arXiv subjects

Alireza Akbari

Publications and source records attributed to Alireza Akbari.

83 records · Page 5Linked to original sources

arXiv2012

Spin Exciton Formation inside the Hidden Order Phase of CeB6

The heavy fermion metal CeB6 exhibits hidden order of antiferroquadrupolar (AFQ) type below T_Q=3.2K and subsequent antiferromagnetic (AFM) order at T_N=2.3K. It was interpreted as ordering of the quadrupole and dipole moments of a $Γ_8$ quartet of localised Ce $4f^1$ electrons. This established picture has been profoundly shaken by recent inelastic neutron scattering (G. Friemel et al., arXiv:1111.4151) that found the evolution of a feedback spin exciton resonance within the hidden order phase at the AFQ wave vector which is stabilized by the AFM order. We develop an alternative theory based on a fourfold degenerate Anderson lattice model, including both order parameters as particle-hole condensates of itinerant heavy quasiparticles. This explains in a natural way the appearance of the spin exciton resonance and the momentum dependence of its spectral weight, in particular around the AFQ vector and its rapid disappearance in the disordered phase. Analogies to the feedback effect in unconventional heavy fermion superconductors are pointed out.

cond-mat.str-el↗

arXiv2011

RKKY interaction in SDW phase of iron-based superconductors

Using the multiband model we analyze the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the magnetic impurities in layered ferropnictide superconductors. In the normal state the interaction is spin isotropic and is dominated by the nesting features of the electron and hole bands separated by the antiferromagnetic momentum, ${\bf Q}_{AF}$. In the AF state the RKKY interaction maps into an effective anisotropic XXZ-type Heisenberg exchange model. The anisotropy originates from the breaking of the spin-rotational symmetry induced by the AF order and its strength depends on the size of the AF gap and the structure of the folded Fermi surface. We discuss our results in connection to the recent experiments.

cond-mat.str-el↗

arXiv2011

Quasiparticle interference in heavy Fermion superconductor CeCoIn_5

We investigate the quasiparticle interference in the heavy Fermion superconductor CeCoIn_5 as direct method to confirm the d-wave gap symmetry. The ambiguity between d_{xy} and d_{x^2-y^2} symmetry remaining from earlier specific heat and thermal transport investigations has been resolved in favor of the latter by the observation of a spin resonance that can occur only in d_{x^2-y^2} symmetry. However these methods are all indirect and depend considerably on theoretical interpretation. Here we propose that quasiparticle interference (QPI) spectroscopy by scanning tunneling microscopy (STM) can give a direct fingerprint of the superconducting gap in real space which may lead to a definite conclusion on its symmetry for CeCoIn_5 and related 115 compounds. The QPI pattern for both magnetic and nonmagnetic impurities is calculated for the possible d-wave symmetries and characteristic differences are found that may be identified by STM method.

cond-mat.str-el↗

arXiv2009

Magnetic impurity resonance states and symmetry of the superconducting order parameter in iron-based superconductors

We investigate the effect of magnetic impurities on the local quasiparticle density of states (LDOS) in iron-based superconductors. Employing the two-orbital model where 3$d$ electron and hole conduction bands are hybridizing with the localized $f$-orbital of the impurity spin, we investigate how various symmetries of the superconducting gap and its nodal structure influence the quasiparticle excitations and impurity bound states. We show that the bound states behave qualitatively different for each symmetry. Most importantly we find that the impurity-induced bound states can be used to identify the nodal structure of the extended s-wave symmetry ($S^{\pm}$) that is actively discussed in ferropnictides.

cond-mat.supr-con↗

arXiv2009

Metamagnetic phase transition in the Ising plus Dzyaloshinskii-Moriya model

We study the 1D ferromagnetic Ising (spin-1/2) model with the Dzyaloshinskii-Moriya (DM) interaction. We analyze the low energy excitation spectrum and the ground state magnetic phase diagram using the Lanczos method. The DM interaction-dependency is calculated for the low-energy excitation spectrum, spiral order parameter and spin-spin correlation functions. We show that a metamagnetic quantum phase transition occurs between the ferromagnetic and spiral phases. The existence of the metamagnetic phase transition is confirmed, using the variational matrix product states approach.

cond-mat.str-el↗

arXiv2009

Bound state of 4$f$-excitation and magnetic resonance in unconventional superconductors

We analyze the influence of unconventional superconductivity on crystalline electric field (CEF) excitations of rare earth ions in novel superconductors. We show that resonant magnetic excitations of the conduction electrons that have been observed in these systems below T$_c$ may result in the formation of the bound state in the 4$f$-electron susceptibility. This occurs at energies well below the CEF excitation energy. The effect is discussed as a function of temperature and the strength of the coupling between the 4$f$ and $d$-electrons. We argue that these effects may be present in the layered cuprates and ferropnictides which contain rare-earth ions.

cond-mat.supr-con↗

arXiv2009

Theory of Spin Exciton in the Kondo Semiconductor $Yb B_{12}$

The Kondo semiconductor $Yb B_{12}$ exhibits a spin and charge gap of approximately 15 meV. Close to the gap energy narrow dispersive collective excitations were identified by previous inelastic neutron scattering experiments. We present a theoretical analysis of these excitations. Starting from a periodic Anderson model for crystalline electric field (CEF) split 4f states we derive the hybridized quasiparticle bands in slave boson mean-field approximation and calculate the momentum dependent dynamical susceptibility in random phase approximation (RPA). We show that a small difference in the hybridization of the two CEF (quasi-) quartets leads to the appearance of two dispersive spin resonance excitations at the continuum threshold. Their intensity is largest at the antiferromagnetic (AF) zone boundary point and they have an upward dispersion which merges with the continuum less than halfway into the Brillouin zone. Our theoretical analysis explains the most salient features of previously unexplained experiments on the magnetic excitations of $Yb B_{12}$.

cond-mat.str-el↗

arXiv2008

Heat capacity of Schottky type in low-dimensional spin system

The heat capacity of low-dimensional spin systems is studied using theoretical and numerical techniques. Keeping only two energy states, the system is mapped onto the two -level-system (TLS) model. Using the low temperature Lanczos method, it is confirmed that the behavior of $T_{M}$ and the energy gap as functions of the control parameter is the same in the two models studied; a conclusion that can probably be extrapolated to the general case of any system that possesses an energy gap.

cond-mat.str-el↗

arXiv2008

Quadrupole Effect on the Heat Conductivity of Cold Glasses

At very low temperatures, the tunnelling theory for amorphous solids predicts a thermal conductivity $κ\propto T^p$, with $p = 2$. We have studied the effect of the Nuclear Quadrupole moment on the thermal conductivity of glasses at very low temperatures. We developed a theory that couples the tunnelling motion to the nuclear quadrupoles moment in order to evaluate the thermal conductivity. Our result suggests a cross over between two different regimes at the temperature close to the nuclear quadrupoles energy. Below this temperature we have shown that the thermal conductivity is larger than the standard tunneling result and therefore we have $p < 2$. However, for temperatures higher than the nuclear quadrupoles energy, the result of standard tunnelling model has been found.

cond-mat.dis-nn↗

arXiv2007

Alternating Heisenberg Spin-1/2 Chains in a Transverse Magnetic Field

The ground state phase diagram of the alternating spin-1/2 chains with anisotropic ferromagnetic coupling under the influence of a symmetry breaking transverse magnetic field is studied. We have used the exact diagonalization technique. In the limit where the antiferromagnetic coupling is dominant, we have identified two Ising-type quantum phase transitions. We have calculated two critical fields $h_{c_{1}}$ and $h_{c_{2}}$, corresponding to the transition between different magnetic phases of the system. It is found that the intermediate state ($h_{c_{1}}<h<h_{c_{2}}$) is gapful, describing the stripe-antiferromagnetic phase.

cond-mat.str-el↗

arXiv2007

Dielectric Susceptibility and Heat Capacity of Ultra-Cold Glasses in Magnetic Field

Recent experiments demonstrated unexpected, even intriguing properties of certain glassy materials in magnetic field at low temperatures. We have studied the magnetic field dependence of the static dielectric susceptibility and the heat capacity of glasses at low temperatures. We present a theory in which we consider the coupling of the tunnelling motion to nuclear quadrupoles in order to evaluate the static dielectric susceptibility. In the limit of weak magnetic field we find the resonant part of the susceptibility increasing like $B^2$ while for the large magnetic field it behaves as 1/B. In the same manner we consider the coupling of the tunnelling motion to nuclear quadrupoles and angular momentum of tunnelling particles in order to find the heat capacity. Our results show the Schotky peak for the angular momentum part, and $B^2$ dependence for nuclear quadrupoles part of heat capacity, respectively. We discuss whether or not this approach can provide a suitable explanation for such magnetic properties.

cond-mat.dis-nn↗