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A. Bjelis

Publications and source records attributed to A. Bjelis.

16 recordsLinked to original sources
arXiv2002

Photo-emission properties of quasi-one-dimensional conductors

We calculate the self-energy of one-dimensional electron band with the three-dimensional long range Coulomb interaction within the random phase approximation, paying particular attention to the contribution coming from the electron scatterings on the collective plasmon mode. It is shown that the spectral density has a form of wide feature at thr frequency scale of the plasmon frequency, without the presence of quasi-particle delta-peaks. The relevance of this result with respect to experimental findings and to the theory of Luttinger liquids is discussed.

cond-mat↗
arXiv2002

Magnetic oscillations and field induced spin density waves in (TMTSF)_2ClO_4

We report an analysis of the effects of magnetic field on a quasi-one-dimensional band of interacting electrons with a transverse dimerizing potential. One-particle problem in bond-antibond representation is solved exactly. The resulting propagator is used to calculate the spin-density-wave (SDW) response of the interacting system within the matrix RPA for the SDW susceptibility. We predict the magnetic field induced transition of the first order between interband SDW_0 and intraband SDW_(+-) phases. We reproduce the rapid oscillations with a period of 260 Tesla and the overal profile of the TMTSF_2ClO_4 phase diagram.

cond-mat.str-el↗
arXiv2002

Discrete approach to incoherent excitations in conductors

Keeping the discretness of the reciprocal space we calculate the spectrum of incoherent electron-hole excitations in the conducting Fermi liquids. The metod is illustrated on the well-known jellium model within the random phase approximation. It also leads to the formulation os a sum rule from which we get the details os dispersion curve for the collective plasmon mode. The notion of time averaging in the discrete approach is briefly recalled.

cond-mat.soft↗
arXiv2001

New SDW phases in quasi-one-dimensional systems dimerized in the transverse direction

The spin density wave instabilities in the quasi-one-dimensional metal (TMTSF)_2ClO_4 are studied in the framework a matrix random phase approximation for intra-band and inter-band order parameters. Depending on the anion ordering potential V which measures the lattice doubling in the transverse direction, two different instabilities are possible. The SDW_0 state at low values of V is antiferromagnetic in b direction and has the critical temperature that decreases rapidly with V. The degenerated states SDW_(+-), stable at higher values of V, are superpositions of two magnetic orders, each one on its subfamily of chains. As V increases the ratio between two components of SDW_(+-) tends to zero and the critical temperature increases asymptotically towards that of SDW instability for a system having perfect nesting and no anion order. At intermediate V the metallic state can persist down to T=0.

cond-mat.str-el↗
arXiv2000

Collective modes in uniaxial incommensurate-commensurate systems with the real order parameter

The basic Landau model for uniaxial systems of the II class is nonintegrable, and allows for various stable and metastable periodic configurations, beside that representing the uniform (or dimerized) ordering. In the present paper we complete the analysis of this model by performing the second order variational procedure, and formulating the combined Floquet-Bloch approach to the ensuing nonstandard linear eigenvalue problem. This approach enables an analytic derivation of some general conclusions on the stability of particular states, and on the nature of accompanied collective excitations. Furthermore, we calculate numerically the spectra of collective modes for all states participating in the phase diagram, and analyze critical properties of Goldstone modes at all second order and first order transitions between disordered, uniform and periodic states. In particular it is shown that the Goldstone mode softens as the underlying soliton lattice becomes more and more dilute.

cond-mat.mtrl-sci↗
arXiv2000

Acoustic collective excitations and static dielectric response in incommensurate crystals with real order parameter

Starting from the basic Landau model for the incommensurate-commensurate materials of the class II, we derive the spectrum of collective modes for all (meta)stable states from the corresponding phase diagram. It is shown that all incommensurate states posses Goldstone modes with acoustic dispersions. The representation in terms of collective modes is also used in the calculation and discussion of static dielectric response for systems with the commensurate wave number in the center of the Brillouin zone.

cond-mat.mtrl-sci↗
arXiv2000

Landau Model for Commensurate-Commensurate Phase Transitions in Uniaxial Improper Ferroelectric Crystals

We propose the Landau model for lock-in phase transitions in uniaxially modulated improper ferroelectric incommensurate-commensurate systems of class I. It includes Umklapp terms of third and fourth order and secondary order parameter representing the local polarization. The corresponding phase diagram has the structure of harmless staircase, with the allowed wave numbers obeying the Farey tree algorithm. Among the stable commensurate phases only those with periods equal to odd number of lattice constants have finite macroscopic polarizations. These results are in excellent agreement with experimental findings in some A2BX4 compounds.

cond-mat.mtrl-sci↗
arXiv1999

Pauli and orbital effects of magnetic field on charge density waves

Taking into account both Pauli and orbital effects of external magnetic field we compute the mean field phase diagram for charge density waves in quasi-one-dimensional electronic systems. The magnetic field can cause transitions to CDW states with two types of the shifts of wave vector from its zero-field value. It can also stabilize the field-induced charge density wave. Furthermore, the critical temperature shows peaks at a new kind of magic angles.

cond-mat.str-el↗
arXiv1998

Crystal stability and optical properties of organic chain compounds

The solution to the long standing problem of the cohesion of organic chain compounds is proposed. We consider the tight-binding dielectric matrix with two electronic bands per chain, determine the corresponding hybridized collective modes, and show that three among them are considerably softened due to strong dipole-dipole and monopole-dipole interactions. By this we explain the unusual low frequency optical activity of TTF-TCNQ, including the observed 10meV anomaly. The softening of the modes also explains the cohesion of the mixed-stack lattice, the fractional charge transfer almost independent of the material, and the formation of the charged sheets in some compounds.

cond-mat↗
arXiv1998

Landau model for uniaxial systems with complex order parameter

We study the Landau model for uniaxial incommensurate-commensurate systems of the I class by keeping Umklapp terms of third and fourth order in the expansion of the free energy. It applies to systems in which the soft mode minimum lies between the corresponding commensurate wave numbers. The minimization of the Landau functional leads to the sine-Gordon equation with two nonlinear terms, equivalent to the equation of motion for the well-known classical mechanical problem of two mixing resonances. We calculate the average free energies for periodic, quasiperiodic and chaotic solutions of this equation, and show that in the regime of finite strengths of Umklapp terms only periodic solutions are absolute minima of the free energy, so that the phase diagram contains only commensurate configurations. The phase transitions between neighboring configurations are of the first order, and the wave number of ordering goes through harmless staircase with a finite number of steps. These results are the basis for the interpretation of phase diagrams for some materials from the I class of incommensurate-commensurate systems, in particular of those for A$_2$BX$_4$ and BCCD compounds. Also, we argue that chaotic barriers which separate metastable periodic solutions represent an intrinsic mechanism for observed memory effects and thermal hystereses.

cond-mat.mtrl-sci↗
arXiv1996

Dielectric properties of multiband electron systems: II - Collective modes

Starting from the tight-binding dielectric matrix in the random phase approximation we examine the collective modes and electron-hole excitations in a two-band electronic system. For long wavelengths (${\bf q}\rightarrow0$), for which most of the analysis is carried out, the properties of the collective modes are closely related to the symmetry of the atomic orbitals involved in the tight-binding states. In insulators there are only inter-band charge oscillations. If atomic dipolar transitions are allowed, the corresponding collective modes reduce in the asymptotic limit of vanishing bandwidths to Frenkel excitons for an atomic insulator with weak on-site interactions. The finite bandwidths renormalize the dispersion of these modes and introduce a continuum of incoherent inter-band electron-hole excitations. The possible Landau damping of collective modes due to the presence of this continuum is discussed in detail.

cond-mat↗
arXiv1996

Dielectric properties of multiband electron systems: I - Tight-binding formulation

The screened electron-electron interaction in a multi-band electron system is calculated within the random phase approximation and in the tight-binding representation. The obtained dielectric matrix contains, beside the usual site-site correlations, also the site-bond and bond-bond correlations, and thus includes all physically relevant polarization processes. The arguments are given that the bond contributions are negligible in the long wavelength limit. We analyse the system with two non-overlapping bands in this limit, and show that the corresponding dielectric matrix reduces to a $2\times2$ form. The intra-band and inter-band contributions are represented by diagonal matrix elements, while the off-diagonal elements contain the mixing between them. The latter is absent in insulators but may be finite in conductors. Performing the multipole expansion of the bare long-range interaction, we show that this mixing is directly related to the symmetry of the atomic orbitals participating in the tight-binding electronic states. In systems with forbidden atomic dipolar transitions, the intra-band and inter-band polarizations are separated. However, when the dipolar transitions are allowed, the off-diagonal elements of the dielectric matrix are of the same order as diagonal ones, due to a finite monopole-dipole interaction between the intra-band and inter-band charge fluctuations.

cond-mat↗
arXiv1995

THE PHASE DIAGRAM FOR THE SINE-GORDON MODEL WITH TWO UMKLAPP TERMS

We study the Landau free energy for a uniaxial ordering, taking into account two Umklapp terms of comparable strengths (those of the third and fourth order). Exploring the analogy with the well-known nonintegrable classical mechanical problem of two mixed nonlinear resonances, we complete the previous studies of the corresponding phase portrait by calculating numerically periodic solutions, including those far from the separatrices. It is shown that in the physical range of parameters only periodic configurations are absolutely stable. We determine for the first time the complete thermodynamic phase diagram and show that, in contrast to some earlier claims, the wave number of the ordering does not pass through the devil's staircase, but through a finite number of steps which decreases as the amplitudes of the Umklapp terms increase.

cond-mat↗
arXiv1995

THE TIGHT-BINDING APPROACH TO THE DIELECTRIC RESPONSE IN THE MULTIBAND SYSTEMS

Starting from the random phase approximation for the weakly coupled multiband tightly-bounded electron systems, we calculate the dielectric matrix in terms of intraband and interband transitions. The advantages of this representation with respect to the usual plane-wave decomposition are pointed out. The analysis becomes particularly transparent in the long wavelength limit, after performing the multipole expansion of bare Coulomb matrix elements. For illustration, the collective modes and the macroscopic dielectric function for a general cubic lattice are derived. It is shown that the dielectric instability in conducting narrow band systems proceeds by a common softening of one transverse and one longitudinal mode. Furthermore, the self-polarization corrections which appear in the macroscopic dielectric function for finite band systems, are identified as a combined effect of intra-atomic exchange interactions between electrons sitting in different orbitals and a finite inter-atomic tunneling.

cond-mat↗
arXiv1994

Domain Patterns in Incommensurate Systems with the Uniaxial Real Order Parameter

The basic Landau model for the incommensurate-commensurate transition to the uniform or dimerized uniaxial ordering is critically reexamined. The previous analyses identified only sinusoidal and homogeneous solutions as thermodynamically stable and proposed a simple phase diagram with the first order phase transition between these configurations. By performing the numerical analysis of the free energy and the Euler-Lagrange equation we show that the phase diagram is more complex. It also contains a set of metastable solutions present in the range of coexistence of homogeneous and sinusoidal solutions. These new configurations are periodic patterns of homogeneous domains connected by sinusoidal segments. They are Lyapunov unstable, very probably due to the nonintegrability of the free energy functional. We also discuss some other mathematical aspects of the model, and compare it with the essentially simpler sine-Gordon model for the transitions to the states with higher commensurabilities. The present results might be a basis for the explanation of phenomena like thermal hystereses, cascades of phase transitions, and memory effects, observed in materials exhibiting the transitions to uniform or dimerized state. Some particular examples are discussed in detail.

cond-mat↗