arXiv subjects

P. Fazekas

Publications and source records attributed to P. Fazekas.

At least 19 recordsLinked to original sources

arXiv2007

The electronic structure and the phases of BaVS3

BaVS3 is a moderately correlated d-electron system with a rich phase diagram. To construct the corresponding minimal electronic model, one has to decide which d-states are occupied, and to which extent. The ARPES experiment presented here shows that the behavior of BaVS3 is governed by the coexistence of wide-band (A_1g) and narrow-band (twofold degenerate E) d-electrons. We sketch a lattice fermion model which may serve as a minimal model of BaVS3. This serves foremost for the understanding of the metal-insulator in pure BaVS3 and its absence in some related compounds. The nature of the low temperature magnetic order differs for several systems which may be described in terms of the same electron model. We describe several recent experiments which give information about magnetic order at high pressures. In particular, we discuss field-induced insulator-to-metal transition at slightly subcritical pressures, and an evidence for magnetic order in the high-pressure metallic phase. The phase diagram of Sr-doped BaVS3 is also discussed. The complexity of the phases of BaVS3 arises from the fact that it is simultaneously unstable against several kinds of instabilities.

cond-mat.str-el↗

arXiv2007

Magnetic-field-induced transition in BaVS3

The metal-insulator transition (MIT) of BaVS3 is suppressed under pressure and above the critical pressure of p~2GPa the metallic phase is stabilized. We present the results of detailed magnetoresistivity measurements carried out at pressures near the critical value, in magnetic fields up to B=12T. We found that slightly below the critical pressure the structural tetramerization -- which drives the MIT -- is combined with the onset of magnetic correlations. If the zero-field transition temperature is suppressed to a sufficiently low value (T_MI<15K), the system can be driven into the metallic state by application of magnetic field. The main effect is not the reduction of T_MI with increasing B, but rather the broadening of the transition due to the applied magnetic field. We tentatively ascribe this phenomenon to the influence on the magnetic structure coupled to the bond-order of the tetramers.

cond-mat.str-el↗

arXiv2006

The emergence of Resonating Valence Bond physics in spin-orbital models

We discuss how orbital degeneracy, which is usually removed by a cooperative Jahn-Teller distortion, could under appropriate circumstances lead rather to a Resonating Valence Bond spin-orbital liquid. The key points are: i) The tendency to form spin-orbital dimers, a tendency already identified in several cases; ii) The mapping onto Quantum Dimer Models, which have been shown to possess Resonating Valence Bond phases on the triangular lattice. How this program can be implemented is explained in some details starting from a microscopic model of LiNiO$_2$.

cond-mat.str-el↗

arXiv2006

Quantum critical phase in BaVS$_3$

We study the high-pressure metallic phase of high-purity single crystals of BaVS$_3$ by measuring the temperature, pressure, and magnetic field dependence of the resistivity. Above the critical pressure of $p_{\rm cr}=1.97$GPa an extended non-Fermi liquid $p-T$ regime emerges with resistivity exponent $1.5 \le n<2$, crossing over to a FL only around $p=2.7$GPa. A hysteretic feature indicates that close to the insulator--metal boundary, the system is magnetically ordered. Our findings reveal a close analogy between the extended partially ordered NFL state of non-conventional itinerant magnets and the corresponding state of BaVS$_3

cond-mat.str-el↗

arXiv2005

Experimental Electronic Structure and Interband Nesting in BaVS_3

The correlated 3d sulphide BaVS_3 is a most interesting compound because of the apparent coexistence of one-dimensional and three-dimensional properties. Our experiments explain this puzzle and shed new light on its electronic structure. High-resolution angle-resolved photoemission measurements in a 4eV wide range below the Fermi level explored the coexistence of weakly correlated a_1g wide-band and strongly correlated e_g narrow-band d-electrons that is responsible for the complicated behavior of this material. The most relevant result is the evidence for a_1g--e_g inter-band nesting condition.

cond-mat.str-el↗

arXiv2004

Octupolar ordering of Gamma8 ions in magnetic field

We study f-electron lattice models which are capable of supporting octupolar, as well dipolar and quadrupolar, order. Analyzing the properties of the Gamma8 ground state quartet, we find that (111)-type combinations of the Gamma5 octupoles Tbeta(111)=Tbeta(x)+Tbeta(y)+Tbeta(z) are the best candidates for octupolar order parameters. Octupolar ordering induces Gamma5-type quadrupoles as secondary order parameter. Octupolar order is to some extent assisted, but in its basic nature unchanged, by allowing for the presence of quadrupolar interactions. In the absence of an external magnetic field, equivalent results hold antiferro-octupolar ordering on the fcc lattice. In this sense, the choice of our model is motivated by the recent suggestion of octupolar ordering in NpO2. The bulk of our paper is devoted to a study of the effect of an external magnetic field on ferro-octupolar ordering. We found that octupolar order survives up to a critical magnetic field if the field is lying in specific directions, while for general field directions, the underlying symmetry of the model is destroyed and therefore the phase transition suppressed even in weak fields. Field-induced multipoles and field-induced couplings between various order parameters are discussed on the basis of a group theoretical analysis of the Helmholtz potential. We also studied the effect of octupolar ordering on the non-linear magnetic susceptibility which satisfies Ehrenfest-type relations at continuous octupolar transitions.

cond-mat.str-el↗

arXiv2004

Orbital degeneracy as a source of frustration in LiNiO$_2$

Motivated by the absence of cooperative Jahn-Teller effect and of magnetic ordering in LiNiO$_2$, a layered oxide with triangular planes, we study a general spin-orbital model on the triangular lattice. A mean-field approach reveals the presence of several singlet phases between the SU(4) symmetric point and a ferromagnetic phase, a conclusion supported by exact diagonalizations of finite clusters. We argue that one of the phases, characterized by a large number of low-lying singlets associated to dimer coverings of the triangular lattice, could explain the properties of LiNiO$_2$, while a ferro-orbital phase that lies nearby in parameter space leads to a new prediction for the magnetic properties of NaNiO$_2$.

cond-mat.str-el↗

arXiv2003

Quadrupolar interactions in Pr compounds: PrFe4P12 and PrBa2Cu3O6

We examine trivalent Pr crystal field models with near-degeneracy of the two lowest crystal field levels, where interaction-induced quadrupolar and dipolar moments are as important as the permanent moments of the crystal field ground state. We find that the Gamma1-Gamma4 level scheme yields a successful description of the antiferroquadrupolar ordering of PrFe4P12. For PrBa2Cu3O6, we argue that quadrupolar interaction is important for understanding the Pr ordering transition at 11K.

cond-mat.str-el↗

arXiv2003

The electrical properties of Cd2Re2O7 under pressure

We examine the resistivity and thermopower of single crystal specimens of the pyrochlore oxide Cd2Re2O7 at pressures up to 2GPa. Thermopower proves to be a sensitive tool in the study of the phase diagram of Cd2Re2O7. The 200K metal-to-metal phase transition is accompanied by a strong increase of the absolute value of the thermopower. A weaker anomaly allows to identify a second phase transition at 125K. Following the temperature dependence of this anomaly, we obtain the corresponding phase boundary up to 1.2GPa, and argue that it must drop to T=0 before p reaches 1.8GPa. There is a wide temperature range where the electrical properties are fairly sensitive to pressure, indicating the strong coupling of the electronic degrees of freedom to the lattice.

cond-mat.str-el↗

arXiv2002

Competition and coexistence of magnetic and quadrupolar ordering

The large number of low-lying states of d- and f-shells supports a variety of order parameters. The effective dimensionality of the local Hilbert space depends on the strength, and kind, of intersite interactions. This gives rise to complicated phase diagrams, and an enhanced role of frustration and fluctuation effects. The general principles are illustrated on the example of the effect of a magnetic field on quadrupolar phase transitions in some Pr-based skutterudite compounds.

cond-mat.str-el↗

arXiv2000

Spin and Orbital Order in Itinerant Ferromagnets

The long-standing problem of the effect of correlations on the ferromagnetism of is apparently nearing solution. The ferromagnetism of transition metals compounds, for instance doped manganites, poses a new question: is there some kind of orbital order coexisting with itinerant ferromagnetism? The ideas and techniques introduced by Gutzwiller should be of use again.

cond-mat.str-el↗

arXiv2000

Pressure dependence of the spin gap in BaVS_3

We carried out magnetotransport experiments under hydrostatic pressure in order to study the nature of the metal-insulator transition in BaVS$_3$. Scaling relations for $ρ(T,H,p)$ are established and the pressure dependence of the spin gap is determined. Our new results, in conjunction with a re-analysis of earlier specific heat and susceptibility data, demonstrate that the transition is weakly second order. The nature of the phase diagram in the $T$--$p$--$H$ space is discussed.

cond-mat.str-el↗

arXiv2000

Orbitally Driven Spin Pairing in the 3D Non-Magnetic Mott Insulator BaVS3: Evidence from Single Crystal Studies

Static electrical and magnetic properties of single crystal BaVS_3 were measured over the structural (T_S=240K), metal-insulator (T_MI=69K), and suspected orbital ordering (T_X=30K) transitions. The resistivity is almost isotropic both in the metallic and insulating states. An anomaly in the magnetic anisotropy at T_X signals a phase transition to an ordered low-T state. The results are interpreted in terms of orbital ordering and spin pairing within the lowest crystal field quasi-doublet. The disordered insulator at T_X<T<T_MI is described as a classical liquid of non-magnetic pairs.

cond-mat.str-el↗

arXiv2000

Pressure Induced Quantum Critical Point and Non-Fermi-Liquid Behavior in BaVS3

The phase diagram of BaVS3 is studied under pressure using resistivity measurements. The temperature of the metal to nonmagnetic Mott insulator transition decreases under pressure, and vanishes at the quantum critical point p_cr=20kbar. We find two kinds of anomalous conducting states. The high-pressure metallic phase is a non-Fermi liquid described by Delta rho = T^n where n=1.2-1.3 at 1K < T < 60K. At p<p_cr, the transition is preceded by a wide precursor region with critically increasing resistivity which we ascribe to the opening of a soft Coulomb gap.

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arXiv1996

Band ferromagnetism versus collective Kondo state in lattice fermion models

It is becoming widely recognized that, contrary to earlier expectations, the usual one-band Hubbard model does not give an explanation for itinerant ferromagnetism. After reviewing the status of magnetic ordering in the one-band model, we discuss the possibility of ferromagnetism in some recently introduced two-band Hubbard models, and in generalized Anderson lattices. It is argued that these two classes of models are closely related and that it is their common feature that the ferromagnetic phase has to compete with a collective Kondo state.

cond-mat.str-el↗

arXiv1996

Collective Kondo effect in the Anderson-Hubbard lattice

The periodic Anderson model is extended by switching on a Hubbard U for the conduction electrons. We use the Gutzwiller variational method to study the nearly integral valent limit. The lattice Kondo energy contains the U-dependent chemical potential of the Hubbard subsystem in the exponent, and the correlation-induced band narrowing in the prefactor. Both effects tend to suppress the Kondo scale, which can be understood to result from the blocking of hybridization. At half-filling, we find a Brinkman-Rice-type transition from a Kondo insulator to a Mott insulator.

cond-mat.str-el↗

arXiv1996

Low density ferromagnetism in the Hubbard model

A single-band Hubbard model with nearest and next-nearest neighbour hopping is studied for $d=1$, 2, 3, using both analytical and numerical techniques. In one dimension, saturated ferromagnetism is found above a critical value of $U$ for a band structure with two minima and for small and intermediate densities. This is an extension of a scenario recently proposed by Müller--Hartmann. For three dimensions and non-pathological band structures, it is proven that such a scenario does not work.

cond-mat↗

arXiv1996

Interaction Effect in the Kondo Energy of the Periodic Anderson-Hubbard Model

We extend the periodic Anderson model by switching on a Hubbard $U_d$ for the conduction electrons. The nearly integral valent (Kondo) limit of the Anderson--Hubbard model is studied with the Gutzwiller variational method. The new formula for the Kondo energy contains the $U_d$-dependent chemical potential of the Hubbard subsystem in the exponent, and the correlation-induced band narrowing in the prefactor. Both effects tend to suppress the Kondo scale, which can be understood to result from the blocking of hybridization (this behaviour is the opposite of that found for Kondo--Hubbard models). At half-filling, we find a Brinkman--Rice-type transition which leads from a small-gap Kondo insulator to a Mott insulator.

cond-mat↗