Single Impurity Effects Research Articles

Impurity effect as a probe for the pairing symmetry of graphene-based superconductors

We study theoretically the single impurity effect on graphene-based superconductors. Four different pairing symmetries are discussed. Sharp in-gap resonant peaks are found near the impurity site for the d + id pairing symmetry and the p + ip pairing symmetry when the chemical potential is large. As the chemical potential decreases, the in-gap states are robust for the d + id pairing symmetry while they disappear for the p + ip pairing symmetry. Such in-gap peaks are absent for the fully gapped extended s-wave pairing symmetry and the nodal f-wave pairing symmetry. The existence of the ingap resonant peaks can be explained well based on the sign-reversal of the superconducting gap along different Fermi pockets and by analyzing the denominator of the T-matrix. All of the features may be checked by the experiments, providing a useful probe for the pairing symmetry of graphene-based superconductors.

Resolving different pairing states in Weyl superconductors through the single-particle spectrum

We study theoretically single-particle spectra of Weyl superconductors. Three different superconducting pairing states are addressed, which are the BCS-type states with the $s$-wave pairing symmetry and the $p+ip$-wave pairing symmetry, and the FFLO pairing state. We elaborate that these three states can be resolved well based on the bulk and surface spectral functions as well as the local density of states. The single impurity effect is also explored, which may help us to differentiate the BCS-type pairing states and the FFLO state further.

Impurity Effect as a Probe of the Pairing Symmetry in BiS2-Based Superconductors

The impurity effects are studied for the BiS_{2}- layered superconductors based on a two orbital model with the Bogoliubov-de-Gennes technique. The superconducting critical temperature (T_{c}) as a function of the mpurity concentration is calculated. Significant reduction of T_{c} is found for spin singlet nearest-neighboring paring d-wave state and the spin triplet next-nearest-neighboring (NNN) paring p-wave state, while no depression of T_{c} for isotropic s-wave state. The single impurity effects for various paring states are also explored. The impurity resonance peak in the local density of states spectrum is found only for the d-wave state. For the spin triplet NNN paring p-wave state, two in-gap peaks occur in the case of positive impurity potential; and a single in-gap peak is found for the negative impurity potential, which shifts towards to lower energy with decreasing the potential strength. These results can be used to detect the paring symmetry of the BiS_{2}- based superconductors.

Melting of Lennard-Jones rare-gas clusters doped with a single impurity atom

Single impurity effect on the melting process of magic number Lennard-Jones, rare gas, clusters of up to 309 atoms is studied on the basis of Parallel Tempering Monte Carlo simulations in the canonical ensemble. A decrease on the melting temperature range is prevalent, although such effect is dependent on the size of the impurity atom relative to the cluster size. Additionally, the difference between the atomic sizes of the impurity and the main component of the cluster should be considered. We demonstrate that solid-solid transitions due to migrations of the impurity become apparent and are clearly differentiated from the melting up to cluster sizes of 147 atoms.

Single Impurity Effects in Multiband Superconductors with Different Sign Order Parameters

A single impurity problem is investigated for multiband s -wave superconductors with different sign order parameters ( s ± -wave superconductors) suggested in Fe-pnictide superconductors. Not only intraband but also interband scattering is considered at the impurity. The latter gives rise to impurity-induced local boundstates close to the impurity. We present an exact form of the energy of the local boundstates as a function of strength of the two types of impurity scattering. The essential role of the impurity is unchanged in finite number of impurities. The main conclusions for a single impurity problem help us understand effects of dense impurities in the s ± -wave superconductors. Local density of states around the single impurity is also investigated. We suggest impurity site nuclear magnetic resonance as a suitable experiment to probe the local boundstates that is peculiar to the s ± -wave state. We find that the s ± -wave model is mapped to a chiral d x 2 - y 2 ±i d x y -wave, reflecting the unconv...

The Role of Simulations in Nanoscience, a Case Study: Gold Nanowires

Suspended gold nanowires can be made atomically thin with as many as five atoms, showing extremely large Au-Au bond distances. Using tools derived from Density Functional Theory (DFT) we study many questions posed by the experiments. First we use realistic molecular dynamics simulation to study the mechanisms of formation, evolution and breaking of these atomically thin Au nanowires under stress. We show how defects induce the formation one-atom chains that can grow as long as five-atoms before breaking. Results are in excellent agreement with experiments, except for the resulting shorter bond distances. In order to address this question, we use ab initio electronic structure calculations to show that the exceedingly large Au-Au interatomic distances experimentally obtained could be the effect of impurities. We studied the effect of single impurities H, B, C, N, O, S and small molecules as H2 on the nanowire's electronic and structural properties, in particular how they affect the maximum Au-Au bond length.

Single Impurity Effect on the Melting of Nanoclusters

We show by molecular dynamics simulations that the melting temperature of clusters can be tuned by selective doping. In fact, a single Ni or Cu impurity in Ag icosahedral clusters considerably increases the melting temperature even for sizes of more than a hundred atoms. The upward shift is correlated to the strain relaxation induced by a small central impurity in icosahedral clusters.

Quasiparticle scattering and local density of states in thed-density-wave phase

We study the effects of single-impurity scattering on the local density of states in the high-$T_c$ cuprates. We compare the quasiparticle interference patterns in three different ordered states: d-wave superconductor (DSC), d-density wave (DDW), and coexisting DSC and DDW (DSC-DDW). In the coexisting state, at energies below the DSC gap, the patterns are almost identical to those in the pure DSC state with the same DSC gap. However, they are significantly different for energies greater than or equal to the DSC gap. This transition at an energy around the DSC gap can be used to test the nature of the superconducting state of the underdoped cuprates by scanning tunneling microscopy. Furthermore, we note that in the DDW state the effect of the coherence factors is stronger than in the DSC state. The new features arising due to DDW ordering are discussed.

Local density of states induced by anisotropic impurity scattering in a d-wave superconductor

We study a single impurity effect on the local density of states in a d-wave superconductor accounting for the momentum-dependent impurity potential. We show that the anisotropy of the scattering potential can alter significantly the spatial dependence of the quasiparticle density of states in the vicinity of the impurity.

Single impurity effects in the mixed state ofd-wave superconductors

The effects of a single impurity in the mixed state of a d-wave superconductor is studied by solving the lattice Bogoliubov--de Gennes equations self-consistently. The mechanism for the vortex pinning by the impurity is carefully elucidated. It is shown that the impurity sitting at the core center is not repulsive to the vortex 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.

Magnetotransport ofCeRhIn5

We report measurements of the temperature-dependent anisotropic resistivity and in-plane magnetoresistance on single crystals of the tetragonal heavy-fermion antiferromagnet (TN = 3.8 K) CeRhIn5. The measurements are reported in the temperature range 1.4 K to 300 K and in magnetic fields to 18 tesla. The resistivity is moderately anisotropic, with a room-temperature c-axis to in-plane resistivity ratio rho_c/rho_a(300 K) = 1.7. rho(T) measurements on the non-magnetic analog LaRhIn5 indicate that the anisotropy in the CeRhIn5 resistivity stems predominately from anisotropy in Kondo-derived magnetic scattering. In the magnetically ordered regime an applied field H reduces TN only slightly due to the small ordered moment (0.37mu_B) and magnetic anisotropy. The magnetoresistance (MR) below TN is positive and varies linearly with H. In the paramagnetic state a positive MR is present below 7.5 K, while a high-field negative contribution is evident at higher temperatures. The positive contribution decreases in magnitude with increasing temperature. Above 40 K the positive contribution is no longer observable, and the MR is negative. The low-T positive MR results from interactions with the Kondo-coherent state, while the high-T negative MR stems from single-impurity effects. The H and T-dependent magnetotransport reflects the magnetic anisotropy and Kondo interactions at play in CeRhIn5.

Neutron-diffraction studies and interatomic distances in CaPr doped NdBa 2Cu 3O 7−δ

Sintered samples of Nd 1−2 x Ca x Pr x Ba 2Cu 3O 7−δ with x = 0, 0.025, 0.050, and 0.10 have been studied by electrical-resistivity measurements, X-ray and neutron diffraction and from calculated bond-valence sums (BVS). Doping of equal amounts of CaPr suppresses a parabolic term in the decrease of the superconducting T c and results in a strong linear suppression of −d T c/d x = 206 K. This observation indicates a single-impurity effect and suggests that CaPr is a charge neutral doping in Nd-123. The c-axis length and cell volume decrease with doping and the Cu2O4 distance increases. BVS calculations suggest that T c in Pr doped 123 may be governed by small changes in interatomic distances and corresponding charge transfers, rather than a contribution from magnetic pair breaking as previously conjectured. Similar calculations of published data for oxygen-deficient Y-123 and Pr doped Y-123, show that the hole density in the planes →0 at about the concentration where T c vanishes. The average CuO 2 plane distance supports that Pr has valence +4 at low dopings. Results are discussed which suggest that the Pr valence at higher doping levels is +3.

Low temperature and high field studies on Yb x Y1−x Cu4Ag

Yb/Y substitutions in YbCu4Ag up to about 50% preserve the Kondo lattice character of the Yb borderphase, whereas beyond this concentration single impurity effects dominate. Magnetoresistance measurements and semi-empirical analyses are compared with model calculations, yielding good qualitative agreement.

Size effect in the electrical resistance of a ballistic point contact with single impurities

We calculate the effect of single impurities in a ballistic, normal metal point contact on the electrical resistance. Both the cases of ordinary and magnetic impurities are considered. For small point contact diameters d comparable with the average impurity—impurity distance r 0 the discreteness of impurity positions is taken into account. In a quasiclassical approximation, the expression for the contact resistance is obtained, which is valid for an arbitrary relation between d and r 0. The dependence of the point contact resistance on the ratio d/ r 0 is studied.

Low-frequency noise in multiple-quantum-point-contact systems.

Electrical-transport measurements were performed on a system consisting of two quantum point contacts (QPC's) in series, where one of the point contacts exhibited low-frequency telegraph noise due to single-impurity effects. We show this system to be ideal for the study of equilibration between the edge states that form at high magnetic fields. We investigate both the adiabatic and nonadiabatic regimes and show that in the transitional magnetic-field range the conductance is compatible with a complete equilibration but between a limited number of edge states only. We also discuss the equivalent experiment at zero field, where one can study the effect of the potential fluctuations on the collimation of the ballistic electrons. In addition, we confirm the observation of 4${\mathit{e}}^{2}$/h quantization in the conductance of a ``wide'' QPC.

Effects of single impurity scattering on one-dimensional quantum-effect devices

Summary form only given. It is shown that a severe degradation in the quantum-effect features occurs when only a single impurity exists in the device. Transport in different lateral regions of the waveguide is studied for a 0.5- mu m-long, 0.3- mu m-wide waveguide. As the waveguide is opened to include a single impurity, an immediate and drastic degradation in the conductance steps is observed, because the presence of the impurity in the channel makes the transmission probability energy dependent and less than unity. By analyzing which 2e/sup 2//h conductance step becomes distorted, the location of two single impurities is pinpointed. One lies very near the thin middle common barrier, while the other lies closer to the outer side barrier. Moreover, in a waveguide containing two impurities, the familiar oscillations in the tunneling current corresponding to the sweeping of the 1D subbands through the Fermi level are completely washed out, due to mode mixing in the vicinity of the impurities. >

Optical conductivity of the anderson impurity model

We use the auxiliary boson and 1/N-expansion methods to calculate the optical conductivity of the Anderson impurity model for all temperatures and frequencies. A qualitative agreement between our results and experiments on dense Kondo systems indicates that the optical properties of these systems are dominated by single impurity effects.

Phase transitions in NM-TM spinglasses

The co-operative spin-freezing caused by RKKY and DM-interactions leads to finite discontinuities in the EA-order parameter because of the co-existence of an effective single impurity effect and a spinglass effect. Derived expressions for specific heat and susceptibility yield a multiple order (first and third) phase transition. This third order transition may be reconciled with the observed “knees” in the specific heat and discontinuities in the susceptibility slope at T f.

Long range interactions and spin wave excitations in Fe xPd 82− xSi 18 metallic glasses

The low temperature magnetization of amorphous Fe x Pd 82− x Si 18, 2 ⩽ x ⩽ 25, approaches saturation like 1/ H. RKKY-type cluster-cluster interactions and single impurity effects are indicated. The effective strength of the RKKY interaction decreases with increasing x. The magnetization of both spin glass-like and weakly ferromagnetic alloys exhibits Bloch's T 3 2 dependence. The spin wave stiffness constants increase linearly with 2 ⩽ x ⩽ 25. The range of exchange interactions decreases with increasing x.

Magnetic susceptibility and single-impurity effects in dilute AgMn alloys

The magnetic susceptibility, chi , of AgMn alloys with concentrations, c, between 10 at ppm and 4 at.% has been measured in the temperature and field ranges 1.4-300K and 0.9-8.5 kOe. The impurity susceptibility has been analysed according to a modified Curie-Weiss law chi = chi 0+(C/(T+ theta )). The effective magnetic Mn moment is p=(5.1+or-0.1) mu B and theta =(0.05+or-0.04)K for c or approximately=0.5 at.%, theta is dependent on the concentration and p=(5.4+or-0.1) mu B. The susceptibility is independent of concentration over the whole temperature range for c<or approximately=100 at ppm and a characteristic temperature TK approximately=20-30 mK has been estimated from a comparison with the calculations of the temperature dependence of chi by Krishna-Murthy et al. (1975) in which a spin value S=5/2 was used as a scaling parameter. Special attention has been given to the sample control and a reliable determination of the magnetic solute concentration.