora of lower energy dark states. In these formation and relaxation processes between different exciton species, non-equilibrium exciton and phonon populations play a dominant role, but remain so far largely unexplored as most states are inaccessible by regular spectroscopies. Here, on the example of homobilayer 2H-MoS$_2$, we realize direct access to the full exciton relaxation cascade from experiment and theory. By measuring the energy- and in-plane momentum-resolved photoemission spectral function, we reveal a distinct fingerprint for dark excitons in a non-equilibrium excitonic occupation distribution. In excellent agreement with microscopic many-particle calculations, we quantify the timescales for the formation of a non-equilibrium dark excitonic occupation and its subsequent thermalization to 85~fs and 150~fs, respectively. Our results provide a previously inaccessible view of the complete exciton relaxation cascade, which is of paramount importance for the future characterization of non-equilibrium excitonic phases and the efficient design of optoelectronic devices based on two-dimensional materials.

cond-mat.mes-hall↗

arXiv2025

Landauer resistivity dipole at one dimensional defect revealed via near-field photocurrent nanoscopy

The fundamental question how to describe Ohmic resistance at the nanoscale has been answered by Landauer in his seminal picture of the so-called Landauer resistivity dipole. This picture has been theoretically well understood, however experimentally there are only few studies due to the need for a non-invasive local probe. Here we use the nanometer lateral resolution of near-field photocurrent imaging to thoroughly characterize a buried monolayer - bilayer graphene interface as an ideal one dimensional defect for the Landauer resistivity dipole. Via systematic tuning of the overall charge carrier density and the current flow we are able to detect the formation of Landauer resistivity dipoles due to charge carrier accumulation around the one dimensional defects. We found that, for Fermi energy values near the charge neutrality point (i.e. at low hole or electron doping), the photocurrent exhibits the same polarity as the applied source-drain voltage, which is consistent with changes in carrier concentration induced by the Landauer resistivity dipoles. This signature is no longer evident at higher charge carrier density in agreement with the performed numerical calculations. Photocurrent nanoscopy can thus serve as non-invasive technique to study local dissipation at hidden interfaces.

cond-mat.mes-hall↗

arXiv2024

Hybrid Frenkel-Wannier excitons facilitate ultrafast energy transfer at a 2D-organic interface

Two-dimensional transition metal dichalcogenides (TMDs) and organic semiconductors (OSCs) have emerged as promising material platforms for next-generation optoelectronic devices. The combination of both is predicted to yield emergent properties while retaining the advantages of their individual components. In OSCs the optoelectronic response is typically dominated by localized Frenkel-type excitons, whereas TMDs host delocalized Wannier-type excitons. However, much less is known about the spatial and electronic characteristics of excitons at hybrid TMD/OSC interfaces, which ultimately determine the possible energy and charge transfer mechanisms across the 2D-organic interface. Here, we use ultrafast momentum microscopy and many-body perturbation theory to elucidate a hybrid exciton at an TMD/OSC interface that forms via the ultrafast resonant Förster energy transfer process. We show that this hybrid exciton has both Frenkel- and Wannier-type contributions: Concomitant intra- and interlayer electron-hole transitions within the OSC layer and across the TMD/OSC interface, respectively, give rise to an exciton wavefunction with mixed Frenkel-Wannier character. By combining theory and experiment, our work provides previously inaccessible insights into the nature of hybrid excitons at TMD/OSC interfaces. It thus paves the way to a fundamental understanding of charge and energy transfer processes across 2D-organic heterostructures.

cond-mat.mes-hall↗

arXiv2024

Signatures of sliding Wigner crystals in bilayer graphene at zero and finite magnetic fields

AB-stacked bilayer graphene has emerged as a fascinating yet simple platform for exploring macroscopic quantum phenomena of correlated electrons. Unexpectedly, a phase with negative dR/dT has recently been observed when a large electric displacement field is applied and the charge carrier density is tuned to the vicinity of an ultra-low-density van Hove singularity. This phase exhibits features consistent with Wigner crystallization, including a characteristic temperature dependence and non-linear current bias behavior. However, more direct evidence for the emergence of an electron crystal in AB-stacked bilayer graphene at zero magnetic field remains elusive. Here we explore the low-frequency noise consistent with depinning and sliding of a Wigner crystal lattice. The current bias and frequency dependence of these noise spectra align well with findings from previous experimental and theoretical studies on the quantum electron solids. Our results offer transport signatures consistent with Wigner crystallization in AB-stacked bilayer graphene at zero and finite magnetic fields, paving the way for further substantiating an anomalous Hall crystal in its original form.

cond-mat.mes-hall↗

arXiv2024

Interlayer charge transfer in graphene 2D polyimide heterostructures

The vertical integration of multiple two-dimensional (2D) materials in heterostructures, held together by van der Waals forces, has opened unprecedented possibilities for modifying the (opto-)electronic properties of nanodevices. Graphene, with its remarkable opto-electronic properties, is an ideal candidate for such applications. Further candidates are 2D polymers, crystalline polymeric materials with customizable structure and electronic properties that can be synthesized in all mathematically possible Bravais lattices. In this study, we investigated the optoelectronic properties of a heterostructure created by pristine graphene and a rectangular 2D polyimide (2DPI) film. This imprints a new superlattice on graphene in conjunction with a direct influence on its electronic properties. Theoretical and experimental analyses reveal that interlayer charge exchange between the 2D polymer and graphene induces hole doping in the graphene layer. We have also observed that the properties of the heterostructure are dependent on the substrate used in experiments, likely due to the porous character of the 2DPI allowing direct interaction of graphene with the support. These findings highlight the unique ability to tailor functionalities in 2D polymers-based heterostructures, allowing the development of optoelectronic devices with precisely engineered properties and stimulating further exploration of the diverse phenomena accessible through tailored designs of the 2D polymers.

cond-mat.mes-hall↗

arXiv2024

Layer-selective spin-orbit coupling and strong correlation in bilayer graphene

Spin-orbit coupling (SOC) and electron-electron interaction can mutually influence each other and give rise to a plethora of intriguing phenomena in condensed matter systems. In pristine bilayer graphene, which has weak SOC, intrinsic Lifshitz transitions and concomitant van-Hove singularities lead to the emergence of many-body correlated phases. Layer-selective SOC can be proximity induced by adding a layer of tungsten diselenide (WSe2) on its one side. By applying an electric displacement field, the system can be tuned across a spectrum wherein electronic correlation, SOC, or a combination of both dominates. Our investigations reveal an intricate phase diagram of proximity-induced SOC-selective bilayer graphene. Not only does this phase diagram include those correlated phases reminiscent of SOC-free doped bilayer graphene, but it also hosts unique SOC-induced states allowing a compelling measurement of valley g-factor and a seemingly impossible correlated insulator at charge neutrality, thereby showcasing the remarkable tunability of the interplay between interaction and SOC in WSe2 enriched bilayer graphene.

cond-mat.mes-hall↗

arXiv2023

Probing the tunable multi-cone bandstructure in Bernal bilayer graphene

Controlling the bandstructure of Dirac materials is of wide interest in current research but has remained an outstanding challenge for systems such as monolayer graphene. In contrast, Bernal bilayer graphene (BLG) offers a highly flexible platform for tuning the bandstructure, featuring two distinct regimes. In one regime, which is well established and widely used, a tunable bandgap is induced by a large enough transverse displacement field. Another is a gapless metallic band occurring near charge neutrality and at not too strong fields, featuring rich 'fine structure' consisting of four linearly-dispersing Dirac cones with opposite chiralities in each valley and van Hove singularities. Even though BLG was extensively studied experimentally in the last two decades, the evidence of this exotic bandstructure is still elusive, likely due to insufficient energy resolution. Here, rather than probing the bandstructure by direct spectroscopy, we use Landau levels as markers of the energy dispersion and carefully analyze the Landau level spectrum in a regime where the cyclotron orbits of electrons or holes in momentum space are small enough to resolve the distinct mini Dirac cones. We identify the presence of four distinct Dirac cones and map out complex topological transitions induced by electric displacement field. These findings introduce a valuable addition to the toolkit for graphene electronics.

cond-mat.mes-hall↗

arXiv2023

What can the activation energy tell about the energetics at grain boundaries in polycrystalline organic films?

Charge-carrier transport at the semiconductor-gate dielectric interface in organic field-effect transistors is critically dependent on the degree of disorder in the typically semi-crystalline semiconductor layer. Charge trapping can occur at the interface as well as in the current-carrying semiconductor layer itself. A detailed and systematic understanding of the role of grain boundaries between crystallites and how to avoid their potentially detrimental effects is still an important focus of research in the organic electronics community. A typical macroscopic measurement technique to extract information about the energetics of the grain boundaries is an activation energy measurement. Here, we compare detailed experiments on the energetic properties of monolayer thin films implemented in organic field-effect transistors, having controlled numbers of grain boundaries within the channel region to kinetic Monte-Carlo simulations of charge-carrier transport to elucidate the influence of grain boundaries on the extracted activation energies. Two important findings are: 1) whereas the energy at the grain boundary does not change with the number of grain boundaries in a thin film, both the measured and simulated activation energy increases with the number of grain boundaries. 2) In simulations where both energy barriers and valleys are present at the grain boundaries there is no systematic relation between the number of grain boundaries and extracted activation energies. We conclude, that a macroscopic measurement of the activation energy can serve as general quality indicator of the thin film, but does not allow microscopic conclusions about the energy landscape of the thin film.

cond-mat.mtrl-sci↗

arXiv2023

Interaction-driven (quasi-) insulating ground states of gapped electron-doped bilayer graphene

Bernal bilayer graphene has recently been discovered to exhibit a wide range of unique ordered phases resulting from interaction-driven effects and encompassing spin and valley magnetism, correlated insulators, correlated metals, and superconductivity. This letter reports on a novel family of correlated phases characterized by spin and valley ordering, observed in electron-doped bilayer graphene. The novel correlated phases demonstrate an intriguing non-linear current-bias behavior at ultralow currents that is sensitive to the onset of the phases and is accompanied by an insulating temperature dependence, providing strong evidence for the presence of unconventional charge carrying degrees of freedom originating from ordering. These characteristics cannot be solely attributed to any of the previously reported phases, and are qualitatively different from the behavior seen previously on the hole-doped side. Instead, our observations align with the presence of charge- or spin-density-waves state that open a gap on a portion of the Fermi surface or fully gapped Wigner crystals. The resulting new phases, quasi-insulators in which part of the Fermi surface remains intact or valley-polarized and valley-unpolarized Wigner crystals, coexist with previously known Stoner phases, resulting in an exceptionally intricate phase diagram.

cond-mat.str-el↗

arXiv2023

Ferroelectric and anomalous quantum Hall states in bare rhombohedral trilayer graphene

Nontrivial interacting phases can emerge in elementary materials. As a prime example, continuing advances in device quality have facilitated the observation of a variety of spontaneous quantum Hall-like states, a cascade of Stoner-like magnets, and an unconventional superconductor in bilayer graphene. Its natural extension, rhombohedral trilayer graphene is predicted to be even more susceptible to interactions given its even flatter low-energy bands and larger winding number. Theoretically, five spontaneous quantum Hall phases have been proposed to be candidate ground states. Here, we provide transport evidence for observing four of the five competing ordered states in interaction-maximized, dually-gated, rhombohedral trilayer graphene. In particular, at vanishing but finite magnetic fields, two states with Chern numbers 3 and 6 can be stabilized at elevated and low electric fields, respectively, and both exhibit clear magnetic hysteresis. We also reveal that the quantum Hall ferromagnets of the zeroth Landau level are ferroelectrics with spontaneous layer polarizations even at zero electric field, as evidenced by electric hysteresis. Our findings exemplify the possible birth of rich interacting electron physics in a simple elementary material.

cond-mat.mes-hall↗

arXiv2023

A Cu3BHT-Graphene van der Waals Heterostructure with Strong Interlayer Coupling

Two dimensional van der Waals heterostructures (2D are of significant interest due to their intriguing physical properties that are critically defined by the constituent monolayers and their interlayer coupling . However, typical inorganic 2 D vdWhs fall into the weakly coupled region, limiting efficient interfacial charge flow crucial for developing high performance quantum opto electronics. Here, we demonstrate strong interlayer coupling in Cu3 BHT (BHT = benzenehexathiol) graphene vdWhs an organic inorganic bilayer characterized by prominent interlayer charge transfer Monolayer Cu3 BHT with a Kagome lattice is synthesized on the water surface and then coupled with graphene to produce a cm2 scale 2D vdWh. Spectroscopic and electrical studies, along with theoretical calculation s show significant hole transfer from monolayer Cu3 BHT to graphene upon contact , being characteristic fingerprints for strong interlayer coupling This study unveils the great potential of integrating highly pi-conjugated 2D coordination polymers (2DCPs) into 2D vdWhs to explor e intriguing physical phenomena.

physics.app-ph↗

arXiv2023

Ultrafast nano-imaging of dark excitons

The role and impact of spatial heterogeneity in two-dimensional quantum materials represents one of the major research quests regarding the future application of these materials in optoelectronics and quantum information science. In the case of transition-metal dichalcogenide heterostructures, in particular, direct access to heterogeneities in the dark-exciton landscape with nanometer spatial and ultrafast time resolution is highly desired, but remains largely elusive. Here, we introduce ultrafast dark field momentum microscopy to spatio-temporally resolve dark exciton formation dynamics in a twisted WSe$_2$/MoS$_2$ heterostructure with 55 femtosecond time- and 500~nm spatial resolution. This allows us to directly map spatial heterogeneity in the electronic and excitonic structure, and to correlate these with the dark exciton formation and relaxation dynamics. The benefits of simultaneous ultrafast nanoscale dark-field momentum microscopy and spectroscopy is groundbreaking for the present study, and opens the door to new types of experiments with unprecedented spectroscopic and spatiotemporal capabilities.

cond-mat.mes-hall↗

arXiv2023

Ultrafast dynamics of bright and dark excitons in monolayer WSe$_2$ and heterobilayer WSe$_2$/MoS$_2$

The energy landscape of optical excitations in mono- and few-layer transition metal dichalcogenides (TMDs) is dominated by optically bright and dark excitons. These excitons can be fully localized within a single TMD layer, or the electron- and the hole-component of the exciton can be charge-separated over multiple TMD layers. Such intra- or interlayer excitons have been characterized in detail using all-optical spectroscopies, and, more recently, photoemission spectroscopy. In addition, there are so-called hybrid excitons whose electron- and/or hole-component are delocalized over two or more TMD layers, and therefore provide a promising pathway to mediate charge-transfer processes across the TMD interface. Hence, an in-situ characterization of their energy landscape and dynamics is of vital interest. In this work, using femtosecond momentum microscopy combined with many-particle modeling, we quantitatively compare the dynamics of momentum-indirect intralayer excitons in monolayer WSe$_2$ with the dynamics of momentum-indirect hybrid excitons in heterobilayer WSe$_2$/MoS$_2$, and draw three key conclusions: First, we find that the energy of hybrid excitons is reduced when compared to excitons with pure intralayer character. Second, we show that the momentum-indirect intralayer and hybrid excitons are formed via exciton-phonon scattering from optically excited bright excitons. And third, we demonstrate that the efficiency for phonon absorption and emission processes in the mono- and the heterobilayer is strongly dependent on the energy alignment of the intralayer and hybrid excitons with respect to the optically excited bright exciton. Overall, our work provides microscopic insights into exciton dynamics in TMD mono- and bilayers.

cond-mat.mes-hall↗

arXiv2023

Probing correlations in the exciton landscape of a moiré heterostructure

Excitons are two-particle correlated bound states that are formed due to Coulomb interaction between single-particle holes and electrons. In the solid-state, cooperative interactions with surrounding quasiparticles can strongly tailor the exciton properties and potentially even create new correlated states of matter. It is thus highly desirable to access such cooperative and correlated exciton behavior on a fundamental level. Here, we find that the ultrafast transfer of an exciton's hole across a type-II band-aligned moiré heterostructure leads to a surprising sub-200-fs upshift of the single-particle energy of the electron being photoemitted from the two-particle exciton state. While energy relaxation usually leads to an energetic downshift of the spectroscopic signature, we show that this unusual upshift is a clear fingerprint of the correlated interactions of the electron and hole parts of the exciton quasiparticle. In this way, time-resolved photoelectron spectroscopy is straightforwardly established as a powerful method to access exciton correlations and cooperative behavior in two-dimensional quantum materials. Our work highlights this new capability and motivates the future study of optically inaccessible correlated excitonic and electronic states in moiré heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

Formation of moiré interlayer excitons in space and time

Moiré superlattices in atomically thin van-der-Waals heterostructures hold great promise for an extended control of electronic and valleytronic lifetimes, the confinement of excitons in artificial moiré lattices, and the formation of novel exotic quantum phases. Such moiré-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure. In order to exploit the full potential of correlated moiré and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement in the moiré potential is indispensable. However, direct experimental access to these parameters is limited since most excitonic quasiparticles are optically dark. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moiré interlayer excitons. We find that interlayer excitons are dominantly formed on the sub-50~fs timescale via interlayer tunneling at the K valleys of the Brillouin zones. In addition, we directly measure energy-momentum fingerprints of the moiré interlayer excitons by mapping their spectral signatures within the mini Brillouin zone that is built up by the twisted heterostructure. From these momentum-fingerprints, we gain quantitative access to the modulation of the exciton wavefunction within the moiré potential in real-space. Our work provides the first direct access to the interlayer moiré exciton formation dynamics in space and time and reveals new opportunities to study correlated moiré and exciton physics for the future realization of exotic quantum phases of matter.

cond-mat.mes-hall↗

arXiv2021

Quantum cascade of new correlated phases in trigonally warped bilayer graphene

Divergent density of states offers the unique opportunity to explore a wide variety of correlated electron physics. In the thinnest limit, this has been predicted and verified in the ultra-flat bands of magic-angle twisted bilayer graphene, the band touching points of few-layer rhombohedral graphite, and the lightly doped rhombohedral trilayer graphene. The simpler and seemingly better understood Bernal bilayer graphene is also susceptible to orbital magnetism-driven phases at charge neutrality, such as layer antiferromagnet and quantum anomalous Hall octet. Here we report the discovery of a cascade of novel correlated phases in the vicinity of electric-field-controlled Lifshitz transitions and van Hove singularities in trigonally warped bilayer graphene. We provide compelling evidence for the observation of Stoner ferromagnets - half and quarter metals. More prominently, we identify signatures consistent with a topologically nontrivial Wigner-Hall crystal at zero magnetic field and its transition to a trivial Wigner crystal, as well as two correlated metals whose behavior deviates from standard Fermi liquids. Our results in this reproducible, tunable, simple system opens a new chapter for studying strongly correlated electrons.

cond-mat.mes-hall↗

arXiv2021

Interplay between topological protected valley and quantum Hall edge transport

An established way of realizing topologically protected states in a two-dimensional electron gas is by applying a perpendicular magnetic field thus creating quantum Hall edge channels. In electrostatically gapped bilayer graphene intriguingly, even in the absence of a magnetic field topologically protected electronic states can emerge at naturally occurring stacking domain walls. While individually both types of topologically protected states have been investigated, their intriguing interplay remains poorly understood. Here, we focus on the interplay between topological domain wall states and quantum Hall edge transport within the eight-fold degenerate zeroth Landau level of high-quality suspended bilayer graphene. We find that the two-terminal conductance remains approximately constant for low magnetic fields throughout the distinct quantum Hall states since the conduction channels are traded between domain wall and device edges. For high magnetic fields, however, we observe evidence of transport suppression at the domain wall, which can be attributed to the emergence of spectral minigaps. This indicates that stacking domain walls do potentially do not correspond to a topological domain wall in the order parameter.

cond-mat.mes-hall↗

arXiv2021

Tunable quantum anomalous Hall octet driven by orbital magnetism in bilayer graphene

The quantum anomalous Hall (QAH) effect - a macroscopic manifestation of chiral band topology at zero magnetic field - has only been experimentally realized by magnetic doping of topological insulators (1 - 3) and delicate design of Moire heterostructures (4 - 8). However, the seemingly simple bilayer graphene without magnetic doping or Moire engineering has long been predicted to host competing ordered states with QAH effects (9 - 11). Here, we explore states in bilayer graphene with conductance of 2 e2/h that not only survive down to anomalously small magnetic fields and up to temperatures of 5 K, but also exhibit magnetic hysteresis. Together, the experimental signatures provide compelling evidence for orbital magnetism driven QAH behavior with a Chern number tunable via electric and magnetic fields as well as carrier sign. The observed octet of QAH phases is distinct from previous observations due to its peculiar ferrimagnetic and ferrielectric order that is characterized by quantized anomalous charge, spin, valley, and spin-valley Hall behavior.

cond-mat.mes-hall↗