The recently discovered nickelate superconductors represent one of the raising hot topic in the research area of strongly-correlated superconductors. A recent experiment [X. Ding et al., Nature 615, 50 (2023)] indicates that superconductivity in nickelates is restricted to a narrow window of hydrogen concentration: 0.22 < x < 0.28 in Nd0.8Sr0.2NiO2Hx. This reported necessity of hydrogen...
Antiferromagnets with spin-split bands in the momentum space emerged as promising materials for technologies based on antiferromagnetic spintronics using spin-orbit or spin-splitting torque. The absence of bulk or structural inversion symmetry along with spin-orbit interaction leads to such spin-splitting known as Dresselhaus or Rashba effect, respectively. Spin-splitting phenomena occur even...
The DMF2RG has been introduced to overcome the weak-coupling limitation of the fRG. This approach builds on the idea to exploit the dynamical mean-field theory (DMFT) as starting point for the flow, thus capturing local non-perturbative correlations via DMFT together with perturbative nonlocal correlations generated during the flow. We discuss how the DMF2RG can be extended to describe...
We present numerical evidence for the existence of a quantum liquid state in one spatial dimension, which combines traits from both the standard Luttinger liquid and Fermi liquid phenomenologies. The state is stabilized in a quantum lattice of interacting spinless fermions by selectively turning off marginal interactions, while remaining irrelevant interactions define the unique features of...
The fundamental properties of superfluids and superconductors are determined by the spatial coherence of the macroscopic condensate. Its fluctuations are pivotal to supercurrent flow, the functionality of superconducting nanostructures, and the response superconducting matter shows to magnetic fields. Central to a theoretical description is the coherence length which sets the relevant length...
Spectroscopy constitutes an important family of experiments to study condensed matter systems, where one obtains information about a material by shining photons at it and measuring the intensity of the scattered photons. However the modern AMO toolbox allows one to measure not just the intensity, but also other observables such as second order coherence (g(2)) and homodyne signal. What these...
Moirรฉ materials produced by stacking monolayers with small relative twist angles are of intense current interest for the range of correlated electron phenomena they exhibit and for their high degree of experimental controllability. Controlling moirรฉ to realize exotic quantum phase of matter is important for both fundamental science research and future application to quantum information...
The formalism for composite fermions, initially developed for bosons at filling factor ฮฝ = 1 [1, 2], has been a cornerstone in understanding the fractional quantum Hall effect (FQHE). We derive the Dirac composite fermion theory for a half-filled Landau level from first principles [3]. In this talk, we present novel insights into this phenomenon by employing a dipole representation for...
We study the interplay of disorder and Heisenberg interactions in the Kitaev model on a honeycomb lattice. The effect of disorder on the transition between Kitaev spin liquid and magnetic ordered states as well as the stability of magnetic ordering is investigated. Using Lanczos exact diagonalization we discuss the consequences of two types of disorder: (i) random-coupling disorder and (ii)...
The Anomalous Hall effect (AHE) is a transport phenomenon in ferromagnets, which exhibit currents even in the absence of a magnetic field. Their inner magnetization breaks Time Reversal Symmetry, allowing the Berry Curvature (BC) to be finite. As a result, topological features close to the Fermi energy have a deep impact in the transport properties, leading to huge Anomalous Hall...
We study the effects of a periodically varying electric field on the Hubbard model at half-filling on a triangular lattice. The driving electric field is incorporated through the phase of the nearest-neighbor hopping amplitude via the Peierls prescription. When $U$ is much larger than the hopping, the system is a Mott insulator and the effective Hamiltonian $H_{eff}$ describing the spin sector...
Topological quantum phase transitions (TQPTs) describe a change in the electronic band structure (as in topological insulators), or in the shape of the Fermi surface (Lifshitz). It is commonly assumed that TQPTs donโt follow the conventional Ginzburg-Landau scheme, nonetheless this is not at odds with the possibility of a description based on the relevant observables of the system, beside the...
The functional renormalisation group has played an important role in providing a tool for unbiased investigation of strongly correlated systems in condensed matter. To aid for a quantitative investigation, a full, yet efficient, momentum [4] and frequency [5] treatment of the vertex and the self-energy is needed. Such methods have been developed in [4] and [5] for local Hubbard interactions,...
Quantum spin liquids are an exotic phase of matter characterized by the presence of fractionalized excitations(spinons) and emergent gauge fields. One of the difficulties in probing experimentally a QSL phase comes from the fact that the spinons do not carry an electric charge, ruling out the possibility of using conventional electrical probes. Going beyond conventional transport, we propose...
We use dynamical mean-field theory to study the interplay of electron correlations and spontaneous symmetry-breaking in magic-angle twisted bilayer graphene. We find that there is no genuine Mott stateโlong-range order from spontaneous symmetry-breaking is essential to opening a gap at the Fermi level. We find that dynamic correlations strongly suppress ordering temperatures (by a factor of 10...
The superconducting diode effect refers to an asymmetry in the critical supercurrent $J_c(\hat{n})$ along opposite directions, $J_c(\hat{n})\neq J_c(-\hat{n})$. While the basic symmetry requirements for this effect are known, it is, for junction-free systems, difficult to capture within current theoretical models the large current asymmetries $J_c(\hat{n})/J_c(-\hat{n})$ recently observed in...
Many current and future quantum technologies rely on amorphous materials, where translational symmetry is broken, but short-range order with well-defined structural length scales persists ([1]). This brings forward the fundamental question whether long range order is a necessary condition to establish coherence and structured momentum-dependent electronic state, and how to characterize it in...
We show how the stability conditions for a system of interacting fermions can be also expressed in terms of local two-particle correlators, instead of conventional derivatives of thermodynamic potentials. By inspecting the spectral representation of the generalized local charge susceptibility and its lowest negative eigenvalues, we first illustrate the applicability of this stability...
Majorana zero modes (MZMs) emerge as edge states in topological superconductors and are promising for topological quantum computation, but their detection has so far been elusive. Here we show that non-Hermiticity can be used to obtain dramatically more robust MZMs. The enhanced properties appear as a result of an extreme instability of exceptional points to superconductivity, such that even a...
We address the sudden reconstruction of the Fermi surface (FS) at the Kondo breakdown (KB) quantum critical point (QCP) in heavy fermion systems. We focus on results on the periodic Anderson model, obtained using a two-site cellular dynamical mean-field theory (CDMFT) approach. By employing the Numerical Renormalization Group to solve the effective impurity model, we are able to dispose of the...
To capture the universal low-energy physics of metals within effective field theories, one has to generalize the usual notion of scale invariance and renormalizable field theory due to the presence of intrinsic scales (Fermi momenta). In this work, we develop a field-theoretic functional renormalization group formalism for full low-energy effective field theories of non-Fermi liquids that...
We derive two fundamental laws of chiral band crossings: (i) a local constraint relating the Chern number to phase jumps of rotation eigenvalues; and (ii) a global constraint determining the number of chiral crossings on rotation axes. Together with the fermion doubling theorem, these laws describe all conditions that a network of chiral band crossing must satisfy. We apply the fundamental...
According to the big bang theory, the universe began as an extremely hot, dense, and small point known as singularity. Approximately 13.8 billion years ago, this singularity began to expand and cool, eventually forming subatomic particles, atoms, stars, galaxies, and eventually the structure of the universe as we know it today. At that time, the universe was formed from the stage of...
At n=3/4 filling of the moirรฉ flat band, transition metal dichalcogenide moirรฉ bilayers will develop kagome charge order. We derive an effective spin model for the resulting localized spins and find that its further neighbor spin interactions can be much less suppressed than the corresponding electron hopping strength. Using density matrix renormalization group simulations, we study its phase...
We propose a Kohn-Luttinger-like mechanism for charge density waves in correlated electron systems with higher symmetries SU(N).The mechanism is responsible for an instability with finite transfer momentum in the particle-hole direct channel which emerges due to the feedback from the particle-hole crossed channel. Like in the original Kohn-Luttinger mechanism, the separation of momentum...
The breakdown of the lattice Kondo effect in local-moment metals can lead to non-trivial forms of quantum criticality and a variety of non-Fermi-liquid phases. Given indications that Kondo-breakdown transitions involve criticality not only in the spin but also in the charge sector, we investigate the interplay of Kondo breakdown and strong valence fluctuations in generalized Anderson lattice...
Non-Abelian anyons typically emerge in gapped topological models, and give rise to a fusion space which scales with a fractional quantum dimension to the power of the number of anyons. It is also well established that a single spin-1/2 impurity coupled to $k$ multiple fermionic channels results in a fractional entropy equal to the quantum dimension of a single $SU(2)_k$ anyon. Generalizing...
The formation of a heavy Fermi liquid in metals with local moments is characterized by multiple energy and temperature scales, most prominently the Kondo temperature and the coherence temperature, characterizing the onset of Kondo screening and the emergence of Fermi-liquid coherence, respectively. In the standard setting of a wide conduction band, both scales depend exponentially on the Kondo...
We investigate Landau levels in an electron band that exhibits a topological Lifshitz transition. We focus on Bernal-stacked bilayer graphene, a system that has drawn a lot of attention recently. A dual gated experimental setup allows to tune the out-of-plane displacement field and the charge carrier density independently, giving insights in exotic correlation effects [1]. Depending on the...
Many-electron systems undergo a collective Larmor precession in the presence of a magnetic field. In a paramagnetic metal, the resulting spin wave provides insight into the correlation effects generated by the electron-electron interaction. Here, we use dynamical mean-field theory to investigate the collective Larmor precession in the strongly correlated regime, where dynamical correlation...
Modern scanning probe techniques, like scanning tunneling microscopy (STM), provide access to a large amount of data encoding the underlying physics of quantum matter. In this work, we analyze how convolutional neural networks (CNN) can be employed to learn effective theoretical models from STM data on correlated moirรฉ superlattices. These engineered systems are particularly well suited for...
We analyze the interplay of fluctuating antiferromagnetism and pairing in the two dimensional Hubbard model with a moderate repulsive interaction. In particular, we present a gauge theory description for fluctuations where the stiffnesses are computed in the coexisting phase from renormalization mean-field equations via the functional renormalization group (fRG).
We show a sizable doping...
Recent experiments on moirรฉ transition metal dichalcogenides have established this class of compounds as a highly tunable platform for the study of correlated electronic phenomena such as the correlation-driven Mott metal-insulator transition, quantum criticality and superconductivity. At the same time these materials can be approximately described in terms of the single-band moirรฉ Hubbard...
Motivated by a recent experiment on a square-lattice Rydberg atom array realizing a long-range dipolar XY model [Chen et al., Nature (2023)], we numerically study the model's equilibrium properties. We obtain the phase diagram, critical properties, entropies, variance of the magnetization, and site-resolved correlation functions. We consider both ferromagnetic and antiferromagnetic...
In the pursuit towards targeted material design leveraging strong electronic correlation, computationally inexpensive yet qualitatively reliable methods play a fundamental role. These approaches should allow for a rapid mapping of phase space, unveiling a first impression of possible phases of matter, which can then be explored in selected regions of parameter space with more accurate yet...
We investigate the role of non-local electronic correlations at finite temperatures in the half-filled anisotropic triangular lattice Hubbard model using the dynamical vertex approximation (DฮA), a diagrammatic extension [1] of the dynamical mean-field theory (DMFT). We analyze the impact of (quantum) phase transitions on finite temperature properties at the one- and two-particle level. We...
Magic-angle twisted bilayer graphene exhibits diverse, fascinating phases when the four flat bands around the charge neutrality point are partially filled. These phases include magnetic Chern insulators with predominantly orbital magnetization and adjacent superconducting regions. Recent scanning tunneling microscopy (STM) measurements on low strain samples have provided evidence of a local...
The optical conductivity in a wide variety of correlated models has been shown to be dominated by pi-ton vertex corrections, which describe the coupling of light with antiferromagnetic or charge density wave fluctuations with a wave vector close to q = (pi, pi, ...) [1]. While the analysis [2] of the pi-ton vertex corrections in two-dimensional (2D) weakly correlated systems show that they...
The optical conductivity of systems with strong interactions is one of the most studied quantities experimentally, yet its computation from microscopic models remains challenging. In the context of linear response theory and of the Kubo formula for conductivity, this difficulty is embedded in the momentum and energy dependence of the electron self-energy and of the vertex corrections. One...
Tharathep Plienbumrung,1,2 Maria Daghofer,1,2 Michael Schmid,3 and Andrzej M. Oleล 4,5
1 Institute for Functional Matter and Quantum Technologies, University of Stuttgart, D-70550 Stuttgart, Germany
2 Center for Integrated Quantum Science and Technology, University of Stuttgart, D-70550 Stuttgart, Germany
3 Waseda Research Institute for Science and Engineering, Waseda University,...
Using the recently introduced combination of the fRG with the DMFT, coined DMF2RG, we compute the frequency- and momentum-dependent self-energy of the two-dimensional Hubbard model at strong coupling. For this, we extend the single-boson exchange formulation of the fRG for the computation of the self-energy flow with the Schwinger-Dyson equation. This has shown to be essential to capture the...
PT-symmetric structures have received particular interest due to their multiple applications in optics and synthetic materials. PT-symmetric superconductivity is a rather new, developing field. Here, I will show theoretically how such superconductor can be obtained due to spatiotemporal modulations of a material, that can lead to asymmetric electron-electron interaction. I will discuss Andreev...
Multivariate functions of continuous variables arise in countless branches of science. Numerical computations with such functions typically involve a compromise between two contrary desiderata: accurate resolution of the functional dependence, versus parsimonious memory usage. Recently, two promising strategies have emerged for satisfying both requirements: (i) The quantics representation,...
The study of the simple spin models provides an insight into the nature
of phase transitions. Given the growth of the attention to the
non-Hermitian systems in the recent years, it is interesting to look
into phase transitions in non-Hermitian systems.
Here, I present a theoretical study of quantum phases and quantum phase
transitions occurring in non-Hermitian transverse-field Ising...
Fermionic Symmetry Protected Topological (SPT) phases can be destroyed by spontaneous symmetry breaking. However, particularly in low dimensions, strong quantum fluctuations may destroy local order parameters and thereby, potentially restoring the SPT phase. To illustrate this phenomenon, we present a study of a model comprising SSH chains coupled to Cooper pair boxes at different fillings...
Superconductivity in two-dimensional (2D) materials has sparked great interest due to the emergence of various novel quantum phenomena [1]. Recent experimental studies of 2D superconductivity in anisotropic layered materials propose both conventional and unconventional electron pairing. In this series, layered transition metal dichalcogenides (TMDs) are an intriguing class of materials which...
We identify an exotic quasiuniversal behavior near the all-in-all-out Weyl quantum critical point in three-dimensional Luttinger semimetals, such as the pyrochlore iridates $R_2$Ir$_2$O$_7$, with $R$ a rare-earth element. The quasiuniversal behavior is characterized by power laws with exponents that vary slowly over several orders of magnitude in energy or length. However, in contrast to the...
Magnon scattering in quantum hall heterojunctions has been introduced as a new experimental setup to probe the spin structure of the rich variety of phases in quantum Hall systems. We introduce a model to study magnon scattering in skyrmion crystals, sandwiched between ferromagnets which act as the source of magnons. Skyrmions are topological objects while skyrmion crystals break internal and...
The functional renormalization group (fRG) is already established as a powerful tool to study many-electron systems. Furthermore, the single-boson-exchange (SBE) decomposition of two-particle vertices, which consists in a bosonization in all channels, has proven to be a particularly efficient parametrization of fermionic problems, both within parquet- and fRG-based approaches. We discuss here...
Motivated by the recent experimental realization of ABCB stacked quadlayer graphene [Wirth et al., ACS Nano 16, 16617 (2022)], we study correlated phenomena in moirรฉ-less graphene tetralayers for realistic interaction profiles using an orbital resolved, weak coupling random phase approximation approach. We demonstrate that spin fluctuations originating from local interactions are crucial...
Quantum spin liquids are tantalizing phases of quantum matter, but experimental evidence of their existence has remained elusive. Even theoretically, it is unclear whether many phases permitted by a mean-field classification can be realized as the stable ground states of a physical model. Recent theoretical and numerical studies have provided evidence that triangular-lattice Heisenberg...
We consider dressing of excitonic properties by strongly correlated electrons in gate-controlled twisted homo-bilayer heterostructures. The combined effect of the moirรฉ potential and the Coulomb interaction supports the formation of different strongly correlated phases depending on the filling, including charge-ordered metals or incompressible insulators at integer occupation. The coupling...
In this paper, we present computations of the optical absorption spectra of T graphene quantum dots (TGQD) employing a ฯ-electron method and long-range Coulomb interactions within the Pariser-Parr-Pople (PPP) model Hamiltonian [1]{2]. The Configuration-Interaction (CI)[3][4] approach is used at various levels to incorporate electron-correlation effects on the ground and excited states. The...
The superconducting symmetry of Sr$_2$RuO$_4$ is one of the long standing problems of material science. Even though the normal state of the material can be well described in terms of density functional theory plus dynamical mean-field theory calculations, the character of the superconducting state remains elusive. In this paper we investigate the behavior of the superconducting order parameter...
In this work, we study symmetry-enforced Z2 topology in non-magnetic centrosymmetric materials in both regimes of strong and negligible spin-orbit coupling. We provide a classification of space groups, enforcing non-trivial topological Z$_2$ invariant. For the space groups, we list planes in the Brillouin zone that are topological at half-filling. The classification implies the existence of...
Carbon quantum dots (CQD) represent one of the recently discovered allotropic forms of carbon whose quantum confinement properties give it certain peculiarities in terms of its behavior. In these structures, the close relationship established between optoelectronic properties such as absorbance, fluorescence, and the presence of functional groups at their edges is well known. Associated with...
We study the half-filled two-dimensional Hubbard model on a square lattice in cellular dynamical mean-field theory (CDMFT), a real-space cluster extension [1] of the dynamical mean-field theory. By increasing the number of cluster sites up to 6x6 we observe a progressive reduction of the onset interaction U* of a metal-insulator crossover. In particular, in the case of 4x4 sites, we observe a...
We use real-space Hartree-Fock theory to construct a magnetic phase diagram of the two-dimensional Hubbard model as a function of temperature and doping. We are able to detect various spin- and charge order patterns including Nรฉel, stripe and spiral order without biasing the system towards one of them. For an intermediate interaction strength we predominatly find Nรฉel order close to...
We study the roles of Coulomb interactions in transport properties of interacting Dirac electrons in two-dimensions. We study it from a weak-coupling and a strong-coupling perspective. We demonstrate that long-range Coulomb interactions play two independent roles. (i) In the weak-coupling analysis, they provide the inelastic and momentum-conserving scattering mechanism that leads to fast...
Floating topological superconductors coupled to conduction electrons can realize unconventional O(N), Sp(2N), or multi-channel Kondo effects. Here, we introduce a new topological superconducting mesoscopic device, a time-reversal invariant version of the Majorana Cooper pair box in the Coulomb blockade regime. In this setup of Cartan-Altland-Zirnbauer class DIII, spinful Majorana zero modes...
In the triangular lattice Hubbard model, the interplay of strong correlations and geometrical frustration gives rise to a variety of emergent phenomena. At intermediate coupling a metal-insulator transition is observed and spin liquid physics have been proposed, whereas at strong coupling a magnetic insulator is found. Triangular lattice structures can be realized in a variety of materials,...
We study possible patterns for spontaneous symmetry breaking in a Dirac fermion model, which is applicable to twisted bilayer graphene at charge neutrality. We show how a chiral SU(4) symmetry emerges and construct the corresponding low-energy model that includes a Fierz-complete set of symmetry-allowed four-fermion interactions. We employ an unbiased renormalization group treatment to...
The electron Green's function is a powerful tool that describes single-particle excitations in correlated systems, commonly associated with poles in the complex frequency plane. Intriguingly, when strong interactions come into play, the Green's function determinant can also have bands of zeros, corresponding to poles of the self-energy. However, these zeros have long been for long overlooked...
We investigate topological superconductivity in the Rashba-Hubbard model, describing heavy-atom superlattice and van der Waals materials with broken inversion. We focus in particular on fillings close to the van Hove singularities, where a large density of states enhances the superconducting transition temperature. To determine the topology of the superconducting gaps and to analyze the...
