arXiv subjects

S. Sykora

Publications and source records attributed to S. Sykora.

14 recordsLinked to original sources

arXiv2026

Nematicity in LaFeAsO single crystals studied by elastoresistance, high-resolution thermal expansion and shear-modulus measurements

Nematicity in LaFeAsO single crystals is studied by means of high-resolution thermal expansion, shear modulus, and elastoresistivity measurements. A softening of the shear modulus $C_{\rm 66}$ towards the structural phase transition at $T_{\rm S}$ is observed. In addition, a similar Curie-Weiss-like divergence of the nematic susceptibilities is found in the temperature dependence of both $\chi^{sh}$ and $\chi^{er}$, which are deduced from the shear modulus (sh) and the elastoresistivity (er) studies, respectively. These observations provide evidence for an electronic origin of nematicity in LaFeAsO. The characteristic energy of the coupling between the lattice and the electronic degrees of freedom is deduced to $\sim$30~K. The comparison to corresponding measurements on BaFe$_2$As$_2$ single crystals reveals a very similar temperature dependence of the shear modulus but yields contrasting results for $\chi^{er}$ : In BaFe$_2$As$_2$, $\chi^{er}$ diverges similarly as the uncoupled nematicity deduced from the shear modulus data as it is expected from the underlying Landau theory. In contrast, the Weiss temperatures of $\chi^{er}$ and $\chi^{sh}$ are significantly different in LaFeAsO. This difference is at odds with the commonly anticipated theories of resistivity anisotropy and electronic nematicity in iron pnictides.

cond-mat.supr-con↗

arXiv2012

Incommensurate magnetic fluctuations and Fermi surface topology in LiFeAs

Using the angle-resolved photoemission spectroscopy (ARPES) data accumulated over the whole Brillouin zone (BZ) in LiFeAs we analyze the itinerant component of the dynamic spin susceptibility in this system in the normal and superconducting state. We identify the origin of the incommensurate magnetic inelastic neutron scattering (INS) intensity as scattering between the electron pockets, centered around the $(\pi,\pi)$ point of the BZ and the large two-dimensional hole pocket, centered around the $\Gamma$-point of the BZ. As the magnitude of the superconducting gap within the large hole pocket is relatively small and angle dependent, we interpret the INS data in the superconducting state as a renormalization of the particle-hole continuum rather than a true spin exciton. Our comparison indicates that the INS data can be reasonably well described by both the sign changing symmetry of the superconducting gap between electron and hole pockets as well as sign preserving gap, depending on the assumptions made for the fermionic damping.

cond-mat.supr-con↗

arXiv2012

Phase separation in the Edwards model

The nature of charge transport within a correlated background medium can be described by spinless fermions coupled to bosons in the model introduced by Edwards. Combining numerical density matrix renormalization group and analytical projector-based renormalization methods we explore the ground-state phase diagram of the Edwards model in one dimension. Below a critical boson frequency any long-range order disappears and the system becomes metallic. If the charge carriers are coupled to slow quantum bosons the Tomonaga-Luttinger liquid is attractive and finally makes room for a phase separated state, just as in the t-J model. The phase boundary separating repulsive from the attractive Tomonaga-Luttinger liquid is determined from long-wavelength charge correlations, whereas fermion segregation is indicated by a vanishing inverse compressibility. On approaching phase separation the photoemission spectra develop strong anomalies.

cond-mat.str-el↗

arXiv2012

Probing the Unconventional Superconducting State of LiFeAs by Quasiparticle Interference

A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high Tc~18K. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van-Hove singularity at the center of the Brillouin zone which is in stark contrast with other pnictide superconductors where nesting is crucial for both scattering and s+- superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the hole-like bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries s++, s+-, and d-wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary p-wave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g. s+id-wave symmetry).

cond-mat.supr-con↗

arXiv2010

Charge-density-wave formation in a half-filled fermion-boson transport model: A projective renormalization approach

We study the metal-insulator transition in a very general two-channel transport model, where charge carriers are coupled to a correlated background medium. The fluctuations of the background were described as bosonic excitations, having the ability to relax. Employing an analytical projector-based renormalization technique, we calculate the ground-state and spectral properties of this fermion-boson model and corroborate recent numerical results, which indicate---in dependence on the `stiffness' of the background medium---a Luttinger-liquid to charge-density-wave transition for the one-dimensional half-filled band case. In particular, we determine the renormalized electron and boson dispersion relations and show that the quantum phase transition is not triggered by a softening of the boson modes. Thus the charge density wave is different in nature from an usual Peierls distorted state.

cond-mat.str-el↗

arXiv2009

Microscopic approach to high-temperature superconductors: Pseudogap phase

Despite the intense theoretical and experimental effort, an understanding of the superconducting pairing mechanism of the high-temperature superconductors is still lacking. An additional puzzle is the unknown connection between the superconducting gap and the so-called pseudogap which is a central property of the most unusual normal state. Angle-resolved photoemission spectroscopy (ARPES) measurements have revealed a gap-like behavior on parts of the Fermi surface, leaving a non-gapped segment known as Fermi arc around the diagonal of the Brillouin zone. Starting from the $t$-$J$ model, in this paper we present a microscopic approach to investigate physical properties of the pseudogap phase in the framework of a novel renormalization scheme called PRM. This approach is based on a stepwise elimination of high-energy transitions using unitary transformations. We arrive at a renormalized 'free' Hamiltonian for correlated electrons. The ARPES spectral function along the Fermi surface turns out to be in good agreement with experiment: We find well-defined excitation peaks around $ω=0$ near the nodal direction, which become strongly suppressed around the antinodal point. The origin of the pseudogap can be traced back to a suppression of spectral weight from incoherent excitations in a small $ω$-range around the Fermi energy. In a subsequent paper, also the supercunducting phase at moderate hole doping will be discussed within the PRM approach.

cond-mat.supr-con↗

arXiv2009

Microscopic approach to high-temperature superconductors: Superconducting phase

Despite the intense theoretical and experimental effort, an understanding of the superconducting pairing mechanism of the high-temperature superconductors, leading to an unprecedented high transition temperature $T_c$, is still lacking. Starting from the $t$-$J$ model, we present a microscopic approach to the physical properties of the superconducting phase at moderate hole-doping in the framework of a novel renormalization scheme, called PRM. Our microscopic approach allows us to explain the experimental findings in the underdoped as well as in the optimal hole doping regime. In good agreement with experiments, we find no superconducting solutions for very small hole doping. In the superconducting phase, the order parameter turns out to have d-wave symmetry with a coherence length of a few lattice constants. The spectral function, obtained from angle-resolved photoemission spectroscopy (ARPES) along the Fermi surface, is also in good agreement with experiment: The spectra display peak-like structures which are caused alone by coherent excitations in a small range around the Fermi energy.

cond-mat.supr-con↗

arXiv2008

Projector-based renormalization method (PRM) and its application to many-particle systems

Despite the advances in the development of numerical methods analytical approaches play a key role on the way towards a deeper understanding of strongly interacting systems. In this regards, renormalization schemes for Hamiltonians represent an important new direction in the field. Among these renormalization schemes the projector-based renormalization method (PRM) reviewed here might be the approach with the widest range of possible applications: As demonstrated in this review, continuous unitary transformations, perturbation theory, non-perturbative phenomena, and quantum-phase transitions can be understood within the same theoretical framework. This review starts from the definition of an effective Hamiltonian by means of projection operators that allows the evaluation within perturbation theory as well as the formulation of a renormalization scheme. The developed approach is then applied to three different many-particle systems: At first, we study the electron-phonon problem to discuss several modifications of the method and to demonstrate how phase transitions can be described within the PRM. Secondly, to show that non-perturbative phenomena are accessible by the PRM, the periodic Anderson model is investigated to describe heavy-fermion behavior. Finally, we discuss the quantum-phase transition in the one-dimensional Holstein model of spinless fermions where both metallic and insulating phase are described within the same theoretical framework.

cond-mat.str-el↗

arXiv2008

Coexistence of superconductivity and charge-density waves in a two-dimensional Holstein model at half-filling

In one dimension the coupling of electrons to phonons leads to a transition from a metallic to a Peierls distorted insulated state if the coupling exceeds a critical value. On the other hand, in two dimensions the electron-phonon interaction may also lead to the formation of Cooper pairs. This competition of superconductivity and charge order (in conjunction with a lattice distortion) is studied in this letter by means of the projector-based renormalization method (PRM). Increasing the electron-phonon interaction, we find a crossover behavior between a purely superconducting state and a charge-density wave where a well-defined parameter range of coexistence of superconductivity and lattice distortion exists.

cond-mat.supr-con↗

arXiv2007

Static and dynamic properties of the spinless Falicov-Kimball model

The spinless Falicov-Kimball model is studied by the use of a recently developed projector-based renormalization method (PRM) for many-particle Hamiltonians. The method is used to evaluate static and dynamic quantities of the one-dimensional model at half-filling. To these belong the quasiparticle excitation energy $\tildeε_k$ and the momentum distribution $n_k$ of the conduction electrons and spatial correlation functions of the localized electrons. One of the most remarkable results is the appearance of a gap in $\tildeε_k$ at the Fermi level of the order of the Coulomb repulsion $U$, which is accompanied by a smooth behavior for $n_k$. The density of states for the conduction electrons and the one-particle spectral functions for the localized electrons are also discussed. In both quantities a gap opens with increasing $U$.

cond-mat.str-el↗

arXiv2006

Dominant particle-hole contributions to the phonon dynamics in the spinless one-dimensional Holstein model

In the spinless Holstein model at half-filling the coupling of electrons to phonons is responsible for a phase transition from a metallic state at small coupling to a Peierls distorted insulated state when the electron-phonon coupling exceeds a critical value. For the adiabatic case of small phonon frequencies, the transition is accompanied by a phonon softening at the Brillouin zone boundary whereas a hardening of the phonon mode occurs in the anti-adiabatic case. The phonon dynamics studied in this letter do not only reveal the expected renormalization of the phonon modes but also show remarkable additional contributions due to electronic particle-hole excitations.

cond-mat.str-el↗

arXiv2006

Analytical approach to the quantum-phase transition in the one-dimensional spinless Holstein model

We study the one-dimensional Holstein model of spinless fermions interacting with dispersion-less phonons by using a recently developed projector-based renormalization method (PRM). At half-filling the system shows a metal-insulator transition to a Peierls distorted state at a critical electron-phonon coupling where both phases are described within the same theoretical framework. The transition is accompanied by a phonon softening at the Brillouin zone boundary and a gap in the electronic spectrum. For different filling, the phonon softening appears away from the Brillouin zone boundary and thus reflects a different type of broken symmetry state.

cond-mat.str-el↗

arXiv2005

Single-particle excitations and phonon softening in the one-dimensional spinless Holstein model

We investigate the influence of the electron-phonon coupling in the one-dimensional spinless Holstein model at half-filling using both a recently developed projector-based renormalization method (PRM) and an refined exact diagonalization technique in combination with the kernel polynomial method. At finite phonon frequencies the system shows a metal-insulator transition accompanied by the appearance of a Peierls distorted state at a finite critical electron-phonon coupling. We analyze the opening of a gap in terms of the (inverse) photoemission spectral functions which are evaluated in both approaches. Moreover, the PRM approach reveals the softening of a phonon at the Brillouin-zone boundary which can be understood as precursor effect of the gap formation.

cond-mat.str-el↗

arXiv2004

Computer aided perturbation theory by cumulants: dimerized and frustrated spin 1/2 chain

This paper demonstrates that a computer aided perturbation theory can easily be realized by use of a cumulant approach. In contrast to a recent alternative formulation on the basis of Wegner's flow equation method the present approach can be applied to systems with arbitrary Hilbert space. In particular an equidistant spectrum of the unperturbed part of the Hamiltonian is not needed. The method is illustrated in detail for dimerized and frustrated spin 1/2 chains for which the ground state energy is calculated up to seventh order perturbation theory.

cond-mat.str-el↗