45 research outputs found
Thermodynamics and optical conductivity of unconventional spin density waves
We consider the possibility of formation of an unconventional spin density
wave (USDW) in quasi-one dimensional electronic systems. In analogy with
unconventional superconductivity, we develop a mean field theory of SDW
allowing for the momentum dependent gap on the Fermi surface.
Conditions for the appearence of such a low temperature phase are investigated.
The excitation spectrum and basic thermodynamic properties of the model are
found to be very similar to those of d-wave superconductors in spite of the
different topology of their Fermi surfaces. Several correlation functions are
calculated, and the frequency dependent conductivity is evaluated for various
gap functions. The latter is found to reflect the maximum gap value, however
with no sharp onset for absorbtion.Comment: 13 pages, 11 figures, submitted to Phys. Rev.
Európai Graduális Kollégium = International Research Training Group
A kutatási támogatást a Budapest-Marburg nemzetközi doktori iskola kĂ©t magyar PhD hallgatĂłjának ösztöndĂjára fordĂtottuk. A hallgatĂłk szemĂ©lye a futamidĹ‘ alatt tanĂ©venkĂ©nt változott, Ăgy a beszámolásköteles tevĂ©kenysĂ©g a doktori iskola több tĂ©máját is Ă©rinti. Az ''Elektron spin rezonancia szilárdtestekben'' cĂmű tĂ©makörben kifejlesztettĂĽnk kĂ©t, speciális feladatok elvĂ©gzĂ©sĂ©re alkalmas ESR spektromĂ©tert, melyeknek kiválĂł tulajdonságait antiferromágneses, szupravezetĹ‘ Ă©s fullerid polimer anyagokon vĂ©gzett mĂ©rĂ©sekkel demonstráltuk. A ''Waveletek Ă©s változĂł felbontásĂş analĂzis'' cĂmű kutatási program keretĂ©ben olyan elektronszerkezet számĂtási eljárást dolgoztunk ki, melynek erĹ‘forrás igĂ©nye kvantumkĂ©miai problĂ©mák esetĂ©n az elektronszámnak csupán a logaritmusával skálázĂłdik. Az ''Amorf fĂ©lvezetĹ‘k növekedĂ©si modelljei'' cĂmű tĂ©makör keretĂ©n belĂĽl molekuladinamikai szimuláciĂłkkal sikeresen Ă©rtelmeztĂĽk a kĂĽlönbözĹ‘ beesĂ©si szögek alatt növesztett szelĂ©n vĂ©konyrĂ©tegek eltĂ©rĹ‘ viselkedĂ©sĂ©t a fĂ©ny hatására bekövetkezĹ‘ tĂ©rfogatváltozást illetĹ‘en. A ''Nemkonvencionális sűrűsĂ©ghullámok'' cĂmű kutatási irány keretĂ©n belĂĽl elmĂ©leti eredmĂ©nyeinket kĂsĂ©rletekkel összevetve meggyĹ‘zĹ‘en demonstráltuk szamos anyag (egyes magashĹ‘mĂ©rsĂ©kletű szupravezetĹ‘k, töltĂ©sátviteli sĂłk Ă©s nehĂ©zfermionos anyagok) esetĂ©ben, hogy anomális viselkedĂ©sĂĽk (pszeudogap, termomágneses transzport) hátterĂ©ben a fázisdiagrammjuk egyes tartományaiban megjelenĹ‘ nemkonvencionális kondenzátum áll. | The grant support was used to finance the fellowships of two hungarian PhD students of the Budapest-Marburg international graduate school. During the grant period the persons supported by the fellowships changed from schoolyear to schoolyear, therefore the present report touches upon several projects of the graduate school. Within the framework of the project ''Electron spin resonance in solids'' we have developed two ESR spectrometers able to perform special tasks. The exceptional qualities of these devices were demonstrated by measurements on antiferromagnetic, superconducting and fullerid polymer materials. Work on the project ''Wavelets and multiresolution analysis'' led to a procedure for quantum chemical electron structure calculations, which requires resources proportional only to the logarithm of the number of electrons. We have performed molecular dynamics simulations within the topic ''Growth models of amorphous semiconductors''. Our research explained the different photoinduced volume changes in chalcogenid glasses, depending on the angle of deposition. While working on the project ''Unconventional density waves'', we compared our theoretical results with experiments, and convincingly demonstrated that a number of materials (certain high temperature superconductors, charge transfer salts and heavy fermion materials) owe their anomalous behavior (pseudogap, thermomagnetic transport) to unconventional condensates in regions of their phase diagrams
Magnetotransport in d-wave density waves
Angle dependent magnetoresistance (ADMR) and giant Nernst effect are
hallmarks of unconventional density waves (UDW). Here these transport
properties for d-wave density wave (d-DW) are computed for
quasi-two-dimensional systems. The present theory describes ADMR observed in
the pseudogap phase of Y_0.68Pr_0.32Ba_2Cu_3O_7 and CeCoIn_5 single crystals
very satisfactorily.Comment: 7 pages, 6 figure
Optical conductivity of d-wave superconductors
We study theoretically the optical conductivity of d-wave superconductors
like in high temperature cuprates in the presence of impurities. We limit
ourselves at T=0K and focus on the frequency dependence of both sigma_1(omega)
and \omega*\sigma_2(omega) for omega<2Delta. When the impurity scattering is in
the unitary limit, we find a peak in sigma_1(omega) with omega/Delta\simeq
0.1\sim 0.5$, which may account for the peak seen by Basov et al. in
Zn-substituted YBCO.Comment: 7 pages, 9 figures, submitted to Europhys. Let
Unconventional charge density wave driven by electron-phonon coupling
We report our study on unconventional charge density waves (UCDW) (i.e. a
charge density wave with wavevector dependent gap) in pure quasi-one
dimensional conductors. We develop a new possible mechanism of establishment of
such a low temperature phase, in which the driving force of the phase
transition is the electron-phonon interaction with coupling depending on both
the momentum transfer q and the momentum of the scattered electron k. Mean
field treatment is applied to obtain the excitation spectrum, correlation
functions such as the density correlator and the optical conductivity, and the
effective mass of the phase excitation. The fluctuation of the order parameter
leads to the sliding of the UCDW as a whole. In the absence of impurities, we
calculated the effect of this fluctuation on the optical properties. The
inclusion of the collective mode significantly alters the optical conductivity,
and leads to an effective mass which is nonmonotonic in temperature as opposed
to conventional CDWs.Comment: 11 figures, 13 pages, Revtex4 styl
Nemkonvencionális kondenzátumok szilárdtestekben = Unconventional Condensates in Solids
A kutatás során a kölcsönhatĂł elektronrendszerek nemkonvencionális kondenzátumokkal jellemezhetĹ‘ alacsony hĹ‘mĂ©rsĂ©kleti fázisait vizsgáltuk, kĂĽlönös tekintettel a szupravezetĹ‘ Ă©s sűrűsĂ©ghullám állapotokra. Egy alkalmasan választott átlagtĂ©r elmĂ©let segĂtsĂ©gĂ©vel meghatároztunk számos, kĂsĂ©rleti relevanciával bĂrĂł fizikai mennyisĂ©get ezekben a rendszerekben. SzámĂtásokat vĂ©geztĂĽnk többek között a nemkonvencionális sűrűsĂ©ghullámok mágneses tĂ©rbeli transzport tulajdonságaira vonatkozĂłan, megvizsgáltuk a Raman szĂłrás Ă©s az elektron-fonon csatolás jellegzetessĂ©geit, valamint tanulmányoztuk a kondenzátum Ă©s kĂĽlönfĂ©le szennyezĹ‘k kölcsönhatását. EredmĂ©nyeinket összevetettĂĽk több magashĹ‘mĂ©rsĂ©kletű szupravezetĹ‘n, szerves töltĂ©sátviteli sĂłn Ă©s nehĂ©zfermionos anyagon vĂ©gzett mĂ©rĂ©sekkel, Ă©s a legtöbb esetben nem csak kvalitatĂv, hanem kvantitatĂv egyezĂ©st találtunk. Vizsgálataink tovább erĹ‘sĂtettĂ©k azt a feltĂ©telezĂ©st, hogy az emlĂtett anyagok fázisdiagrammjának egyes tartományaiban Ă©szlelt anomális viselkedĂ©s egy nemkonvencionális sűrűsĂ©ghullám kondenzátum jelenlĂ©tĂ©nek tulajdonĂthatĂł. | We have investigated the low temperature phases of the interacting electron systems characterized by unconventional condensates. We payed particular attention to superconductors and density waves. Using a suitable mean field theory we have determined a number of physical quantities of experimental interest in these systems. We have calculated among others the magnetotransport properties of unconventional density waves, investigated the peculiarities of Raman scattering and electron-phonon coupling, and studied the interaction of the condensate with various kinds of impurities. We have compared our results with measurements on several high temperature superconductors, organic charge transfer salts and heavy fermion materials, and in most cases we have found not only qualitative, but quantitative agreement. Our investigations have further strengthened the case, that the anomalous behavior in certain regions of the phase diagram of these materials is due to an unconventional density wave condensate
Local density of states and Friedel oscillations around a non-magnetic impurity in unconventional density wave
We present a mean-field theoretical study on the effect of a single
non-magnetic impurity in quasi-one dimensional unconventional density wave. The
local scattering potential is treated within the self-consistent -matrix
approximation. The local density of states around the impurity shows the
presence of resonant states in the vicinity of the Fermi level, much the same
way as in -density waves or unconventional superconductors. The assumption
for different forward and backscattering, characteristic to quasi-one
dimensional systems in general, leads to a resonance state that is double
peaked in the pseudogap. The Friedel oscillations around the impurity are also
explored in great detail, both within and beyond the density wave coherence
length . Beyond we find power law behavior as opposed to the
exponential decay of conventional density wave. The entropy and specific heat
contribution of the impurity are also calculated for arbitrary scattering
strengths.Comment: 13 pages, 4 figure