Transport in two-dimensional topological materials: recent developments in experiment and theory

We review theoretical and experimental highlights in transport in two-dimensional materials focussing on key developments over the last five years. Topological insulators are finding applications in magnetic devices, while Hall transport in doped samples and the general issue of topological protection remain controversial. In transition metal dichalcogenides valley-dependent electrical and optical phenomena continue to stimulate state-of-the-art experiments. In Weyl semimetals the properties of Fermi arcs are being actively investigated. A new field, expected to grow in the near future, focuses on the non-linear electrical and optical responses of topological materials, where fundamental questions are once more being asked about the intertwining roles of the Berry curvature and disorder scattering. In topological superconductors the quest for chiral superconductivity, Majorana fermions and topological quantum computing is continuing apace.Comment: Topical review commissioned by 2D Materials, 57 pages, 33 figures. Your suggestions and comments are welcom

Highly Ordered Organic Layers and Wires

This thesis deals with the synthesis of highly ordered organic thin films and the characterization of the molecule-substrate interaction through spectroscopy and diffraction. Organic devices, such as organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs) have become ubiquitous in modern times. The transistor and high frequency performance of such organic devices crucially depends on charge carrier mobility. In inorganic semiconductors, which are bonded covalently, the band masses are typically lower and their crystallinity higher in comparison to their organic counterparts. In a simple Drude model, the charge carrier mobility is inversely proportional to the effective charge carrier mass. The low effective mass and high crystallinity of inorganic semiconductors result in large carrier mobilities of up to 1E7 cm^2/Vs for GaAs at low temperatures. The large effective mass in van der Waals bonded organic materials and their poorer molecular order decrease the carrier mobility. This thesis addresses the limitations of the inherently low mobility and disorder in organic thin films in a twofold way. The first is the introduction of a novel synthesis method for graphene nanoribbons, which are covalently bonded long stripes of graphene. This new method, developed in this thesis, is based on laser induced photothermal heating. It allows for the synthesis of atomically precise graphene nanoribbons with a higher degree of control over the reaction than conventional methods and is shown to work in a multitude of different nanoribbon species. The growth takes place in an area that is solely governed by the spotsize of the incoming laser light (4 µm). This method has an advantage over present methods through the exact control of the growth kinetics with regards to chemical uniformity and local area distribution. Additionally, the physical properties and growth kinetics of photothermally grown graphene nanoribbons are investigated by means of Raman spectroscopy. In a second way, the growth of organic moire structures on a topological insulator is studied. We show the growth of C60 thin films on the topological insulator Bi4Te3 through electron diffraction and observe a moire pattern. This indicates very long range order in the form of a (4x4) on (9x9) superstructure that is observable on the entire 1x1 cm^2 sample surface. The growth of the structure is performed using molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) in ultra-high vacuum (UHV) conditions and the properties of the interface are studied using low energy electron diffraction (LEED), angle resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). We find that a C60 induced surface reconstruction and the softness of the underlying, layered topological insulator are responsible for the high order. The theoretical calculations find that the structure bonds mostly through physisorption and both the theory and band structure measurements show no perturbation of the electronic states of the topological insulator by the overlayer. Finally, we extend the concept of well ordered growth of organic thin films on topological insulators to superconducting alkali metal doped C60. These organic films are metallic at room temperature but turn into s-wave superconductors at a critical temperature of 28 K. The combination of this relatively high transition temperature in combination with the well defined growth opens up a new playground for both experimental and theoretical studies. The van der Waals bond nature of the interface protects the interface from alloying, which can be a problem for inorganic topological insulator--superconductor interfaces. We show a novel synthesis route for the growth of well ordered superconducting alkali metal doped fullerenes on the topological insulator Bi4Te3. The growth process is studied using LEED and ultra violet photoemission spectroscopy (UPS) and makes the phase pure synthesis of thin film Rb3C60 possible, which is crucial to avoid contamination through an insulating Rb6C60 phase. ARPES spectra confirm the intactness of the interface by measurements of both the Fermi surface of the topological insulator as well as the newly formed Rb3C60 metallic film

Entropy-driven inductance on the surface of a topological insulator

New opportunities for energy harvesting can be found in a century-old thought experiment known as Maxwell's demon, which suggests that energy can be extracted from information. Although systems resembling Maxwell's demon have been experimentally realized, scalability remains a challenge. A scalable solution can be found in the interfacial states of topological insulators, which could resemble Maxwell's demon due to the interaction between spin-momentum locked electrons and nuclear spins. In these systems, information - nuclear spin polarization - can be directly used to store and harvest energy. In this thesis, we focused on thin films of the three-dimensional topological insulator (Bi1-xSbx)2Te3 (BST), which can act as an ‘information engine’ to extract electrical work. Theoretical predictions reveal that the interaction between surface states and nuclear spins in BST leads to an inductive effect, directly coupled to the entropy of the system. This ‘entropic inductance’ has potential applications in microelectronics. Experiments with Hall bar devices fabricated from molecular beam epitaxy (MBE)-grown BST were conducted to investigate this phenomenon under various source-drain bias voltages and temperatures. Despite challenges with high current densities and Joule heating, the study demonstrated finite current-induced nuclear polarization in BST, but the expected inductive signal was small. To enhance the inductive signal, the study explored magnetic doping using vanadium-doped BST (VBST), and searched for the quantum spin Hall effect in ultrathin BST where diffusive losses would be reduced. In future research, the principle of the information engine can be applied to other material systems such as Fermi arc surface states, and future developments in electronic materials could therefore provide the key to a scalable information engine. <br/

STM probe on the surface electronic states of spin-orbit coupled materials

Thesis advisor: Vidya MadhavanSpin-orbit coupling (SOC) is the interaction of an electron's intrinsic angular momentum (spin) with its orbital momentum. The strength of this interaction is proportional to Z4 where Z is the atomic number, so generally it is stronger in atoms with higher atomic number, such as bismuth (Z=83) and iridium (Z=77). In materials composed of such heavy elements, the prominent SOC can be sufficient to modify the band structure of the system and lead to distinct phase of matter. In recent years, SOC has been demonstrated to play a critical role in determining the unusual properties of a variety of compounds. SOC associated materials with exotic electronic states have also provided a fertile platform for studying emergent phenomena as well as new physics. As a consequence, the research on these interesting materials with any insight into understanding the microscopic origin of their unique properties and complex phases is of great importance. In this context, we implement scanning tunneling microscopy (STM) and spectroscopy (STS) to explore the surface states (SS) of the two major categories of SOC involved materials, Bi-based topological insulators (TI) and Ir-based transition metal oxides (TMO). As a powerful tool in surface science which has achieved great success in wide variety of material fields, STM/STS is ideal to study the local density of states of the subject material with nanometer length scales and is able to offer detailed information about the surface electronic structure. In the first part of this thesis, we report on the electronic band structures of three-dimensional TIs Bi2Te3 and Bi2Se3. Topological insulators are distinct quantum states of matter that have been intensely studied nowadays. Although they behave like ordinary insulators in showing fully gapped bulk bands, they host a topologically protected surface state consisting of two-dimensional massless Dirac fermions which exhibits metallic behavior. Indeed, this unique gapless surface state is a manifestation of the non-trivial topology of the bulk bands, which is recognized to own its existence to the strong SOC. In chapter 3, we utilize quasiparticle interference (QPI) approach to track the Dirac surface states on Bi2Te3 up to ~800 meV above the Dirac point. We discover a novel interference pattern at high energies, which probably originates from the impurity-induced spin-orbit scattering in this system that has not been experimentally detected to date. In chapter 4, we discuss the topological SS evolution in (Bi1-xInx)2Se3 series, by applying Landau quantization approach to extract the band dispersions on the surface for samples with different indium content. We propose that a topological phase transition may occur in this system when x reaches around 5%, with the experimental signature indicating a possible formation of gapped Dirac cone for the surface state at this doping. In the second part of this thesis, we focus on investigating the electronic structure of the bilayer strontium iridate Sr3Ir2O7. The correlated iridate compounds belong to another domain of SOC materials, where the electronic interaction is involved as well. Specifically, the unexpected Mott insulating state in 5d-TMO Sr2IrO4 and Sr3Ir2O7 has been suggested originate from the cooperative interplay between the electronic correlations with the comparable SOC, and the latter is even considered as the driving force for the extraordinary ground state in these materials. In chapter 6, we carried out a comprehensive examination of the electronic phase transition from insulating to metallic in Sr3Ir2O7 induced by chemical doping. We observe the subatomic feature close to the insulator-to-metal transition in response with doping different carriers, and provide detailed studies about the local effect of dopants at particular sites on the electronic properties of the system. Additionally, the basic experimental techniques are briefly described in chapter 1, and some background information of the subject materials are reviewed in chapter 2 and chapter 5, respectively.Thesis (PhD) — Boston College, 2014.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics

STM probe on the surface electronic states of spin-orbit coupled materials

Thesis advisor: Vidya MadhavanSpin-orbit coupling (SOC) is the interaction of an electron's intrinsic angular momentum (spin) with its orbital momentum. The strength of this interaction is proportional to Z4 where Z is the atomic number, so generally it is stronger in atoms with higher atomic number, such as bismuth (Z=83) and iridium (Z=77). In materials composed of such heavy elements, the prominent SOC can be sufficient to modify the band structure of the system and lead to distinct phase of matter. In recent years, SOC has been demonstrated to play a critical role in determining the unusual properties of a variety of compounds. SOC associated materials with exotic electronic states have also provided a fertile platform for studying emergent phenomena as well as new physics. As a consequence, the research on these interesting materials with any insight into understanding the microscopic origin of their unique properties and complex phases is of great importance. In this context, we implement scanning tunneling microscopy (STM) and spectroscopy (STS) to explore the surface states (SS) of the two major categories of SOC involved materials, Bi-based topological insulators (TI) and Ir-based transition metal oxides (TMO). As a powerful tool in surface science which has achieved great success in wide variety of material fields, STM/STS is ideal to study the local density of states of the subject material with nanometer length scales and is able to offer detailed information about the surface electronic structure. In the first part of this thesis, we report on the electronic band structures of three-dimensional TIs Bi2Te3 and Bi2Se3. Topological insulators are distinct quantum states of matter that have been intensely studied nowadays. Although they behave like ordinary insulators in showing fully gapped bulk bands, they host a topologically protected surface state consisting of two-dimensional massless Dirac fermions which exhibits metallic behavior. Indeed, this unique gapless surface state is a manifestation of the non-trivial topology of the bulk bands, which is recognized to own its existence to the strong SOC. In chapter 3, we utilize quasiparticle interference (QPI) approach to track the Dirac surface states on Bi2Te3 up to ~800 meV above the Dirac point. We discover a novel interference pattern at high energies, which probably originates from the impurity-induced spin-orbit scattering in this system that has not been experimentally detected to date. In chapter 4, we discuss the topological SS evolution in (Bi1-xInx)2Se3 series, by applying Landau quantization approach to extract the band dispersions on the surface for samples with different indium content. We propose that a topological phase transition may occur in this system when x reaches around 5%, with the experimental signature indicating a possible formation of gapped Dirac cone for the surface state at this doping. In the second part of this thesis, we focus on investigating the electronic structure of the bilayer strontium iridate Sr3Ir2O7. The correlated iridate compounds belong to another domain of SOC materials, where the electronic interaction is involved as well. Specifically, the unexpected Mott insulating state in 5d-TMO Sr2IrO4 and Sr3Ir2O7 has been suggested originate from the cooperative interplay between the electronic correlations with the comparable SOC, and the latter is even considered as the driving force for the extraordinary ground state in these materials. In chapter 6, we carried out a comprehensive examination of the electronic phase transition from insulating to metallic in Sr3Ir2O7 induced by chemical doping. We observe the subatomic feature close to the insulator-to-metal transition in response with doping different carriers, and provide detailed studies about the local effect of dopants at particular sites on the electronic properties of the system. Additionally, the basic experimental techniques are briefly described in chapter 1, and some background information of the subject materials are reviewed in chapter 2 and chapter 5, respectively.Thesis (PhD) — Boston College, 2014.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics

STM probe on the surface electronic states of spin-orbit coupled materials

Thesis advisor: Vidya MadhavanSpin-orbit coupling (SOC) is the interaction of an electron's intrinsic angular momentum (spin) with its orbital momentum. The strength of this interaction is proportional to Z4 where Z is the atomic number, so generally it is stronger in atoms with higher atomic number, such as bismuth (Z=83) and iridium (Z=77). In materials composed of such heavy elements, the prominent SOC can be sufficient to modify the band structure of the system and lead to distinct phase of matter. In recent years, SOC has been demonstrated to play a critical role in determining the unusual properties of a variety of compounds. SOC associated materials with exotic electronic states have also provided a fertile platform for studying emergent phenomena as well as new physics. As a consequence, the research on these interesting materials with any insight into understanding the microscopic origin of their unique properties and complex phases is of great importance. In this context, we implement scanning tunneling microscopy (STM) and spectroscopy (STS) to explore the surface states (SS) of the two major categories of SOC involved materials, Bi-based topological insulators (TI) and Ir-based transition metal oxides (TMO). As a powerful tool in surface science which has achieved great success in wide variety of material fields, STM/STS is ideal to study the local density of states of the subject material with nanometer length scales and is able to offer detailed information about the surface electronic structure. In the first part of this thesis, we report on the electronic band structures of three-dimensional TIs Bi2Te3 and Bi2Se3. Topological insulators are distinct quantum states of matter that have been intensely studied nowadays. Although they behave like ordinary insulators in showing fully gapped bulk bands, they host a topologically protected surface state consisting of two-dimensional massless Dirac fermions which exhibits metallic behavior. Indeed, this unique gapless surface state is a manifestation of the non-trivial topology of the bulk bands, which is recognized to own its existence to the strong SOC. In chapter 3, we utilize quasiparticle interference (QPI) approach to track the Dirac surface states on Bi2Te3 up to ~800 meV above the Dirac point. We discover a novel interference pattern at high energies, which probably originates from the impurity-induced spin-orbit scattering in this system that has not been experimentally detected to date. In chapter 4, we discuss the topological SS evolution in (Bi1-xInx)2Se3 series, by applying Landau quantization approach to extract the band dispersions on the surface for samples with different indium content. We propose that a topological phase transition may occur in this system when x reaches around 5%, with the experimental signature indicating a possible formation of gapped Dirac cone for the surface state at this doping. In the second part of this thesis, we focus on investigating the electronic structure of the bilayer strontium iridate Sr3Ir2O7. The correlated iridate compounds belong to another domain of SOC materials, where the electronic interaction is involved as well. Specifically, the unexpected Mott insulating state in 5d-TMO Sr2IrO4 and Sr3Ir2O7 has been suggested originate from the cooperative interplay between the electronic correlations with the comparable SOC, and the latter is even considered as the driving force for the extraordinary ground state in these materials. In chapter 6, we carried out a comprehensive examination of the electronic phase transition from insulating to metallic in Sr3Ir2O7 induced by chemical doping. We observe the subatomic feature close to the insulator-to-metal transition in response with doping different carriers, and provide detailed studies about the local effect of dopants at particular sites on the electronic properties of the system. Additionally, the basic experimental techniques are briefly described in chapter 1, and some background information of the subject materials are reviewed in chapter 2 and chapter 5, respectively.Thesis (PhD) — Boston College, 2014.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics

Surface characterization of topological superconductor materials using scanning tunneling microscopy and spectroscopy

Realizing and understanding topological superconductors that can host exotic quasiparticles such as the Majorana fermions is currently an exciting challenge in condensed-matter physics. However, not many materials have been conclusively identified as topological superconductors. Here, we examine topological superconductivity in two different systems - the family of doped Bi2Se3 superconductors and in the heterostructure of a conventional superconductor [Pb(111) thin film] grown on a topological insulator TlBiSe2. We do so by using the surface-sensitive technique of scanning tunneling microscopy (STM) and spectroscopy (STS) under ultra high vacuum (UHV) and down to 350 mK. We present the growth and characterization of three different members of the doped Bi2Se3 family. In this class of materials, bulk superconducting properties consistently show a peculiar twofold symmetry that is only compatible with a gap structure that has odd-parity and hence is considered to be topological. However, we find that superconductivity on the surface of these crystals is not robust and this constitutes a formidable challenge for our goal. Nevertheless, we have made several unexpected observations in each case. On the superconducting area on the surface of CuxBi2Se3, we measure a tunneling spectrum that suggests that the density of states around the Fermi energy is gapped but with a twofold anisotropy. This anisotropy also manifests in a 10 percent difference in the average superconducting gap measured as a function of the orientation of the in-plane magnetic field with respect to the crystal lattice. The minima in the gap structure is found to coincide with a a crystallographic mirror plane. However, contrary to expectation that one should find a mirror-symmetry protected pair of point nodes, we find a minima and not a node in the gap structure since zero integrated density of states at the Fermi level is measured in the absence of a magnetic field. For a superconducting region on CPSBS, we observe elliptical areas of enhanced quasiparticle density of states due to magnetic vortices penetrating the sample when the external magnetic field is applied normal to the sample plane. The anisotropy in the vortex profile is shown to be a consequence of the anisotropic gap structure. However, we discover that the orientation of the gap minima here is rotated by 60 degree compared to the point nodes in the bulk and that the expected gap nodes are most likely lifted in this case as well. The above results are understood by employing a symmetry-based phenomenological analysis. The latter shows that to find out the expected gap structure on the surface one must consider the effective symmetry, in particular the broken inversion symmetry at the surface as well as any other differences in crystal symmetry on the surface in comparison to the bulk. Any change in the effective symmetry on the surface will allow for a change in the superconducting gap structure. In this case the observed lifting of the gap nodes and the rotation of the gap minima are consistent with the symmetry changes. Among the three members of the doped Bi2Se3 family considered here, SrxBi2Se3 is the only one to show a near 100 percent superconducting volume fraction in the bulk. However, on the surface of these crystals no gap in the density of states is observed around the Fermi energy, as long as clean metallic probe tips are used. Only when micron-sized flakes of the sample are transferred onto the STM probe tip during prolonged scanning a superconducting gap in the density of states appears. To rationalize this observation we have claimed that superconductivity in SrxBi2Se3 crystals does not extend to the surface when the topological surface state (TSS) is intact. The existence of the TSS causes the charge distribution to be different near the surface compared to the bulk, leading to band bending and a consequential local electric field, which can kill superconductivity at the surface. Therefore, in micro-flakes, where the TSS is likely destroyed due to strain from the mechanical transfer, superconductivity can be observed on the surface. The above hypothesis applies on the surface of CuxBi2Se3 as well since no evidence for the TSS is found in the superconducting regions, hinting at the absence of any surface band bending or local electric field. Moreover, majority of the surface on CuxBi2Se3} and CPSBS turned out to be nonsuperconducting, and for such areas we propose that band bending at the surface is tied to the suppression of the superconducting order parameter. Band bending and its impact on the superconducting properties have been discussed for cuprate superconductors where the relevant parameters of the Thomas-Fermi screening length (nanometers) and carrier density are similar to that of doped Bi2Se3. In parallel to our efforts on the doped Bi2Se3 superconductors, the heterostructure of Pb(111) thin film on TlBiSe2 is also investigated for signatures of two-dimensional topological superconductivity. In particular, we searched for a localized Majorana mode inside vortex cores as well as a dispersive one-dimensional Majorana mode at the physical edge of the two-dimensional system. However, no signature of such modes is detected for this system