arXiv subjects

Jian-Xin Zhu

Publications and source records attributed to Jian-Xin Zhu.

At least 253 records · Page 14Linked to original sources

arXiv2003

Inelastic Tunneling Spectroscopy in a D-wave Superconductor

We propose a mechanism to use inelastic tunneling spectrosopy STM to detect a single spin in a d-wave superconductor and in a pseudogap state, based on a direct exchange coupling J between the surface electrons and the local spin S in a magnetic field. This coupling will produce a kink in a dI/dV characteristic at Zeeman energy of the spin ω_0. We find that for relevant values of parameters signal scales as dI^2/dV^2 \simeq (JN_0)^2 Θ(eV - ω_0) and could be in the range of 10^{-2} of the bare density of states where N_0 is the density of states for surface electrons. Scattering in superconductor with the coherence peak at gap maximum Δleads also to strong features at Δ+ ω_0. This suggests a new technique for a detection of a local spin excitation with STM. We also consider a detection of a local vibrational mode as a simple extension of the spin case.

cond-mat.str-el↗

arXiv2003

Theory of the NMR relaxation rates in cuprate superconductors with field induced antiferromagnetic order

Based on a model Hamiltonian with a d-wave pairing interaction and a competing antiferromagnetic interaction, we numerically study the site dependence of the nuclear spin resonance (NMR) relaxation rate T_1^{-1} as a function of temperature for a d-wave superconductor(DSC) with magnetic field induced spin density wave (SDW) order. In the presence of the induced SDW, we find that there exists no simple direct relationship between NMR signal rate T_1^{-1} and low energy local density of states while these two quantities are linearly proportional to each other in a pure DSC. In the vortex core region, T_1^{-1} on ^{17}O site may exhibit a double-peak behavior, one sharp and one broad, as the temperature is increased to the superconductivity transition temperature T_c, in contrast to a single broad peak for a pure DSC. The existence of the sharp peak corresponds to the disappearance of the induced SDW above a certain temperature T_{AF} which is assumed to be considerably lower than T_c. We also show the differences between T_1^{-1} on ^{17}O and that on ^{63}Cu as a function of lattice site at different temperatures and magnetic fields. Our results obtained from the scenario of the vortex with induced SDW is consistent with recent NMR and scanning tunneling microscopy experiments.

cond-mat.supr-con↗

arXiv2002

Theory of Current and Shot Noise Spectroscopy in Single-Molecular Quantum Dots with Phonon Mode

Using the Keldysh nonequilibrium Green function technique, we study the current and shot noise spectroscopy of a single molecular quantum dot coupled to a local phonon mode. It is found that in the presence of electron-phonon coupling, in addition to the resonant peak associated with the single level of the dot, satellite peaks with the separation set by the frequency of phonon mode appear in the differential conductance. In the ``single level'' resonant tunneling region, the differential shot noise power exhibit two split peaks. However, only single peaks show up in the ``phonon assisted'' resonant-tunneling region. An experimental setup to test these predictions is also proposed.

cond-mat.mes-hall↗

arXiv2002

Vortex Charges in High Temperature Superconductors

Based on an effective model Hamiltonian with competing antiferromagnetic (AF) and d-wave superconductivity (DSC) interactions, the vortex charge in high T_c superconductors is investigated by solving self-consistently the Bogoliubov-de Gennes equations. We found that the vortex charge is always negative when a sufficient strength of AF order is induced inside the vortex core, otherwise, the vortex charge is positive. By tuning the on-site Coulomb repulsion U or the doping parameter $δ$, a transition between the positive and negative vortex charges may occur. The vortex charge at optimal doping has also been studied as a function of magnetic field. Recent NMR and Hall effect experiments may be understood in terms of the present results. New imaging experiments should be able to probe the vortex charge directly.

cond-mat.supr-con↗

arXiv2002

Josephson Current in the Presence of a Precessing Spin

The Josephson current in the presence of a precessing spin between various types of superconductors is studied. It is shown that the Josephson current flowing between two spin-singlet pairing superconductors is not modulated by the precession of the spin. When both superconductors have equal-spin-triplet pairing state, the flowing Josephson current is modulated with twice of the Larmor frequency by the precessing spin. It was also found that up to the second tunneling matrix elements, no Josephson current can occur with only a direct exchange interaction between the localized spin and the conduction electrons, if the two superconductors have different spin-parity pairing states.

cond-mat.supr-con↗

arXiv2002

Quantum Electronic Transport through a Precessing Spin

The conductance through a local nuclear spin precessing in a magnetic field is studied by using the equations-of-motion approach. The characteristics of the conductance is determined by the tunneling matrix and the position of equilibrium chemical potential. We find that the spin flip coupling between the electrons on the spin site and the leads produces the conductance oscillation. When the spin is precessing in the magnetic field at Larmor frequency ($ω_{L}$), the conductance develops the oscillation with the frequency of both $ω_{L}$ and 2$ω_{L}$ components, the relative spectrum weight of which can be tuned by the chemical potential and the spin flip coupling.

cond-mat.mes-hall↗

arXiv2002

Quantum Nyquist Temperature Fluctuations

We consider the temperature fluctuations of a small object. Classical fluctuations of the temperature have been considered for a long time. Using the Nyquist approach, we show that the temperature of an object fluctuates when in a thermal contact with a reservoir. For large temperatures or large specific heat of the object $C_v$, we recover standard results of classical thermodynamic fluctuations $<ΔT^2> = \frac{k_B T^2}{C_v}$. Upon decreasing the size of the object, we argue, one necessarily reaches the quantum regime that we call quantum temperature fluctuations. At temperatures below $T^{*}\sim \hbar/k_{B}τ$, where $τ$ is the thermal relaxation time of the system, the fluctuations change the character and become quantum. For a nano-scale metallic particle in a good thermal contact with a reservoir, $T^{*}$ can be on a scale of a few Kelvin.

cond-mat.stat-mech↗

arXiv2002

In-plane Tunneling Spectrum into a [110]-Oriented High-$T_c$ Superconductor in the Pseudogap Regime

Both the differential tunneling conductance and the surface local density of states (LDOS) of a [110]-oriented high-temperature superconductor in the pseudogap (PG) regime are studied theoretically. As a competing candidate for the mechanism of PG state, the charge-density wave (CDW), spin-density wave (SDW), $d$-density wave (DDW), and d-wave superconducting (DSC) orderings show distinct features in the tunneling conductance. For the CDW, SDW, and DSC orderings, the tunneling conductance approaches the surface LDOS as the barrier potential is increased. For the DDW ordering, we show for the first time that there exist midgap states at the [110] surface, manifesting themselves as a sharp zero-energy peak in the LDOS, as in the case of DSC ordering. However, due to the particle-hole pair nature of the DDW state, these states do not carry current, and consequently the one-to-one correspondence between the tunneling conductance and the surface LDOS is absent.

cond-mat.supr-con↗

arXiv2002

Incommensurate Magnetism around Vortices and Impurities in High-$T_c$ Superconductors

By solving self-consistently an effective Hamiltonian including interactions for both antiferromagnetic spin-density wave (SDW) and d-wave superconducting (DSC) orderings, a comparison study is made for the local magnetic structure around superconducting vortices and unitary impurities. To represent the optimally doped regime of cuprates, the parameter values are chosen such that the DSC is dominant while the SDW is vanishingly small. We show that when vortices are introduced into the superconductor, an oscillating SDW is induced around them. The oscillation period of the SDW is microscopically found, consistent with experiments, to be eight lattice constants ($8a_0$). The associated charge-density wave (CDW) oscillates with a period of one half ($4a_0$) of the SDW. In the case of unitary impurities, we find a SDW modulation with identical periodicity, however without an associated CDW. We propose neutron scattering experiments to test this prediction.

cond-mat.supr-con↗

arXiv2001

Field induced d_x^2-y^2+id_xy state in d-density-wave metals

We argue that the d_{xy} component of the order parameter can be generated to form the d_x^2-y^2+id_xy-density wave state by the external magnetic field. The driving force for this transition is the coupling of the magnetic field with the orbital magnetism. The fully gapped particle spectrum and the magnetically active collective mode of the condensate are discussed as a possible signature of the d+id' density wave state.

cond-mat.supr-con↗

arXiv2001

High Angular Momentum Core States and Impurity Effects in the Mixed State of d-Wave Superconductors

The local quasiparticle spectra around the vortex core of a d-wave superconductor in the mixed state is studied by solving the lattice Bogoliubov-de Gennes equations self-consistently. It is shown that in addition to the zero-energy states, there also exist core states of high angular momentum. A nonmagnetic unitary impurity sitting at the core center is nondestructive to these core states and the zero-energy resonant state induced by itself is still visible in the local quasiparticle spectrum. The calculated imaging shows a fourfold ``star'' shape of the local density of states whose orientation is energy dependent. It is also found that, although the zero-energy states are extended, the core states of finite energy level could be localized.

cond-mat.supr-con↗

arXiv2001

C-axis Response of a High-T_{c} Superconductor with D-Density Wave Order

The microscopic origin of the pseudogap state which exists in the underdoped cuprates remains unknown. The $c$-axis properties in the pseudogap regime are particularly anomalous. We use a recently proposed model of a $d$-density wave which leads to staggered currents and the doubling of the unit cell to investigate the $c$-axis kinetic energy and the optical sum rule. The density of states expected in the model is also considered.

cond-mat.supr-con↗

arXiv2001

Quasiparticle States around a Nonmagnetic Impurity in D-Density-Wave State of High-$T_c$ Cuprates

Recently Chakravarty {\em et al.} proposed an ordered $d$-density wave (DDW) state as an explanation of the pseudogap phase in underdoped high-temperature cuprates. We study the competition between the DDW and superconducting ordering based on an effective mean-field Hamiltonian. We are mainly concerned with the effect of the DDW ordering on the electronic state around a single nonmagnetic impurity. We find that a single subgap resonance peak appears in the local density of state around the impurity. In the unitary limit, the position of this resonance peak is always located at $E_r=-μ$ with respect to the Fermi energy. This result is dramatically different from the case of the pure superconducting state for which the impurity resonant energy is approximately pinned at the Fermi level. This can be used to probe the existence of the DDW ordering in cuprates.

cond-mat.supr-con↗

arXiv2001

Quasiparticle States at a d-Wave Vortex Core in High-Tc Superconductors: Induction of Local Spin Density Wave Order

The local density of states (LDOS) at one of the vortex lattice cores in a high Tc superconductor is studied by using a self-consistent mean field theory including interactions for both antiferromagnetism (AF) and d-wave superconductivity (DSC). The parameters are chosen in such a way that in an optimally doped sample the AF order is completely suppressed while DSC prevails. In the mixed state, we show that the local AF-like SDW order appears near the vortex core and acts as an effective local magnetic field on the quasiparticles. As a result, the LDOS at the core exhibits a double-peak structure near the Fermi level that is in good agreement with the STM observations on YBCO and BSCCO. The presence of local AF order near the votex core is also consistent with the recent neutron scattering experiment on LSCO.

cond-mat.supr-con↗

arXiv2001

Local Quasiparticle States near a Unitary Impurity with Induced Magnetic Moment in a d-Wave Superconductor

The local quasiparticle density of states around a unitary impurity with a Kondo-like magnetic moment induced at its nearest neighbors in a d-wave superconductor is studied within the slave-boson mean-field approach. The Hamiltonian is exactly diagonalized and the results are obtained self-consistently. We show that the interference between the strong impurity potential scattering and the Kondo effect leads to novel quasiparticle spectra around the impurity, which are strikingly different from the case of a single unitary or magnetic impurity. The recent STM image of the local differential tunneling conductance around a Zn impurity in a high-T_c cuprate can be essentially explained if the blocking effect of BiO surface layer between the tip and probed CuO_2 plane [Zhu et al., Phys. Rev. B 62, 6207 (2000)] is taken into account.

cond-mat.supr-con↗

arXiv2000

Quasiparticle Localization in Disordered d-Wave Superconductors

An extensive numerical study is reported on disorder effect in two-dimensional d-wave superconductors with random impurities in the unitary limit. It is found that a sharp resonant peak shows up in the density of states at zero energy and correspondingly the finite-size spin conductance is strongly enhanced which results in a non-universal feature in one-parameter scaling. However, all quasiparticle states remain localized, indicating that the resonant density peak alone is not sufficient to induce delocalization. In the weak disorder limit, the localization length is so long that the spin conductance at small sample size is close to the universal value predicted by Lee (Phys. Rev. Lett. {\bf 71}, 1887 (1993)).

cond-mat.supr-con↗

arXiv2000

Local Quasiparticle States around an Anderson Impurity in a d-Wave Superconductor: Kondo Effects

The Kondo effects of an Anderson impurity embedded into a d-wave superconductor is studied. Within the slave-boson mean-field approach, the derived Bogoliubov-de Gennes equations are solved via exact diagonalization. We show that a critical coupling strength, above which the Kondo effect takes place, exists regardless of whether the band particle-hole symmetry is present or not. The resonant quasiparticle peaks are found in the local density of states (LDOS) both directly at the impurity and around its neighbors, which is in sharp contrast to the case of nonmagnetic unitary impurities, where the LDOS vanishes on the impurity site.

cond-mat.supr-con↗

arXiv2000

Superconductivity in Ferromagnetic RuSr_2GdCu_2O_8

The phase diagram of temperature versus exchange field is obtained within a BCS model for d-wave superconductivity in CuO_2 layers which is coupled to ferromagnetic RuO_2 layers in RuSr_2GdCu_2O_8. It is found that the Fulde-Ferrell-Larkin-Ovchinnikov state is very sensitive to the band filling factor. For strong exchange field, we point out that superconductivity could only exist in the interfaces between ferromagnetic domains. The magnetization curve is calculated and its comparison with experiment is discussed. We also propose the measuring of tunneling conductance near a single unitary impurity to detect the strength of the exchange interaction.

cond-mat.supr-con↗