arXiv subjects

P. Pfeuty

Publications and source records attributed to P. Pfeuty.

8 recordsLinked to original sources

arXiv2002

Inelastic neutron scattering study of magnetic excitations in Sr$_2$RuO$_4$

Magnetic excitations in \srruo ~ have been studied by inelastic neutron scattering. The magnetic fluctuations are dominated by incommensurate peaks related to the Fermi surface nesting of the quasi-one-dimensional $d_{xz}$- and $d_{yz}$-bands. The shape of the incommensurate signal agrees well with RPA calculations. At the incommensurate {\bf Q}-positions the energy spectrum considerably softens upon cooling pointing to a close magnetic instability : \srruo ~does not exhibit quantum criticality but is very close to it. $ω/ T$-scaling may be fitted to the data for temperatures above 30 K. Below the superconducting transition, the magnetic response at the nesting signal is not found to change in the energy range down to 0.4meV.

cond-mat.str-el↗

arXiv1999

Some aspects of electronic topological transition in 2D system on a square lattice. Excitonic ordered states

We study the ordered "excitonic" states which develop around the quantum critical point (QCP) associated with the electronic topological transition (ETT) in a 2D electron system on a square lattice. We consider the case of hopping beyond nearest neighbors when ETT has an unusual character. We show that the amplitude of the order parameter (OP) and of the gap in the electron spectrum increase with increasing the distance from the QCP, δ_c - δ, where δ= 1-n and "n" is an electron concentration. Such a behavior is different from the ordinary case when OP and the gap decrease when going away from the point which is a motor for instability. The gap opens at "hot spots" and extends untill the saddle points (SP) whatever is the doping concentration. The spectrum gets a characteristic flat shape as a result of hybrydization effect in the vicinity of two different SP's. The shape of the spectrum and the angle dependence of the gap have a striking similarity with the features observed in the normal state of the underdoped high-T$_c$ cuprates. We discuss also details about the phase diagram and the behaviour of the density of states.

cond-mat.supr-con↗

arXiv1999

Various ordered states in a 2D interacting electron system close to an electronic topological transition

We consider a 2D electron system on a square lattice with hopping beyond nearest neighbors. The existence of the quantum critical point associated with an electronic topological transition in the noninteracting system results in density wave (DW) and high temperature d-wave superconducting (dSC) instabilities in the presence of an exchange interaction J. We analyse different DW ordering such as isotropic Spin DW (SDW), d-wave SDW, isotropic Charge DW (CDW) and d-wave CDW. The coexistence of dSC and SDW orders leads necessary to the existence of a third order which is a pi triplet superconducting (PTS) order. A new phase diagram with a mixed phase of SDW, dSC and PTS order is found. The theory is applied to high-Tc cuprates.

cond-mat.str-el↗

arXiv1999

SC state in the underdoped high-T_c cuprates as a quantum spin liquid. A microscopic theory

We have discovered a new example of quantum spin liquid which is a superconducting (SC) phase in 2D electron system close to electronic topological transition. As a quantum spin liquid in low dimensional localized spin systems it is characterized by a resonance spin mode and a well separated two-particle continuum. Application of the theory to the high-T_c cuprates allows to shed light on many observed in the underdoped regime features such as the so called 40 meV resonance peak, incommensurability at lower ω, spin gap etc.

cond-mat.supr-con↗

arXiv1999

New scenario for high-T_c cuprates: electronic topological transition as a motor for anomalies in the underdoped regime

We have discovered a new nontrivial aspect of electronic topological transition (ETT) in a 2D free fermion system on a square lattice. The corresponding exotic quantum critical point, δ=δ_c, T=0, (n=1-δis an electron concentration) is at the origin of anomalous behaviour in the interacting system on one side of ETT, δ<δ_c. The most important is an appearance of the line of characteristic temperatures, T^*(δ) \propto δ_c-δ. Application of the theory to high-T_c cuprates reveals a striking similarity to the observed experimentally behaviour in the underdoped regime (NMR and ARPES).

cond-mat.supr-con↗

arXiv1998

The Kondo-Hubbard model at half-filling

We have analyzed the antiferromagnetic (J>0) Kondo-Hubbard lattice with the band at half-filling by means of a perturbative approach in the strong coupling limit, the small parameter is an arbitrary tight-binding band. The results are valid for any band shape and any dimension. We have obtained the energies of elementary charge and spin excitations as well as the magnetic correlations in order to elucidate the magnetic and charge behavior of the Kondo lattice at half-filling. Finally, we have briefly analyzed the ferromagnetic case (J<0), which is shown to be equivalent to an effective antiferromagnetic Heisenberg model.

cond-mat.str-el↗

arXiv1998

Normal state pseudogap and (π,0) feature in the underdoped high-T_c cuprates. A microscopical theory

We show that a simple 2D electron system on a square lattice with hoping between more than nearest neighbors exhibits in the presence of electron spin exchange interaction properties strikingly similar to those observed in the underdoped cuprates It is a normal state pseudogap, its behaviour with doping and T, a form of the spectrum around (0,π) etc. The motor is the electronic topological transition which due to two-dimensionality gets new features with respect to well-studied 3D case.

cond-mat.supr-con↗

arXiv1998

Electronic topological transition in 2D electron system on a square lattice and the line T^*(δ) in the underdoped regime of high - T_c cuprates

It is shown that a 2D system of free fermions on a square lattice with hoping between more than nearest neighbours undergoes a fundamental electronic topological transition (ETT) at some electron concentration n_c. The point δ=δ_c, T=0 (δ=1-n) is an exotic quantum critical point with several aspects of criticality. The first trivial one is related to singularities in thermodynamic properties. An untrivial and never considered aspect is related to the effect of Kohn singularity (KS) in 2D system: this point is the end of a critical line T=0, δ>δ_c of static KS in free fermion polarizability. This ETT is a motor for anomalous behaviour in the system of interacting electrons. The anomalies take place on one side of ETT, δ<δ_c, and have a striking similarity with the anomalies in the high T_c cuprates in the underdoped regime. The most important consequence of ETT is the appearance of the line of characteristic temperatures, T^*(δ) \propto δ_c-δ, which grows from the point δ=δ_c on the side δ<δ_c. Below this line the metal state is anomalous. Some anomalies are considered in the present paper. The anomalies disappear in the case t'=t''=... \to 0.

cond-mat.supr-con↗