Searcharxiv⌕ Search

arXiv subjects

P. A. Lee

Publications and source records attributed to P. A. Lee.

17 recordsLinked to original sources
arXiv2017

Magnetoconductivity in Weyl semimetals: Effect of chemical potential and temperature

We present the detailed analyses of magneto-conductivities in a Weyl semimetal within Born and self-consistent Born approximations. In the presence of the charged impurities, the linear magnetoresistance can happen when the charge carriers are mainly from the zeroth (n=0) Landau level. Interestingly, the linear magnetoresistance is very robust against the change of temperature, as long as the charge carriers mainly come from the zeroth Landau level. We denote this parameter regime as the high-field regime. On the other hand, the linear magnetoresistance disappears once the charge carriers from the higher Landau levels can provide notable contributions. Our analysis indicates that the deviation from the linear magnetoresistance is mainly due to the deviation of the longitudinal conductivity from the $1/B$ behavior. We found two important features of the self-energy approximation: 1. a dramatic jump of $σ_{xx}$, when the $n=1$ Landau level begins to contribute charge carriers, which is the beginning point of the middle-field regime, when decreasing the external magnetic field from high field; 2. In the low-field regime $σ_{xx}$ shows a $B^{-5/3}$ behavior and results the magnetoresistance $ρ_{xx}$ to show a $B^{1/3}$ behavior. The detailed and careful numerical calculation indicates that the self-energy approximation (including both the Born and the self-consistent Born approximations) does not explain the recent experimental observation of linear magnetoresistance in Weyl semimetals.

cond-mat.mes-hall↗
arXiv2014

Intrinsic Anomalous Hall Effect in Magneto-Chiral States

We show that a finite Hall effect in zero applied magnetic field occurs for partially filled bands in certain time-reversal violating states with zero net flux per unit-cell. These states are the Magneto-chiral states with parameters in the effective one-particle Hamiltonian such that they do not satisfy the Haldane-type constraints for topological electronic states. The results extend an earlier discussion of the Kerr effect observed in the cuprates but may be applicable to other experimental situations.

cond-mat.str-el↗
arXiv2010

Magnetic impurity in a $U(1)$-Spin Liquid with a Spinon Fermi-Surface

We address the problem of a magnetic impurity in a two dimensional $U(1)$ spin liquid where the spinons have gap-less excitations near the Fermi-surface and are coupled to an emergent gap-less gauge field. Using a large N expansion we analyze the strong coupling behavior and obtain the Kondo temperature which was found to be the same as for a Fermi-liquid. In this approximation we also study the specific heat and the magnetic susceptibility of the impurity. These quantities present no deviations from the Fermi-liquid ones, consistent with the notion that the magnetic impurity is only sensitive to the local density of fermionic states.

cond-mat.str-el↗
arXiv2009

Coherence and pairing in a doped Mott insulator: Application to the cuprates

The issues of single particle coherence and its interplay with singlet pairing are studied within the slave boson gauge theory of a doped Mott insulator. Prior work by one of us (T. Senthil, arXiv:0804.1555) showed that the coherence scale below which Landau quasiparticles emerge is parametrically lower than that identified in the slave boson mean field theory. Here we study the resulting new non-fermi liquid intermediate temperature regime characterized by a single particle scattering rate that is linear in temperature ($T$). In the presence of a d-wave pair amplitude this leads to a pseudogap state with $T$ dependent Fermi arcs near the nodal direction. Implications for understanding the cuprates are discussed.

cond-mat.str-el↗
arXiv2009

A synthesis of the phenomenology of the underdoped cuprates

The underdoped cuprates have a number of interesting and unusual properties that often seem hard to reconcile with one another. In this paper we show how many of these diverse phenomena can be synthesized into a single coherent theoretical picture. Specifically we present a description where a pseudogap and gapless Fermi arcs exist in the normal state above the superconducting transition temperature ($T_c$), but give way to the observed quantum oscillations and other phenomena at low temperature when the superconductivity is suppressed by a magnetic field. We show the consistency between these phenomena and observations of enhanced Nernst and diamagnetic signals above $T_c$. We also develop a description of the vortex core inside the superconducting state and discuss its relation with the high field phenomena.

cond-mat.str-el↗
arXiv2007

Local moment, itinerancy and deviation from Fermi liquid behavior in Na$_x$CoO$_2$ for $0.71 \leq x \leq 0.84$

Here we report the observation of Fermi surface (FS) pockets via the Shubnikov de Haas effect in Na$_x$CoO$_2$ for $x = 0.71$ and 0.84, respectively. Our observations indicate that the FS expected for each compound intersects their corresponding Brillouin zones, as defined by the previously reported superlattice structures, leading to small reconstructed FS pockets, but only if a precise number of holes per unit cell is \emph{localized}. For $0.71 \leq x < 0.75$ the coexistence of itinerant carriers and localized $S =1/2$ spins on a paramagnetic triangular superlattice leads at low temperatures to the observation of a deviation from standard Fermi-liquid behavior in the electrical transport and heat capacity properties, suggesting the formation of some kind of quantum spin-liquid ground state.

cond-mat.str-el↗
arXiv2006

Shubnikov de Haas effect in the metallic state of Na$_{0.3}$CoO$_2$

Shubnikov de Haas oscillations for two well defined frequencies, corresponding respectively to areas of 0.8 and 1.36% of the first Brillouin zone (FBZ), were observed in single crystals of Na$_{0.3}$CoO$_2$. The existence of Na superstructures in Na$_{0.3}$CoO$_2$, coupled with this observation, suggests the possibility that the periods are due to the reconstruction of the large Fermi surface around the $Γ$ point. An alternative interpretation in terms of the long sought-after $ε_g^\prime$ pockets is also considered but found to be incompatible with existing specific heat data.

cond-mat.str-el↗
arXiv2006

Spin Filtered Edge States and Quantum Hall Effect in Graphene

Electron edge states in graphene in the Quantum Hall effect regime can carry both charge and spin. We show that spin splitting of the zeroth Landau level gives rise to counterpropagating modes with opposite spin polarization. These chiral spin modes lead to a rich variety of spin current states, depending on the spin flip rate. A method to control the latter locally is proposed. We estimate Zeeman spin splitting enhanced by exchange, and obtain a spin gap of a few hundred Kelvin.

cond-mat.mes-hall↗
arXiv2004

Magnetic-field-induced Fermi surface reconstruction in Na$_{0.5}$CoO$_2$

We have performed electrical transport measurements at low temperatures and high magnetic fields in Na$_{0.5}$CoO$_2$ single crystals. Shubnikov de Haas oscillations were observed for two frequencies F_1 150 and F_2 40 T corresponding respectively to 1 and .25% of the area of the orthorhombic Brillouin zone. These small Fermi surface (FS) pockets indicate that most of the original FS vanishes at the charge ordering (CO) transition. Furthermore, in-plane magnetic fields strongly suppress the CO state. For fields rotating within the conducting planes we observe angular magnetoresistance oscillations (AMRO), whose periodicity changes from two- to six-fold at the transition, suggesting that a reconstructed hexagonal FS emerges at a field of about 40 T.

cond-mat.str-el↗
arXiv2004

The Physics Behind High-Temperature Superconducting Cuprates: The "Plain Vanilla" Version Of RVB

One of the first theoretical proposals for understanding high temperature superconductivity in the cuprates was Anderson's RVB theory using a Gutzwiller projected BCS wave function as an approximate ground state. Recent work by Paramekanti, Randeria and Trivedi has shown that this variational approach gives a semi-quantitative understanding of the doping dependences of a variety of experimental observables in the superconducting state of the cuprates. In this paper we revisit these issues using the ``renormalized mean field theory'' of Zhang, Gros, Rice and Shiba based on the Gutzwiller approximation in which the kinetic and superexchange energies are renormalized by different doping-dependent factors $g_{t}$ and $g_{S}$ respectively. We point out a number of consequences of this early mean field theory for experimental measurements which were not available when it was first explored, and observe that it is able to explain the existence of the pseudogap, properties of nodal quasiparticles and approximate spin-charge separation, the latter leading to large renormalizations of the Drude weight and superfluid density. We use the Lee-Wen theory of the phase transition as caused by thermal excitation of nodal quasiparticles, and also obtain a number of further experimental confirmations. Finally, we remark that superexchange, and not phonons, are responsible for d-wave superconductivity in the cuprates.

cond-mat.str-el↗
arXiv2000

Impurity-Induced Quasiparticle Transport and Universal Limit Wiedemann-Franz Violation in d-Wave Superconductors

Due to the node structure of the gap in a d-wave superconductor, the presence of impurities generates a finite density of quasiparticle excitations at zero temperature. Since these impurity-induced quasiparticles are both generated and scattered by impurities, prior calculations indicate a universal limit (Ω-> 0, T -> 0) where the transport coefficients obtain scattering-independent values, depending only on the velocity anisotropy v_f/v_2. We improve upon prior results, including the contributions of vertex corrections and Fermi liquid corrections in our calculations of universal limit electrical, thermal, and spin conductivity. We find that while vertex corrections modify electrical conductivity and Fermi liquid corrections renormalize both electrical and spin conductivity, only thermal conductivity maintains its universal value, independent of impurity scattering or Fermi liquid interactions. Hence, low temperature thermal conductivity measurements provide the most direct means of obtaining the velocity anisotropy for high T_c cuprate superconductors.

cond-mat.supr-con↗
arXiv1998

Metallic Phase and Metal-insulator Transition in 2d Electronic Systems

The recent experimental observation of a metal-insulator transition in two dimensions prompts a re-examination of the theory of disordered interacting systems. We argue that the existing theory permits the existence of a metallic phase and propose a number of experiments such as magnetoconductance and tunnelling in the presence of a parallel field, which should provide diagnostic tests as to whether a given experimental system is in fact in this regime. We also comment on a generic flow diagram which predicts a maximum metallic resistivity.

cond-mat↗
arXiv1997

Localization of Quasiparticles in an NS Structure

We discuss localization of quasiparticles in an extended NS structure in the situation when the reflection from the NS interface is mostly of Andreev kind. The localization of quasiparticle states arises due to trajectory retracing caused by Andreev reflection. This effect is semiclassical in the sense that in the classical limit the states become fully localized, while quantum effects destroy localization behaviour. We derive the criteria for this localization and show that it takes place only for sufficiently smooth disorder with large correlation length, whereas short-range disorder destroys the effect. Localization of quasiparticle states gives rise to resonance peaks in the local tunneling density of states.

cond-mat↗
arXiv1996

The Low-Energy Fixed Points of Random Quantum Spin Chains

The one-dimensional isotropic quantum Heisenberg spin systems with random couplings and random spin sizes are investigated using a real-space renormalization group scheme. It is demonstrated that these systems belong to a universality class of disordered spin systems, characterized by weakly coupled large effective spins. In this large-spin phase the uniform magnetic susceptibility diverges as 1/T with a non-universal Curie constant at low temperatures T, while the specific heat vanishes as T^delta |ln T| for T->0. For broad range of initial distributions of couplings and spin sizes the distribution functions approach a single fixed-point form, where delta \approx 0.44. For some singular initial distributions, however, fixed-point distributions have non-universal values of delta, suggesting that there is a line of fixed points.

cond-mat.stat-mech↗
arXiv1995

Random Exchange Quantum Heisenberg Chains

The one-dimensional quantum Heisenberg model with random $\pm J$ bonds is studied for $S=\frac{1}{2}$ and $S=1$. The specific heat and the zero-field susceptibility are calculated by using high-temperature series expansions and quantum transfer matrix method. The susceptibility shows a Curie-like temperature dependence at low temperatures as well as at high temperatures. The numerical results for the specific heat suggest that there are anomalously many low-lying excitations. The qualitative nature of these excitations is discussed based on the exact diagonalization of finite size systems.

cond-mat↗
arXiv1995

Random Quantum Spin Chains: A Real-Space Renormalization Group Study

Quantum Heisenberg spin chains with random couplings and spin sizes are studied using a real-space renormalization group technique. These systems belong to a new universality class of disordered quantum spin systems realized in {\it e.g.} ${\rm Sr}_3{\rm CuPt}_{1-x}{\rm Ir}_x {\rm O}_6$. The low-energy fixed point is characterized by the formation of weakly coupled large effective spins. At low temperature $T$ the entropy obeys a power law $T^δ$ ($δ\approx0.44$), and the susceptibility follows a Curie-like behavior.

cond-mat↗