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A. Kaminski

Publications and source records attributed to A. Kaminski.

At least 127 records · Page 7Linked to original sources
arXiv2004

Momentum anisotropy of the scattering rate in cuprate superconductors

We examine the momentum and energy dependence of the scattering rate of the high temperature cuprate superconductors using angle resolved photoemission spectroscopy. The scattering rate is of the form a + bw around the Fermi surface for under and optimal doping. The inelastic coefficient "b" is found to be isotropic. The elastic term, "a", however, is found to be highly anisotropic for under and optimally doped samples, with an anisotropy which correlates with that of the pseudogap. This is contrasted with heavily overdoped samples, which show an isotropic scattering rate and an absence of the pseudogap above T_c. We find this to be a generic property for both single and double layer compounds.

cond-mat.str-el↗
arXiv2004

Ferromagnetic and random spin ordering in diluted magnetic semiconductors

In a diluted magnetic semiconductor system, the exchange interaction between magnetic impurities has two independent components: a direct antiferromagnetic interaction and a ferromagnetic interaction mediated by charge carriers. Depending on the system parameters, the ground state of the system may be ordered either ferromagnetically or randomly. In this paper we use percolation theory to find the ferromagnetic transition temperature and the location of the quantum critical point separating the ferromagnetic phase and a valence bond glass phase.

cond-mat.dis-nn↗
arXiv2004

Griffiths phase in diluted magnetic semiconductors

We study the effects of disorder in the vicinity of the ferromagnetic transition in a diluted magnetic semiconductor in the strongly localized regime. We derive an effective polaron Hamiltonian, which leads to the Griffiths phase above the ferromagnetic transition point. The Griffiths-McCoy effects yield non-perturbative contributions to the dynamic susceptibility. We explicitly derive the long-time susceptibility, which has a pseudo-scaling form, with the dynamic critical exponent being expressed through the percolation indices.

cond-mat.mtrl-sci↗
arXiv2004

Momentum anisotropy of the scattering rate in cuprate superconductors

We examine the momentum and energy dependence of the scattering rate of the high temperature cuprate superconductors using angle resolved photoemission spectroscopy. The scattering rate is of the form a + b*w. The inelastic coefficient b is found to be isotropic. The elastic term, a, however, is found to be highly anisotropic in the pseudogap phase of optimal doped samples, with an anisotropy which correlates with that of the pseudogap. This can be contrasted with heavily overdoped samples, which show an isotropic scattering rate in the normal state.

cond-mat.str-el↗
arXiv2003

Identifying the Background Signal in ARPES of High Temperature Superconductors

One of the interesting features of the photoemission spectra of the high temperature cuprate superconductors is the presence of a large signal (referred to as the "background'') in the unoccupied region of the Brillouin zone. Here we present data indicating that the origin of this signal is extrinsic and is most likely due to strong scattering of the photoelectrons. We also present an analytical method that can be used to subtract the background signal.

cond-mat.str-el↗
arXiv2003

Angle Resolved Photemission Experiments to look for Time-reversal Violation in Cuprates

The recent experiments reported by Borisenko et al. (cond-mat/0305179 v2), are examined in light of the conditions to be satisfied in the search for time-reversal violation by circularly polarized ARPES. Two principal problems are found: (1) A lack of any evidence for the magnitude of the pseudogap or the temperature of its onset in the samples studied. (2) A difference in the dichroic signal at low and high temperatures. The difference is greater than the stated error bars and is contrary to the conclusions reached in the paper.

cond-mat↗
arXiv2003

Magnetic and transport percolation in diluted magnetic semiconductors

The ferromagnetic transition in a diluted magnetic semiconductor with localized charge carriers is inevitably a percolation transition. In this work we theoretically study the correlation between this magnetic percolation and transport properties of the sample, including the possibility of a simultaneous transport percolation. We find nontrivial signatures of the percolating magnetic clusters in the transport properties of the system, including interesting non-monotonic temperature dependence of the system resistivity.

cond-mat.mtrl-sci↗
arXiv2003

Reply to Comment on: "Spontaneous breaking of time-reversal symmetry in the pseudogap state of a high-Tc superconductor"

In a recent comment [1], Armitage and Hu have suggested that our experiment observing dichroism in angle resolved photoemission (ARPES) [2] could not be conclusively interpreted as arising from time reversal symmetry breaking, arguing that our observations are likely due to structural effects. The concerns expressed by Armitage and Hu that our results could be due to a change in the mirror plane are as important as they are obvious. In fact the first part of their comment merely restates the results of Simon and Varma [3] about the relationship and contrast of effects due to time reversal symmetry breaking and those caused by crystallographic changes. In any test of time reversal symmetry one must ensure that parity alone is not inducing the observed changes. We have indeed considered this issue very carefully in the course of our study [2] and it is precisely the lack of temperature dependent structural changes significant enough to explain the magnitude of the observed dichroism that forced us to conclude that time reversal symmetry breaking is the only plausible explanation. Furthermore, recent experiments by Borisenko, et al. [4] confirm that changes in the mirror plane are unmeasurably small.

cond-mat.str-el↗
arXiv2003

How to make semiconductors ferromagnetic: A first course on spintronics

The rapidly developing field of ferromagnetism in diluted magnetic semiconductors, where a semiconductor host is magnetically doped by transition metal impurities to produce a ferromagnetic semiconductor (e.g. Ga_{1-x}Mn_xAs with x ~ 1-10 %), is discussed with the emphasis on elucidating the physical mechanisms underlying the magnetic properties. Recent key developments are summarized with critical discussions of the roles of disorder, localization, band structure, defects, and the choice of materials in producing good magnetic quality and high Curie temperature. The correlation between magnetic and transport properties is argued to be a crucial ingredient in developing a full understanding of the properties of ferromagnetic semiconductors.

cond-mat.mtrl-sci↗
arXiv2002

Linear conductance in Coulomb-blockade quantum dots in the presence of interactions and spin

We discuss the calculation of the linear conductance through a Coulomb-blockade quantum dot in the presence of interactions beyond the charging energy. In the limit where the temperature is large compared with a typical tunneling width, we use a rate-equations approach to describe the transitions between the corresponding many-body states. We discuss both the elastic and rapid-thermalization limits, where the rate of inelastic scattering in the dot is either small or large compared with the elastic transition rate, respectively. In the elastic limit, we find several cases where a closed solution for the conductance is possible, including the case of a constant exchange interaction. In the rapid-thermalization limit, a closed solution is possible in the general case. We show that the corresponding expressions for the linear conductance simplify for a Hamiltonian that is invariant under spin rotations.

cond-mat.mes-hall↗
arXiv2002

Temperature-dependent magnetization in diluted magnetic semiconductors

We calculate magnetization in magnetically doped semiconductors assuming a local exchange model of carrier-mediated ferromagnetic mechanism and using a number of complementary theoretical approaches. In general, we find that the results of our mean-field calculations, particularly the dynamical mean field theory results, give excellent qualitative agreement with the experimentally observed magnetization in systems with itinerant charge carriers, such as Ga_{1-x}Mn_xAs with 0.03 < x < 0.07, whereas our percolation-theory-based calculations agree well with the existing data in strongly insulating materials, such as Ge_{1-x}Mn_x. We comment on the issue of non-mean-field like magnetization curves and on the observed incomplete saturation magnetization values in diluted magnetic semiconductors from our theoretical perspective. In agreement with experimental observations, we find the carrier density to be the crucial parameter determining the magnetization behavior. Our calculated dependence of magnetization on external magnetic field is also in excellent agreement with the existing experimental data.

cond-mat.mtrl-sci↗
arXiv2002

Electron energy and phase relaxation on magnetic impurities

We discuss the effect of magnetic impurities on the inelastic scattering and dephasing of electrons. Magnetic impurities mediate the energy exchange between electrons. This mechanism is especially effective at small energy transfers $E$ in the absence of Zeeman splitting, when the two-particle collision integral in the electron kinetic equation has a kernel $K\propto 1/E^2$ in a broad energy range. In a magnetic field, this mechanism is suppressed at $E$ below the Zeeman energy. Simultaneously, the Zeeman splitting of the impurity spin states reduces the electron dephasing rate, thus enhancing the effect of electron interference on conduction. We find the weak localization correction to the conductivity and the magnitude of the conductance fluctuations in the presence of magnetic field of arbitrary strength. Our results can be compared quantitatively with the experiments on energy relaxation in short metallic wires and on Aharonov-Bohm conductance oscillations in wire rings.

cond-mat.mes-hall↗
arXiv2002

Crossover from coherent to incoherent electronic excitations in the normal state of Bi2Sr2CaCu2O8

Angle resolved photoemission spectroscopy (ARPES) and resistivity measurements are used to explore the overdoped region of the high temperature superconductor Bi2Sr2CaCu2O8+delta. We find evidence for a new crossover line in the phase diagram between a coherent metal phase for lower temperatures and higher doping, and an incoherent metal phase for higher temperatures and lower doping. The former is characterized by two well-defined spectral peaks in ARPES due to coherent bilayer splitting and superlinear behavior in the resistivity, whereas the latter is characterized by a single broad spectral feature in ARPES and a linear temperature dependence of the resistivity.

cond-mat.supr-con↗
arXiv2002

Spontaneous time reversal symmetry breaking in the pseudogap state of high-Tc superconductors

When matter undergoes a phase transition from one state to another, usually a change in symmetry is observed, as some of the symmetries exhibited are said to be spontaneously broken. The superconducting phase transition in the underdoped high-Tc superconductors is rather unusual, in that it is not a mean-field transition as other superconducting transitions are. Instead, it is observed that a pseudo-gap in the electronic excitation spectrum appears at temperatures T* higher than Tc, while phase coherence, and superconductivity, are established at Tc (Refs. 1, 2). One would then wish to understand if T* is just a crossover, controlled by fluctuations in order which will set in at the lower Tc (Refs. 3, 4), or whether some symmetry is spontaneously broken at T* (Refs. 5-10). Here, using angle-resolved photoemission with circularly polarized light, we find that, in the pseudogap state, left-circularly polarized photons give a different photocurrent than right-circularly polarized photons, and therefore the state below T* is rather unusual, in that it breaks time reversal symmetry11. This observation of a phase transition at T* provides the answer to a major mystery of the phase diagram of the cuprates. The appearance of the anomalies below T* must be related to the order parameter that sets in at this characteristic temperature .

cond-mat.str-el↗
arXiv2002

Polaron percolation in diluted magnetic semiconductors

We theoretically study the development of spontaneous magnetization in diluted magnetic semiconductors as arising from a percolation of bound magnetic polarons. Within the framework of a generalized percolation theory we derive analytic expressions for the Curie temperature and the magnetization, obtaining excellent quantitative agreement with Monte Carlo simulation results and good qualitative agreement with experimental results.

cond-mat.mtrl-sci↗
arXiv2001

Momentum Distribution Curves in the Superconducting State

We demonstrate that the E vs. k dispersion in the superconducting state extracted from momentum distribution curves differs qualitatively from the traditional dispersion extracted from energy distribution curves. This occurs because of a combination of many-body effects and the presence of an energy gap, along with the associated coherence factors. Analysis of such MDC dispersions can give important information on the microscopics of high temperature superconductors.

cond-mat.supr-con↗
arXiv2001

Electron energy relaxation in the presence of magnetic impurities

We study inelastic electron-electron scattering mediated by the exchange interaction of electrons with magnetic impurities, and find the kernel of the corresponding two-particle collision integral. In a wide region of parameters, the kernel K is proportional to the inverse square of the transferred energy, $K\propto J^4/E^2$. The exchange constant J is renormalized due to the Kondo effect, yielding an additional weak dependence of K on the energies of the colliding electrons. At small energy transfers, the $1/E^2$ divergence is cut off; the cut-off energy is determined by the dynamics of the impurity spins. The obtained results may provide a quantitative explanation of the experiments of Pothier et al. [Phys. Rev. Lett. 79, 3490 (1997)] on anomalously strong energy relaxation in short metallic wires.

cond-mat.mes-hall↗