Physik
Refine
Year of publication
Document Type
- Article (1346)
- Preprint (1077)
- Doctoral Thesis (370)
- Conference Proceeding (228)
- diplomthesis (106)
- Bachelor Thesis (66)
- Master's Thesis (49)
- Contribution to a Periodical (37)
- Working Paper (31)
- Book (28)
Keywords
- Kollisionen schwerer Ionen (47)
- heavy ion collisions (42)
- Quark-Gluon-Plasma (25)
- Heavy Ion Experiments (20)
- LHC (20)
- quark-gluon plasma (17)
- equation of state (16)
- QGP (15)
- Hadron (14)
- Zustandsgleichung (12)
Institute
- Physik (3424)
- Frankfurt Institute for Advanced Studies (FIAS) (1201)
- Informatik (959)
- Präsidium (62)
- ELEMENTS (15)
- Biochemie und Chemie (12)
- Biowissenschaften (11)
- Helmholtz International Center for FAIR (9)
- Geowissenschaften (7)
- Pharmazie (7)
This thesis contains three theoretical works about certain aspects of the interplay of electronic correlations and topology in the Hubbard model.
In the first part of this thesis, the applicability of elementary band representations (EBRs) to diagnose interacting topological phases, that are protected by spatial symmetries and time-reversal-symmetry, in terms of their single-particle Matsubara Green’s functions is investigated. EBRs for the Matsubara Green’s function in the zero-temperature limit can be defined via the topological Hamiltonian. It is found that the Green’s function EBR classification can only change by (i) a gap closing in the spectral function at zero frequency, (ii) the Green’s function becoming singular i.e. having a zero eigenvalue at zero frequency or (iii) the Green’s function breaking a protecting symmetry. As an example, the use of the EBRs for Matsubara Green’s functions is demonstrated on the Su-Schriefer-Heeger model with exact diagonalization.
In the second part the Two-Particle Self-Consistent approach (TPSC) is extended to include spin-orbit coupling (SOC). Time-reversal symmetry, that is preserved in the presence of SOC, is used to derive new TPSC self-consistency equations including SOC. SOC breaks spin rotation symmetry which leads to a coupling of spin and charge channel. The local and constant TPSC vertex then consists of three spin vertices and one charge vertex. As a test case to study the interplay of Hubbard interaction and SOC, the Kane-Mele-Hubbard model is studied. The antiferromagnetic spin fluctuations are the leading instability which confirms that the Kane-Mele-Hubbard model is an XY antiferromagnet at zero temperature. Mixed spin-charge fluctuations are found to be small. Moreover, it is found that the transversal spin vertices are more strongly renormalized than the longitudinal spin vertex, SOC leads to a decrease of antiferromagnetic spin fluctuations and the self-energy shows dispersion and sharp features in momentum space close to the phase transition.
In the third part TPSC with SOC is used to calculate the spin Hall conductivity in the Kane-Mele-Hubbard model at finite temperature. The spin Hall conductivity is calculated once using the conductivity bubble and once including vertex corrections. Vertex corrections for the spin Hall conductivity within TPSC corresponds to the analogues of the Maki-Thompson contributions which physically correspond to the excitation and reabsorption of a spin, a charge or a mixed spin-charge excitation by an electron. At all temperatures, the vertex corrections show a large contribution in the vicinity of the phase transition to the XY antiferromagnet where antiferromagnetic spin fluctuations are large. It is found that vertex corrections are crucial to recover the quantized value of −2e^2/h in the zero-temperature limit. Further, at non-zero temperature, increasing the Hubbard interaction leads to a decrease of the spin Hall conductivity. The results indicate that scattering of electrons off antiferromagnetic spin fluctuations renormalize the band gap. Decreasing the gap can be interpreted as an effective increase of temperature leading to a decrease of the spin Hall conductivity.
In this work I investigate two different systems - spin systems and charge-density-waves. The same theoretical method is used to investigate both types of system. My investigations are motivated by experimental investigations and the goal is to describe the experimental results theoretically. For this purpose I formulate kinetic equations starting from the microscopical dynamics of the systems.
First of all, a method is formulated to derive the kinetic equations diagrammatically. Within this method an expansion in equal-time connected correlation functions is carried out. The generating functional of connected correlations is employed to derive the method.
The first system to be investigated is a thin stripe of the magnetic insulator yttrium-iron-garnet (YIG). Magnons are pumped parametrically with an external microwave field. The motivation of my theoretical investigations is to explain the experimental observations. In a small parameter range close to the confluence field strength where confluence processes of two parametrically pumped magnons with the same wave vector becomes kinematically possible the efficiency of the pumping is reduced or enhanced depending on the pumping field strength. Because it is expected that that confluence and splitting processes of magnons are essential for the experimental observations I go beyond the kinetic theories that are conventionally applied in the context of parametric excitations in YIG and investigate the influence of cubic vertices on the parametric instability of magnons in YIG.
Furthermore, the influence of phonons is investigated. Usually in the literature these are taken into account as heat bath. Here, I want to explain experiments where an accumulation of magnetoelastic bosons - magnon-phonon-quasi-particles - has been observed. I employ the method of kinetic equations to investigate this phenomenon theoretically. The kinetic theory is able to reproduce the experimental observations and it is shown that the accumulation of magnetoelastic bosons is purely incoherent.
Finally, charge-density waves (CDW) in quasi-one-dimensional materials will be investigated. Charge-density waves emerge from a Peierls-instability and are a prime example for spontaneous symmetry breaking in solids. Again, the motivation for my theoretical investigations are an experiment where the spectrum of amplitude and phase phonon modes has been measured. Starting from the Fröhlich-Hamiltonian I derive kinetic equations and from these kinetic equations the equations of motion for the CDW order parameter can be derived. The frequencies and damping rates of amplitude and phase phonon modes will be derived from the linearized equations of motion. I compare my theory with existing methods. Furthermore, I also investigate the influence of Coulomb interaction.
In the novel stoichiometric iron-based material RbEuFe4As4 superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states; their coexistance is puzzling and represents a challenge for both experiment and theory. Using angle-resolved photoemission spectroscopy, resonant photoemission spectroscopy, Andreev reflection spectroscopy and scanning tunneling spectroscopy we have addressed this puzzle and unambigously shown that Fe- and Eu-derived states are largely decoupled and that superconducting and a long range magnetic orders exist almost independently from each other.
We use the topological heavy fermion (THF) model and its Kondo Lattice (KL) formulation to study the symmetric Kondo state in twisted bilayer graphene. Via a large-N approximation, we find a symmetric Kondo (SK) state in KL mode at fillings ν=0,±1,±2. In the SK state, all symmetries are preserved and the local moments are Kondo screened by the conduction electrons. At the mean-field level of the THF model at ν=0,±1,±2,±3, we also find a similar symmetric state. We study the stability of the symmetric state by comparing its energy with the ordered states and find the ordered states to have lower energy. However, moving away from integer fillings by doping holes to the light bands, we find the energy difference is reduced, which suggests the loss of ordering and a tendency towards Kondo screening. In order to include many-body effects beyond the mean-field approximation, we perform dynamical mean-field theory (DMFT) calculations on the THF model. We find the spin susceptibility follows a Curie behavior at ν=0,±1,±2 down to ∼2K where the onset of screening of the local moment becomes visible. This hints to very low Kondo temperatures at these fillings, in agreement with the outcome of our mean-field calculations. At non-integer filling ν=±0.5,±0.8,±1.2 DMFT shows deviations from a 1/T-susceptibility at much higher temperatures, suggesting a more effective screening of local moments with doping. Finally, we study the effect of a C3z-rotational-symmetry-breaking strain via mean-field approaches and find that a symmetric phase (that only breaks C3z symmetry) can be stabilized at sufficiently large strain at ν=0,±1,±2. Our results suggest that a symmetric Kondo phase is strongly suppressed at integer fillings, but could be stabilized either at non-integer fillings or by applying strain.
We demonstrate ultra-sharp (≲10 nm) lateral p-n junctions in graphene using electronic transport, scanning tunneling microscopy, and first principles calculations. The p-n junction lies at the boundary between differentially-doped regions of a graphene sheet, where one side is intrinsic and the other is charge-doped by proximity to a flake of α-RuCl3 across a thin insulating barrier. We extract the p-n junction contribution to the device resistance to place bounds on the junction width. We achieve an ultra-sharp junction when the boundary between the intrinsic and doped regions is defined by a cleaved crystalline edge of α-RuCl3 located 2 nm from the graphene. Scanning tunneling spectroscopy in heterostructures of graphene, hexagonal boron nitride, and α-RuCl3 shows potential variations on a sub-10 nm length scale. First principles calculations reveal the charge-doping of graphene decays sharply over just nanometers from the edge of the α-RuCl3 flake.
The existence of bound states induced by local impurities coupled to an insulating host depends decisively on the global topological properties of the host's electronic structure. In this context, we consider magnetic impurities modelled as classical unit-length spins that are exchange-coupled to the spinful Haldane model on the honeycomb lattice. We investigate the spectral flow of bound states with the coupling strength J in both the topologically trivial and Chern-insulating phases. In addition to conventional k-space topology, an additional, spatially local topological feature is available, based on the space of impurity-spin configurations forming, in case of R impurities, an R-fold direct product of two-dimensional spheres. Global k-space and local S-space topology are represented by different topological invariants, the first (k-space) Chern number and the R-th (S-space) spin-Chern number. We demonstrate that there is a local S-space topological transition as a function of J associated with a change in the spin Chern number and work out the implications of this for the J-dependent local electronic structure close to the impurities and, in particular, for in-gap bound states. The critical exchange couplings' dependence on the parameters of the Haldane model, and thus on the k-space topological state, is obtained numerically to construct local topological phase diagrams for systems with R=1 and R=2 impurity spins.
We demonstrate ultra-sharp (≲10 nm) lateral p-n junctions in graphene using electronic transport, scanning tunneling microscopy, and first principles calculations. The p-n junction lies at the boundary between differentially-doped regions of a graphene sheet, where one side is intrinsic and the other is charge-doped by proximity to a flake of α-RuCl3 across a thin insulating barrier. We extract the p-n junction contribution to the device resistance to place bounds on the junction width. We achieve an ultra-sharp junction when the boundary between the intrinsic and doped regions is defined by a cleaved crystalline edge of α-RuCl3 located 2 nm from the graphene. Scanning tunneling spectroscopy in heterostructures of graphene, hexagonal boron nitride, and α-RuCl3 shows potential variations on a sub-10 nm length scale. First principles calculations reveal the charge-doping of graphene decays sharply over just nanometers from the edge of the α-RuCl3 flake.
Formation of Hubbard-like bands as a fingerprint of strong electron-electron interactions in FeSe
(2017)
We use angle-resolved photo-emission spectroscopy (ARPES) to explore the electronic structure of single crystals of FeSe over a wide range of binding energies and study the effects of strong electron-electron correlations. We provide evidence for the existence of "Hubbard-like bands" at high binding energies consisting of incoherent many-body excitations originating from Fe 3d states in addition to the renormalized quasiparticle bands near the Fermi level. Many high energy features of the observed ARPES data can be accounted for when incorporating effects of strong local Coulomb interactions in calculations of the spectral function via dynamical mean-field theory, including the formation of a Hubbard-like band. This shows that over the energy scale of several eV, local correlations arising from the on-site Coulomb repulsion and Hund's coupling are essential for a proper understanding of the electronic structure of FeSe and other related iron based superconductors.
Type-II multiferroic materials, in which ferroelectric polarization is induced by inversion non-symmetric magnetic order, promise new and highly efficient multifunctional applications based on mutual control of magnetic and electric properties. However, to date this phenomenon is limited to low temperatures. Here we report giant pressure-dependence of the multiferroic critical temperature in CuBr2: at 4.5 GPa it is enhanced from 73.5 to 162 K, to our knowledge the highest TC ever reported for non-oxide type-II multiferroics. This growth shows no sign of saturating and the dielectric loss remains small under these high pressures. We establish the structure under pressure and demonstrate a 60\% increase in the two-magnon Raman energy scale up to 3.6 GPa. First-principles structural and magnetic energy calculations provide a quantitative explanation in terms of dramatically pressure-enhanced interactions between CuBr2 chains. These large, pressure-tuned magnetic interactions motivate structural control in cuprous halides as a route to applied high-temperature multiferroicity.
In the search for novel organic charge transfer salts with variable degrees of charge transfer we have studied the effects of two modifications of the recently synthesized donor–acceptor system [tetramethoxypyrene (TMP)]–[tetracyanoquinodimethane (TCNQ)]. One is of chemical nature by substituting the acceptor TCNQ molecules by F4TCNQ molecules. The second consists in simulating the application of uniaxial pressure along the stacking axis of the system. In order to test the chemical substitution, we have grown single crystals of the TMP–F4TCNQ complex and analyzed its electronic structure via electronic transport measurements, ab initio density functional theory (DFT) calculations and UV/VIS/IR absorption spectroscopy. This system shows an almost ideal geometrical overlap of nearly planar molecules stacked alternately (mixed stack) and this arrangement is echoed by a semiconductor-like transport behavior with an increased conductivity along the stacking direction. This is in contrast to TMP–TCNQ which shows a less pronounced anisotropy and a smaller conductivity response. Our band structure calculations confirm the one-dimensional behavior of TMP–F4TCNQ with pronounced dispersion only along the stacking axis. Infrared measurements illustrating the C[triple bond, length as m-dash]N vibration frequency shift in F4TCNQ suggest however no improvement in the degree of charge transfer in TMP–F4TCNQ with respect to TMP–TCNQ. In both complexes about 0.1e is transferred from TMP to the acceptor. Concerning the pressure effect, our DFT calculations on the designed TMP–TCNQ and TMP–F4TCNQ structures under different pressure conditions show that application of uniaxial pressure along the stacking axis of TMP–TCNQ may be the route to follow in order to obtain a much more pronounced charge transfer.