arXiv subjects

Misha Turlakov

Publications and source records attributed to Misha Turlakov.

7 recordsLinked to original sources

arXiv2005

Coherent-incoherent transition in the sub-Ohmic spin-boson model

We study the spin-boson model with a sub-Ohmic bath using a variational method. The transition from coherent dynamics to incoherent tunneling is found to be abrupt as a function of the coupling strength $α$ and to exist for any power $0 < s< 1$, where the bath coupling is described by $J(ω) \sim αω^{s}$. We find non-monotonic temperature dependence of the two-level gap $\tilde{K}$ and a re-entrance regime close to the transition due to non-adiabatic low-frequency bath modes. Differences between thermodynamic and dynamic conditions for the transition as well as the limitations of the simplified bath description are discussed.

cond-mat.mes-hall↗

arXiv2004

Quantum phase transitions of magnetic rotons

Due to weak spin-orbit coupling, the magnetic excitations of an itinerant ferromagnet become magnetic rotons, excitations with degenerate minima on a hypersphere at finite wavevector. Using self-consistent Hartree and renormalization group calculations, we study weak fluctuation-driven first-order quantum phase transitions, a quantum tricritical point controlled by anisotropy and the non-Fermi liquid behavior associated with the large phase volume of magnetic rotons. We propose that magnetic rotons are essential for the description of the anomalous high-pressure behavior of the itinerant helical ferromagnet MnSi.

cond-mat.str-el↗

arXiv2003

Universal sound absorption in low-temperature glasses

Phenomenologically assuming a sharp decrease of shear relaxation time for large wavevector $k>k_ξ$ density modes (where $k_ξ$ is of order of inverse of several interatomic distances $a$), I develop a general elasto-hydrodynamic theory describing the low-energy excitations of glassy and amorphous solids, which are responsible for anomalous specific heat and thermal conductivity. The theory explains the origin of collective two-level states and Boson peak. The ratio of the wavelength of the phonon, $λ$, to its mean free path, $l$, - universal property of sound absorption in glasses - is derived in this theory to be $λ/l=(2/3) (c_t/c_l)^2 (k_ξa)^3$, where $c_t$ and $c_l$ are transverse and longitudinal sound velocities correspondingly.

cond-mat.dis-nn↗

arXiv2002

Sum rule analysis of Umklapp processes and Coulomb energy: application to cuprate superconductivity

The third moment frequency sum rule for the density-density correlation function is rederived in the presence of Umklapp processes. Upper and lower bounds on the electron-electron Coulomb energy are derived in two-dimensional and three-dimensional media, and the Umklapp processes are shown to be crucial in determining the spectrum of the density fluctuations (especially for the two-dimensional systems). This and other standard sum rules can be used in conjunction with experimental spectroscopies (electron-energy loss spectroscopy, optical ellipsometry, etc.) to analyse changes of the electron-electron Coulomb energy at the superconducting transition in cuprates.

cond-mat.supr-con↗

arXiv2001

Incoherent interlayer conductivity of layered systems

The criterion for the maximum incoherent conductivity $σ_{c,max}$ in the anisotropic system is shown to be $σ_{c,max} σ_{ab}=C (e^2/h)^2 n_{2D}$, where $n_{2D}$ is the two-dimensional electron density, and $C$ is a numerical constant of order one. A model of c-axis interlayer conductivity of layered system in the incoherent regime is developed. The temperature dependence of the c-axis conductivity is derived to be $σ_c (T) \sim T^α$, where $α$ is determined by the effective strength of the interplane voltage fluctuations. The ``pseudogap'' phenomenon of suppressed interplane conductivity at low frequencies (or low voltages) is discussed.

cond-mat.str-el↗

arXiv2000

Interlayer c-axis transport in the normal state of cuprates

A theoretical model of c-axis transport properties in cuprates is proposed. Inter-plane and in-plane charge fluctuations make hopping between planes incoherent and diffusive (the in-plane momentum is not conserved after tunneling). The non-Drude optical conductivity $σ_c(ω)$ and the power-law temperature dependence of the {\it dc} conductivity are generically explained by the strong fluctuations excited in the process of tunneling. Several microscopic models of the charge fluctuation spectrum are considered.

cond-mat.supr-con↗

arXiv2000

Johnson-Nyquist noise in films and narrow wires

The Johnson-Nyquist noise in narrow wires having a transverse size smaller than the screening length is shown to be white up to the frequency $D/L^2$ and to decay at higher frequencies as $ω^{-{1/2}}$. In two-dimensional films having a thickness smaller than the screening length, the Johnson-Nyquist noise is predicted to be frequency independent up to the frequency $σ_{2D}/L$ and to have a {\it universal} $1/ω$ spectrum at higher frequencies. These results are contrasted with the conventional noise spectra in neutral and three-dimensional charged liquids.

cond-mat.mes-hall↗