14 research outputs found
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
The pseudogap phase in (TaSe_4)_2I
We have developed the mean-field theory of coexisting charge-density waves
(CDW) and unconventional charge-density waves (UCDW). The double phase
transition manifests itself in the thermodynamic quantities and in the magnetic
response, such as spin susceptibility and spin-lattice relaxation rate. Our
theory applies to quasi-one dimensional (TaSe_4)_2I, where above the CDW
transition, thermal fluctuations die out rapidly, but robust pseudogap
behaviour is still detected. We argue, that the fluctuations are suppressed due
to UCDW, which partially gaps the Fermi surface, and causes non-Fermi-liquid
(pseudogap) behaviour.Comment: 7 pages, 6 figure
Dirac fermionok szilárdtestekben = Dirac fermions in solids
Kutatásaink során olyan szilárdtestek viselkedésének elméleti leirását tűztük ki célul, melyekben az elektronok dinamikáját egy a Dirac egyenlethez hasonló összefüggés határozza meg. Tömeg nélküli fermionok energia-impulzus összefüggése igy lineáris lesz, melynek jelenleg messze legismertebb példája a szűk évtizede fölfedezett grafén. Ennek az anyagnak rezonancia módszerekkel történő vizsgálata kapcsán kiszámoltuk az NMR élettartamot és a rezonancia helyének eltolódását, valamint meghatároztuk az elektronok spin relaxációs tulajdonságait. A kisérletekkel való összevetés az intrinsic spin-pálya csatolás dominanciájára utal. Megvizsgáltuk a vezetési tulajdonságokat erős elektromos tér esetén, és azt találtuk, hogy a rendszerben folyó áram az elektromos tér 3/2-dik hatványával nő, melyet kisérletek is igazoltak. Tanulmányoztuk továbbá a grafénbeli Friedel-oszcilláció jellegzetességeit erősen lokalizált szennyező körül, és a lassan lecsengő, hosszú hullámhosszú oszcillációk mellett azonositottunk egy rövid hullámhosszú mintázatot is. Meghatároztuk a felületi akusztikus hullámok segitségével mérhető hullámszám függő vezetőképességet, valamint az állapotsűrűség megváltozását szennyezők hatására. Megállapitottuk, hogy az elektronok közötti taszitás miatt grafénban bekövetkező fém-szigetelő átmenet kritikus exponensei jelentősen eltérnek a Landau-elmélet értékeitől. | The aim of our research was to describe theoretically the behavior of solids in which the dynamics of electrons is governed by a formula similar to the Dirac equation. The energy-momentum relation of massless fermions will thus be linear, the most familiar example of which is graphene discovered within the past decade. In connection with the investigation of this material by resonance methods we calculated the NMR lifetime and the shift in the position of the resonance, and determined the spin relaxation properties of the electrons. Comparison with experiments points to the dominance of intrinsic spin-orbit coupling. We investigated the conductance properties in strong electric fields, and found that the current grows wit the 3/2 power of the electric field, which was confirmed by experiments. Moreover we studied the characteristics of Friedel oscillations in graphene around a well localized impurity, and in addition to the slowly decaying long wavelength oscillations we identified a short wavelength pattern as well. We determined the wavenumber dependent conductivity measurable by surface acoustic waves, and the change in the density of states due to impurities. We found that the critical exponents of the metal-insulator transition due to repulsion between electrons in graphene are significantly different from those given by the Landau theory
Unconventional density wave in CeCoIn_5?
Very recently large Nernst effect and Seebeck effect were observed above the
superconducting transition temperature 2.3K in a heavy fermion superconductor
CeCoIn_5. We shall interpret this large Nernst effect in terms of
unconventional density wave (UDW), which appears around T=18K. Also the
temperature dependence of the Seebeck coefficient below T=18K is described in
terms of UDW. Another hallmark for UDW is the angular dependent
magnetoresistance, which should be readily accessible experimentally.Comment: 4 pages, 7 figure
Pseudogap enhancement due to magnetic impurities in d-density waves
We study the effect of quantum magnetic impurities on d-wave spin density
waves (d-SDW). The impurity spins are aligned coherently according to the spin
space anisotropy of the condensate. Both the order parameter and transition
temperature increases due to the coherent interplay between magnetic scatterers
and d-SDW. This can explain the recent experimental data on the pseudogap
enhancement of Ni substituted NdBa_2{Cu_{1-y}Ni_y}O_6.8 from Pimenov et al.
(Phys. Rev. Lett. 94, 227003 (2005)).Comment: 4 pages, 3 figure
Spin- és töltésdinamika szilárd testekben és nanoszerkezetekben = Spin and charge dynamics in solids and nanostructures
Napjaink szilárdtestfizikai kutatásainak központi terĂĽletĂ©t jelentik azok a kvantum-jelensĂ©gek, amelyekben az elektron töltĂ©se Ă©s spinje egyaránt lĂ©nyeges szerepet játszik. A projekt keretĂ©ben ilyen jelensĂ©geket tanulmányoztunk hagyományos mĂłdszerekkel (ESR spektroszkĂłpia, elektromos Ă©s mágneses mĂ©rĂ©sek), valamint olyan Ăşj spektroszkĂłpia eljárásokkal, melyek nanotechnolĂłgiai megoldásokat alkalmaznak (alagĂşt- Ă©s pont-kontaktus-spektroszkĂłpia, Andrejev-spektroszkĂłpia). A kĂsĂ©rleti Ă©s elmĂ©leti mĂłdszerekkel vizsgált anyagcsaládok egzotikus alapállapottal rendelkezĹ‘ erĹ‘sen anizotrop kölcsönhatĂł elektronrendszerek, mint pĂ©ldául: i, szupravezetĹ‘ átalakulás közelĂ©ben lĂ©vĹ‘ kuprátok, ii., ritkaföldfĂ©m vegyĂĽletek nem-konvencionális sűrűsĂ©ghullámai, illetve iii., átmeneti fĂ©m oxidok a kvantum kritikus pont tartományában. A legfontosabb Ăşj eredmĂ©nyek a versengĹ‘ kölcsönhatásĂş rendszerek fázisdiagramjára, az antiferromágneses anyagok spin-gerjesztĂ©seinek közvetlen meghatározására, valamint ferromágneses anyagokban a töltĂ©shordozĂłk spin-polarizáltságának mĂ©rĂ©sĂ©re vonatkoznak. Ez utĂłbbi, az alapvetĹ‘en Ăşj nanotechnolĂłgiai mĂ©rĂ©stechnika alkalmazása mellett, a modern kvantum elmĂ©letek nanoszerkezetekre törtĂ©nĹ‘ kiterjesztĂ©sĂ©t is igĂ©nyelte. Az eredmĂ©nyeket nĂvĂłs nemzetközi folyĂłiratokban közöltĂĽk. A 40 kiemelt publikáciĂł között 6 db. Physical Review Letters Ă©s 27 db. Physical Review B cikk szerepel. | Electrons have spin and charge, and the quantum phenomena where both of these properties are relevant represent the hottest subjects in condensed matter research today. We investigated these features by bulk characterization methods (ESR spectroscopy, transport and magnetization measurements) supplemented with novel spectroscopic ones utilizing nanotechnology (tunneling-, point-contact, and Andreev-spectroscopy). Highly anisotropic interacting electron systems with exotic ground states have been studied both experimentally and theoretically. The prime examples are: i., cuprates in the vicinity of a superconducting transition, ii., rare-earth compounds with unconventional density waves and iii., transition metal oxides close to a quantum critical point. The most important new results deal with the determination of phase diagrams for systems of competing interactions, direct measurement of spin excitations in antiferromagtic structures, and determination of spin polarization of the charge carriers in ferromagnetic structures. The latter required novel nanoscale techniques, and the application of quantum theories for nanostructures. The results were published leading scientific journals. The top 40 publication include 6 Physical Review Letters and 27 Physical Review B papers
Gapped optical excitations from gapless phases: imperfect nesting in unconventional density waves
We consider the effect of imperfect nesting in quasi-one-dimensional
unconventional density waves in the case, when the imperfect nesting and the
gap depends on the same wavevector component.
The phase diagram is very similar to that in a conventional density wave. The
density of states is highly asymmetric with respect to the Fermi energy.
The optical conductivity at T=0 remains unchanged for small deviations from
perfect nesting. For higher imperfect nesting parameter, an optical gap opens,
and considerable amount of spectral weight is transferred to higher
frequencies. This makes the optical response of our system very similar to that
of a conventional density wave. Qualitatively similar results are expected in
d-density waves.Comment: 8 pages, 7 figure
Impurity scattering in unconventional density waves: non-crossing approximation for arbitrary scattering rate
We present a detailed theoretical study on the thermodynamic properties of
impure quasi-one dimensional unconventional charge-, and spin-density waves in
the framework of mean-field theory. The impurities are of the ordinary
non-magnetic type. Making use of the full self-energy that takes into account
all ladder-, and rainbow-type diagrams, we are able to calculate the relevant
low temperature quantities for arbitrary impurity concentration and scattering
rates. These are the density of states, specific heat and the shift in the
chemical potential. Our results therefore cover the whole parameter space: they
include both the self-consistent Born and the resonant unitary limits, and most
importantly give exact results in between.Comment: 11 pages, 8 figure