arXiv subjects

Unjong Yu

Publications and source records attributed to Unjong Yu.

29 records · Page 2Linked to original sources

arXiv2016

Ising antiferromagnet on the Archimedean lattices

Geometric frustration effects were studied systematically with the Ising antiferromagnet on the 11 Archimedean lattices using the Monte-Carlo methods. The Wang-Landau algorithm for static properties (specific heat and residual entropy) and the Metropolis algorithm for a freezing order parameter were adopted. The exact residual entropy was also found. Based on the degree of frustration and dynamic properties, ground states of them were determined. The Shastry-Sutherland lattice and the trellis lattice are weakly-frustrated and have two-dimensional and one-dimensional long-range-ordered ground states, respectively. The bounce, maple-leaf, and star lattices have the spin ice phase. The spin liquid phase appears in the triangular and kagomé lattices.

cond-mat.stat-mech↗

arXiv2016

Ising antiferromagnet on the 2-uniform lattices

The antiferromagnetic Ising model is investigated on the 20 2-uniform lattices using the Monte-Carlo method based on the Wang-Landau algorithm and the Metropolis algorithm to study the geometric frustration effect systematically. Based on the specific heat, the residual entropy, and the Edwards-Anderson freezing order parameter, the ground states of them were determined. In addition to the long-range-ordered phase and the spin ice phase found in the Archimedean lattices, two more phases were found. The partial long-range order is long-range order with exceptional disordered sites, which gives extensive residual entropy. In the partial spin ice phase, the partial freezing phenomenon appears: Majority of sites are frozen without long-range order, but the other sites are fluctuating even at zero temperature. The spin liquid ground state was not found in the 2-uniform lattices.

cond-mat.stat-mech↗

arXiv2013

Unconventional superconductivity on the triangular lattice Hubbard model

Using large-scale dynamical cluster quantum Monte Carlo simulations, we explore the unconventional superconductivity in the hole-doped Hubbard model on the triangular lattice. Due to the interplay of electronic correlations, geometric frustration, and Fermi surface topology, we find a doubly degenerate singlet pairing state at an interaction strength close to the bare bandwidth. Such an unconventional superconducting state is mediated by antiferromagnetic spin fluctuations along the $Γ$-$K$ direction, where the Fermi surface is nested. An exact decomposition of the irreducible particle-particle vertex further confirms the dominant component of the effective pairing interaction comes from the spin channel. Our findings provide support for chiral $d +i d$ superconductivity in water-intercalated sodium cobaltates Na$_{x}$CoO$_{2} \cdot y$H$_{2}$O, as well as insight into the superconducting phases of the organic compounds $κ$-(ET)$_{2}$X and Pd(dmit)$_{2}$.

cond-mat.str-el↗

arXiv2010

Nonlocal effects on magnetism in the diluted magnetic semiconductor Ga_{1-x}Mn_{x}As

The magnetic properties of the diluted magnetic semiconductor Ga_{1-x}Mn_{x}As are studied within the dynamical cluster approximation. We use the k-dot-p Hamiltonian to describe the electronic structure of GaAs with spin-orbit coupling and strain effects. We show that nonlocal effects are essential for explaining the experimentally observed transition temperature and saturation magnetization. We also demonstrate that the cluster anisotropy is very strong and induces rotational frustration and a cube-edge direction magnetic anisotropy at low temperature. With this, we explain the temperature-driven spin reorientation in this system.

cond-mat.str-el↗

arXiv2008

Influence of Band and Orbital Degeneracies on Ferromagnetism in the Periodic Anderson Model

We investigate the influence of degeneracies of the conduction band and the $f$-orbital on the stability of ferromagnetism in the periodic Anderson model. To this end we calculate the temperature dependence of the inverse susceptibility for different degeneracies $D^c, D^f$ and conduction electron densities $n^c$ within the dynamical mean-field theory. A strong increase of the Curie temperature $T_c$ with the degeneracy $D^f$ of the localized $f$-level is found. For $D^c \leq D^f$ a simple \emph{ansatz} based on a mean-field treatment of the RKKY interaction is shown to imply a scaling behavior of $T_c$ as a function of the conduction electron density per band which is well obeyed by the numerical results. In particular, $T_c$ is found to have a maximum at $n^c / D^c\approx 0.3$.

cond-mat.str-el↗

arXiv2008

Ferromagnetism and Kondo Insulator Behavior in the Disordered Periodic Anderson Model

The effect of binary alloy disorder on the ferromagnetic phases of f-electron materials is studied within the periodic Anderson model. We find that disorder in the conduction band can drastically enhance the Curie temperature due to an increase of the local f-moment. The effect may be explained qualitatively and even quantitatively by a simple theoretical ansatz. The emergence of an alloy Kondo insulator at non-integer filling is also pointed out.

cond-mat.str-el↗

arXiv2008

Thermoelectric Power in the Double Exchange Model

Employing the Monte-Carlo method and the exact diagonalization, we have investigated the temperature dependence of the thermoelectric power (TEP) for the double exchange model in the dilute carrier concentration limit. We have found that the TEP follows the Heikes formula in the high temperature regime, whereas, in the intermediate temperature regime, the TEP is suppressed by the exchange coupling between itinerant electrons and local spins. In the low temperature regime, the TEP exhibits an anomalous peak and dip feature near the magnetic transition temperature $T_C$ which can be understood based on the magnetic polaron state. We have also found that the TEP, in the presence of the magnetic field, shows the positive magnetothermoelectric power near $T_C$.

cond-mat.str-el↗

arXiv2004

Spin-polaron model: transport properties of EuB$_6$

To understand anomalous transport properties of EuB$_6$, we have studied the spin-polaron Hamiltonian incorporating the electron-phonon interaction. Assuming a strong exchange interaction between the carriers and the localized spins, the electrical conductivity is calculated. The temperature and magnetic field dependence of the resistivity of EuB$_6$ are well explained. At low temperature, magnons dominate the conduction process, whereas the lattice contribution becomes significant at very high temperature due to the scattering with the phonons. Large negative magnetoresistance near the ferromagnetic transition is also reproduced as observed in EuB$_6$.

cond-mat.str-el↗

arXiv2002

Magnetic phase diagram of doped CMR manganites

The magnetic phase diagram of the colossal magnetoresistance (CMR) manganites is determined based on the Hamiltonian incorporating the double-exchange (DE) interaction between degenerate Mn $e_g$ orbitals and the antiferromagnetic (AF) superexchange interaction between Mn $t_{2g}$ spins. We have employed the rigorous quantum mechanical formalism and obtained the finite temperature phase diagram which describes well the commonly observed features in CMR manganites. We have also shown that the CE-type AF structure cannot be stabilized at $x$=0.5 in this model.

cond-mat.str-el↗

arXiv1999

The small polaron crossover transition in colossal magnetoresistance (CMR) manganites

Based on the combined model of the double exchange and the polaron, we have studied the small-to-large polaron crossover transition and explored its effects on the magnetic and transport properties in colossal magnetoresistance (CMR) manganites. We have used the variational Lang-Firsov canonical transformation, and shown that the magnetic and transport properties of both high and low $T_C$ manganites are well described in terms of a single formalism. We have reproduced the rapid resistivity drop below $T_C$, a realistic CMR ratio, and the {\it first-order-like} sharp magnetic phase transition, which are observed in low $T_C$ manganites.

cond-mat.str-el↗

arXiv1999

Isotope effects and the charge gap formation in the charge ordered phase of colossal magnetoresistance manganites

Giant oxygen isotope effects observed in colossal magnetoresistance manganites are investigated by employing the combined model of the double exchange and interacting lattice polaron mechanism. We have shown that the isotope effects on $ T_C$ in the metallic phase and $ T_{CO}$ in the charge ordered phase of manganites can be explained well in terms of the double exchange and polaron narrowing factors with reasonable physical parameters.

cond-mat.str-el↗