64 research outputs found
Collective Spin Fluctuation Mode and Raman Scattering in Superconducting Cuprates
Although the low frequency electronic Raman response in the superconducting
state of the cuprates can be largely understood in terms of a d-wave energy
gap, a long standing problem has been an explanation for the spectra observed
in the polarization orientations. We present calculations which
suggest that the peak position of the observed spectra is due to a
collective spin fluctuation mode.Comment: 4 pages, 5 eps figure
The Role of Splayed Disorder and Channel Flow on the Dynamics of Driven 3D Vortices
We present the results of three-dimensional molecular dynamics simulations of
vortices which indicate that, for B greater than the matching field, the
enhanced pinning effectiveness of splayed columnar defects relative to vertical
columnar defects can be explained in terms of the existence or absence of
channels through which the vortices can flow without encountering defects.Comment: 2 Pages, 3 Figures, Proceedings from Houston M2S-HTSC-VI References
altered, minor correction
Unconventional electronic Raman spectra of borocarbide superconductors
Borocarbide superconductors, which are thought to be conventional BCS-type
superconductors, are not so conventional in several electronic Raman
properties. Anisotropic gap-like features and finite scattering strength below
the gap were observed for the NiBC ( = Lu, Y) systems in our
previous study. The effects of Co-doping on the 2 gap-like features and
the finite scattering strength below and above the gap are studied in = Lu
(B = B) system. In superconducting states, Co-doping strongly suppresses
the 2 peak in both B and B symmetries. Raman
cross-section calculation which includes inelastic scattering shows a
relatively good fit to the features above the 2 peak, while it does not
fully account for the features below the peak.Comment: 2 pages, 1 figur
Non-resonant Raman scattering through a metal-insulator transition: an exact analysis of the Falicov-Kimball model
For years, theories for Raman scattering have been confined to either the
insulating or fully metallic state. While much can be learned by focusing attention
on the metal or insulator, recent experimental work on the cuprate
systems points to the desirability of formulating a theory for Raman response
which takes one through a quantum critical point – the metalinsulator
transition. Using the Falicov-Kimball model as a canonical model
of a MIT, we employ dynamical mean-field theory to construct an exact theory
for non-resonant Raman scattering. In particular we examine the formation
of charge transfer peaks and pseudogaps as well as the low-energy
dynamics. The results are qualitatively compared to the experimental B₁g
Raman spectra in the cuprates, which probes the hot quasiparticles along
the Brillouin zone axes. The results shed important information on normal
state electronic transport and the pseudo-gap in the cuprates.Упродовж років теорія комбінаційного розсіяння (КР) обмежувалася розглядом або ізоляторів або суто металічного стану. Хоча можна багато довідатися, зосередивши увагу тільки на металах чи ізоляторах, останні експериментальні роботи з купратних систем вказують на бажаність формулювання теорії раманівського відгуку, яке
придатне при проходженні через квантову критичну точку - перехід
метал-ізолятор (ПМІ). Використовуючи модель Фалікова-Кімбала як
канонічну модель ПМІ, ми застосовуємо теорію динамічного середнього поля для побудови точної теорії нерезонансного КР. Зокрема,
ми розглядаємо утворення піків, зумовлених переносом заряду, та
псевдощілин, а також низькоенергетичну динаміку. Результати якісно зіставимі з експериментальними B₁g спектрами КР у купратах, в
яких фіксують “гарячі” квазічастинки вздовж осей зони Брілюена. Результати дають важливу інформацію про електронний транспорт у
нормальному стані та псевдощілину в купратах
Resonant electronic Raman scattering near a quantum critical point
We calculate the resonant electronic Raman scattering for the Falicov-Kimball
model near the Mott transition on a hypercubic lattice. The solution is exact,
and employs dynamical mean field theory.Comment: 2 pages, 2 figures, contribution to the SCES04 conferenc
The Pairing Mechanism in HTSC investigated by Electronic Raman Scattering
By means of electronic Raman scattering we investigated the symmetry of the
energy gap Delta(k), its temperature dependence and its variation with doping
of well characterized Bi2Sr2CaCu2O8+delta single crystals. The oxygen content
delta was varied between the under- and the overdoped regime by subsequently
annealing the same single crystal in Ar and O2, respectively. The symmetry
analysis of the polarized electronic Raman scattering is consistent with a
d_x^2-y^2-wave symmetry of the energy gap in both regimes. The gap ratio
2Delta_max/k_BT_c and its temperature dependence changes with doping similar to
predictions of theories based on paramagnon coupling.Comment: 3 pages, LaTeX, 2 ps figures available on request to
[email protected]
Response, relaxation and transport in unconventional superconductors
We investigate the collision-limited electronic Raman response and the
attenuation of ultrasound in spin-singlet d-wave superconductors at low
temperatures. The dominating elastic collisions are treated within a t-matrix
approximation, which combines the description of weak (Born) and strong
(unitary) impurity scattering. In the long wavelength limit a two-fluid
description of both response and transport emerges. Collisions are here seen to
exclusively dominate the relaxational dynamics of the (Bogoliubov)
quasiparticle system and the analysis allows for a clear connection of response
and transport phenomena. When applied to quasi-2-d superconductors like the
cuprates, it turns out that the transport parameter associated with the Raman
scattering intensity for B1g and B2g photon polarization is closely related to
the corresponding components of the shear viscosity tensor, which dominates the
attenuation of ultrasound. At low temperatures we present analytic solutions of
the transport equations, resulting in a non-power-law behavior of the transport
parameters on temperature.Comment: 22 pages, 3 figure
Interpreting pulse-shape effects in pump-probe spectroscopies
The effect of the pulse-shape on pump-probe spectroscopies is examined for the simplest model of noninteracting fermions on an infinite-dimensional hypercubic lattice. The probe-modified density of states follows the time
evolution of the pump and displays narrowing and Floquet-like sidebands at the pump maximum, whereas the
photoelectron spectra are also strongly affected by the nonequilibrium occupation of the single-particle states
due to the excitation from the pump. The nonequilibrium Raman cross section is derived, and the nonresonant one in both the A1g and B1g symmetries contains a number of peaks at the pump maximum, which can
be attributed to an interference effect or Brillouin scattering off the time variations of the stress tensor. Both
the “measured” occupation of single-particle states and the ratio of Stokes to anti-Stokes peaks are strongly
modified by the probe-pulse width, which must be included in the interpretation of experimental results.Дослiджено вплив форми iмпульсу в експериментах з iмпульсами нагнiтання та вимiру для випадку найпростiшої моделi невзаємодiючих фермiонiв на безмежновимiрнiй гiперкубiчнiй ґратцi. Отримано, що
модифiкована iмпульсом вимiру густина станiв слiдує часовiй еволюцiї iмпульсу нагнiтання. Коли iмпульс
нагнiтання досягає максимуму, пiк на густинi станiв вужчає i з’являються додатковi Флоке-подiбнi боковi
зони. Спектри фотоелектронної емiсiї також зазнають значних змiн внаслiдок нерiвноважного заповнення одночастинкових станiв пiд дiєю iмпульсу нагнiтання. Виведено формулу для розрахунку нерiвноважного перерiзу комбiнацiйного розсiяння свiтла та отримано, що нерiвноважна складова перерiзу розсiяння як для A1g, так i для B1g симетрiй має багатопiкову структуру, що може бути пояснено ефектами
iнтерференцiї чи брiллюенового розсiяння на часових змiнах оператора тензора напружень. Отримано,
що i “вимiряне” заповнення одночастинкових станiв, i вiдношення iнтенсивностi стоксових до антистоксових пiкiв сильно залежать вiд ширини пробного iмпульсу, що необхiдно враховувати при аналiзi результатiв експериментiв
Correlation tuned cross-over between thermal and nonthermal states following ultrafast transient pumping
We examine electron-electron mediated relaxation following excitation of a
correlated system by an ultrafast electric field pump pulse. The results reveal
a dichotomy in the temporal evolution as one tunes through a Mott
metal-to-insulator transition: in the metallic regime relaxation can be
characterized by evolution toward a steady-state electronic distribution well
described by Fermi-Dirac statistics with an increased effective temperature;
however, in the insulating regime this quasithermal paradigm breaks down with
relaxation toward a nonthermal state with a more complicated electronic
distribution that does not vary monotonically as a function of energy. We
characterize the behavior by studying changes in the energy, photoemission
response, and electronic distribution as functions of time. Qualitatively these
results should be observable on short enough time scales that the electrons
behave like an isolated system not in contact with additional degrees of
freedom which can act as a thermal bath. Importantly, proper modeling used to
analyze experimental findings should account for this behavior, especially when
using strong driving fields or studying materials whose physics may manifest
the effects of strong correlations.Comment: Main Text: 5 pages, 4 figures; Supplementary Material: 3 pages, 5
figure
Raman Scattering versus Infrared Conductivity: Evidence for one-dimensional Conduction in La_{2-x}Sr_{x}CuO_{4}
Raman and Infrared (IR) spectra of an underdoped La_{1.90}Sr_{0.10}CuO_{4}
single crystal have been measured as a function of temperature. Both techniques
provide unconventional low-energy spectra. The IR conductivity exhibits
features peaked at finite frequencies which do not have a counterpart in the
Raman response. Below approximately 100 K a transfer of both Raman and IR
spectral weight towards lower energies is found and a new component in the
Raman response builds up being characterized by a very long lifetime of
electrons propagating along the Cu-O bonds.Comment: 4 pages, 3 eps figure
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