12 research outputs found
Coexistence of Tri-Hexagonal and Star-of-David Pattern in the Charge Density Wave of the Kagome Superconductor AVSb
The recently discovered layered kagome metals AVSb(A=K, Rb, Cs) have
attracted much attention because of their unique combination of
superconductivity, charge density wave (CDW) order, and nontrivial band
topology. The CDW order with an in-plane 2x2 reconstruction is found to exhibit
exotic properties, such as time-reversal symmetry breaking and rotational
symmetry breaking. However, the nature of the CDW, including its
dimensionality, structural pattern, and effect on electronic structure, remains
elusive despite intense research efforts. Here, using angle-resolved
photoemission spectroscopy, we unveil for the first time characteristic
double-band splittings and band reconstructions, as well as the band gap
resulting from band folding, in the CDW phase. Supported by density functional
theory calculations, we unambiguously show that the CDW in AVSb
originates from the intrinsic coexistence of Star-of-David and Tri-Hexagonal
distortions. The alternating stacking of these two distortions naturally leads
to three-dimensional 2x2x2 or 2x2x4 CDW order. Our results provide crucial
insights into the nature and distortion pattern of the CDW order, thereby
laying down the basis for a substantiated understanding of the exotic
properties in the family of AVSb kagome metals
Novel phases in long-range many-body systems
Many-body systems with long-range power-law decaying potentials among their microscopic components are known to fall outside the traditional framework of complex systems. While some peculiarities in their behavior had emerged since the
very early days of statistical mechanics, the possibility of engineering such systems
in experimental setups based on molecular and optical systems has stimulated an im-
pressive theoretical activity in the last decades, unveiling a plethora of new physical
phenomena. In particular, depending on the exact form of the couplings, long-range-
interacting systems may avoid thermalization, remaining trapped in long-lived metastable phases, or exhibit new equilibrium phases at thermal equilibrium. The study of such novel phases, emerging both in the equilibrium behavior and in the dynamics, is the subject of the present thesis. In particular, in Part I the equilibrium behavior of long-range XY model and long-range Villain model are studied, while in the second part the quantum dynamics of long-range spin chains, both isolated and subjected to periodic driving, is investigated
Electrical and thermal transport properties of kagome metals AVSb (A=K, Rb, Cs)
The interplay between lattice geometry, band topology and electronic
correlations in the newly discovered kagome compounds AVSb (A=K, Rb,
Cs) makes this family a novel playground to investigate emergent quantum
phenomena, such as unconventional superconductivity, chiral charge density wave
and electronic nematicity. These exotic quantum phases naturally leave
nontrivial fingerprints in transport properties of AVSb, both in
electrical and thermal channels, which are prominent probes to uncover the
underlying mechanisms. In this brief review, we highlight the unusual
electrical and thermal transport properties observed in the unconventional
charge ordered state of AV3Sb5, including giant anomalous Hall, anomalous
Nernst, ambipolar Nernst and anomalous thermal Hall effects. Connections of
these anomalous transport properties to time-reversal symmetry breaking,
topological and multiband fermiology, as well as electronic nematicity, are
also discussed. Finally, a perspective together with challenges of this rapid
growing field are given.Comment: 34 pages,9 figures,an review article published in Tungsten
5,300(2023
Swampland conjectures as generic predictions of quantum gravity
The swampland program aims at answering the question of whether and how effective quantum field theories coupled to gravity can be UV-completed. Our limited understanding of the issues that can arise in this process is reflected in a steadily growing zoo of swampland conjectures. These are supposed to be necessary criteria for such a UV-completion and are motivated from our understanding of black hole thermodynamics, holography and string theory.
The swampland conjectures can potentially have dramatic implications for physical models of real-world phenomena if they are proven in string theory. For example, they could rule out large field inflation, a cosmological constant, or non-vanishing masses of the standard model photon and graviton. The purpose of this work is to contribute to a better understanding of these conjectures by testing them in various corners of string theory. Furthermore, we reveal a complicated network of relations between the swampland conjectures. This hints at the existence of a deeper underlying structure, which is yet to be fully uncovered.
One of the suggested swampland conjectures is the distance conjecture. It states that effective field theories have a finite range of validity in scalar field space, after which they necessarily break down due to an infinite tower of states becoming light. We quantify this range and identify the tower of states in the context of moduli spaces of Calabi-Yau compactifications with N = 2 supersymmetry. We claim that an analogous tower of states appears also in the limit where we send the mass of a spin-2 field to zero. We concretize this expectation in form of a spin-2 swampland conjecture. Finally, we investigate the question of whether the KKLT construction of de Sitter vacua in string theory is consistent. In this way, we challenge a recently proposed de Sitter swampland conjecture, which claims that de Sitter space cannot be a vacuum of string theory
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
Nonstandard finite-size effects at discontinuous phase transitions: Degenerate low-temperature states and boundary conditions
In dieser Dissertation wird das Skalenverhalten derĂśbergangstemperatur
von Systemen an diskontinuierlichen Phasenübergängen aus einem Zwei-
Zustands-Modell abgeleitet und erweitert. Es wird erläutert, wie sich
das Skalenverhalten für periodische Randbedingungen drastisch verändern
kann, sobald der Entartungsgrad der geordneten Phasen von der
Teilchenzahl abhängt. Eswerden Modellsysteme in zwei und drei Dimensionen
betrachtet, deren Zustandssummen mittels analytischer, kombinatorischer
Argumente berechnet werden. FĂĽr das kompliziertere, isotrope
Plaquettemodell in drei Dimensionen können durch diese Rechnungen
Ordnungsparameter definiert werden. Diese werden, zusammen mit dem
veränderten Skalenverhalten selbskonsistent durch anspruchsvolle und
hochpräzise, sogenannte multikanonische Monte-Carlo Simulationen
überprüft und bestätigt
Swampland conjectures as generic predictions of quantum gravity
The swampland program aims at answering the question of whether and how effective quantum field theories coupled to gravity can be UV-completed. Our limited understanding of the issues that can arise in this process is reflected in a steadily growing zoo of swampland conjectures. These are supposed to be necessary criteria for such a UV-completion and are motivated from our understanding of black hole thermodynamics, holography and string theory.
The swampland conjectures can potentially have dramatic implications for physical models of real-world phenomena if they are proven in string theory. For example, they could rule out large field inflation, a cosmological constant, or non-vanishing masses of the standard model photon and graviton. The purpose of this work is to contribute to a better understanding of these conjectures by testing them in various corners of string theory. Furthermore, we reveal a complicated network of relations between the swampland conjectures. This hints at the existence of a deeper underlying structure, which is yet to be fully uncovered.
One of the suggested swampland conjectures is the distance conjecture. It states that effective field theories have a finite range of validity in scalar field space, after which they necessarily break down due to an infinite tower of states becoming light. We quantify this range and identify the tower of states in the context of moduli spaces of Calabi-Yau compactifications with N = 2 supersymmetry. We claim that an analogous tower of states appears also in the limit where we send the mass of a spin-2 field to zero. We concretize this expectation in form of a spin-2 swampland conjecture. Finally, we investigate the question of whether the KKLT construction of de Sitter vacua in string theory is consistent. In this way, we challenge a recently proposed de Sitter swampland conjecture, which claims that de Sitter space cannot be a vacuum of string theory