Kondo effect of single Co adatoms on Cu surfaces

The Kondo resonance of Co adatoms on the Cu(100) and Cu(111) surfaces has been studied by scanning tunneling spectroscopy. We demonstrate the scaling of the Kondo temperature T(K) with the host electron density at the magnetic impurity. The quantitative analysis of the tunneling spectra reveals that the Kondo resonance is dominated by the Cu bulk electrons. While at the Cu(100) surface both tunneling into the hybridized localized state and into the substrate conduction band contribute to the Kondo resonance, the latter channel is found to be dominant for Cu(111).     

Coherence scale of the kondo lattice

It is shown that the large- N approach yields two energy scales for the Kondo lattice model. The single-impurity Kondo temperature, T(K), signals the onset of local singlet formation, while Fermi-liquid coherence sets in only below a lower scale, T small star, filled. At low conduction electron density n(c) ("exhaustion" limit), the ratio T small star, filled/T(K) is much smaller than unity, and is shown to depend only on n(c) and not on the Kondo coupling. The physical meaning of these two scales is demonstrated by computing several quantities as a function of n(c) and temperature.     

Quantum dots with even number of electrons: kondo effect in a finite magnetic field

We show that the Kondo effect can be induced by an external magnetic field in quantum dots with an even number of electrons. If the Zeeman energy B is close to the single-particle level spacing Delta in the dot, the scattering of the conduction electrons from the dot is dominated by an anisotropic exchange interaction. A Kondo resonance then occurs despite the fact that B exceeds by far the Kondo temperature T(K). As a result, at low temperatures T相似文献   

Kondo temperature for the two-channel Kondo models of tunneling centers

A two-channel Kondo (2CK) non-Fermi liquid state in a metal resulting from the interaction between electrons and structural defects modeled by double-well potentials (DWP) is revisited. Account only of the two lowest states in DWP is known to lead to rather low Kondo temperature, T(K). We prove that the contribution of higher excited states reduces T(K), if all of the intermediate states are taken into account. Prefactor in T(K) is shown to be determined by the spacing between the second and the third levels epsilon(3) in DWP rather than by the electron Fermi energy epsilon(F). Since epsilon(3)相似文献   

Measurement of quantum noise in a carbon nanotube quantum dot in the Kondo regime

The current emission noise of a carbon nanotube quantum dot in the Kondo regime is measured at frequencies ν of the order or higher than the frequency associated with the Kondo effect k(B)T (K)/h, with TK the Kondo temperature. The carbon nanotube is coupled via an on-chip resonant circuit to a quantum noise detector, a superconductor-insulator-superconductor junction. We find for hν ≈ k(B)T(K) a Kondo effect related singularity at a voltage bias eV ≈ hν, and a strong reduction of this singularity for hν ≈ 3k(B)T(K), in good agreement with theory. Our experiment constitutes a new original tool for the investigation of the nonequilibrium dynamics of many-body phenomena in nanoscale devices.     

Kondo effect in a magnetic field and the magnetoresistivity of kondo alloys

The effect of a magnetic field on the spectral density of a S = 1/2 Kondo impurity is investigated at zero and finite temperatures by using Wilson's numerical renormalization group method. A splitting of the total spectral density is found for fields larger than a critical value H(c)(T = 0) approximately 0.5T(K), where T(K) is the Kondo scale. The splitting correlates with a peak in the magnetoresistivity of dilute magnetic alloys which we calculate and compare with the experiments on CexLa1-xAl2,x = 0.0063. The linear magnetoconductance of quantum dots exhibiting the Kondo effect is also calculated.     

Kondo universal scaling for a quantum dot coupled to superconducting leads

We study competition between the Kondo effect and superconductivity in a single self-assembled InAs quantum dot contacted with Al lateral electrodes. Because of Kondo enhancement of Andreev reflections, the zero-bias anomaly develops side peaks, separated by the superconducting gap energy Delta. For ten valleys of different Kondo temperature T(K) we tune the gap Delta with an external magnetic field. We find that the zero-bias conductance in each case collapses onto a single curve with Delta/k(B)T(K) as the only relevant energy scale, providing experimental evidence for universal scaling in this system.     

Kondo effect in a few-electron quantum ring

A small quantum ring with less than ten electrons was studied by transport spectroscopy. For strong coupling to the leads a Kondo effect is observed and used to characterize the spin structure of the system in a wide range of magnetic fields. At small magnetic fields Aharonov-Bohm oscillations influenced by Coulomb interaction appear. They exhibit phase jumps by pi at the Coulomb-blockade resonances. Inside Coulomb-blockade valleys the Aharonov-Bohm oscillations can also be studied due to the finite conductance caused by the Kondo effect. Astonishingly, the maxima of the oscillations show linear shifts with increasing magnetic field and gate voltage.     

Kondo model for the "0.7 anomaly" in transport through a quantum point contact

Experiments on quantum point contacts have highlighted an anomalous conductance plateau around 0.7(2e(2)/h), with features suggestive of the Kondo effect. Here, an Anderson model for transport through a point contact analyzed in the Kondo limit. Hybridization to the band increases abruptly with energy but decreases with valence, so that the background conductance and the Kondo temperature T(K) are dominated by different valence transitions. This accounts for the high residual conductance above T(K). The model explains the observed gate-voltage, temperature, magnetic field, and bias-voltage dependences. A spin-polarized current is predicted even for low magnetic fields.     

Nonequilibrium transport through a Kondo dot in a magnetic field: perturbation theory and poor man's scaling

We consider electron transport through a quantum dot described by the Kondo model in the regime of large transport voltage V in the presence of a magnetic field B with max((V,B)>T(K). The electric current I and the local magnetization M are found to be universal functions of V/T(K) and B/T(K), where T(K) is the equilibrium Kondo temperature. We present a generalization of the perturbative renormali-zation group to frequency dependent coupling functions, as necessitated by the structure of bare perturbation theory. We calculate I and M within a poor man's scaling approach and find excellent agreement with experiment.     

Kondo state for a compact Cr trimer on a metallic surface

The ground state of a Cr trimer supported on the Au(111) surface is investigated by means of a variational approach to the Coqblin-Schrieffer Hamiltonian. The temperature of Kondo-resonance formation (T(K)) for equilateral trimers increases drastically as compared to T(K) for a single Cr adatom. The Kondo state of a Cr trimer proves to be very sensitive to geometry and a small shift of any atom from the symmetrical position leads to a rapid decrease in T(K). These results are in good agreement with recent observations of the Kondo response of a single antiferromagnetic chromium trimer [T. Jamneala, Phys. Rev. Lett. 87, 256804 (2001)]].     

Interplay between crystal field splitting and Kondo effect in CeNi(9)Ge(4-x)Si(x)

The pseudo-ternary solid solution CeNi(9)Ge(4-x)Si(x) (0?≤?x?≤?4) has been investigated by means of x-ray diffraction, magnetic susceptibility, specific heat, electrical resistivity, thermopower and inelastic neutron scattering studies. The isoelectronic substitution of germanium by silicon atoms causes a dramatic change of the relative strength of competing Kondo, RKKY and crystal field (CF) energy scales. The strongest effect is the continuous elevation of the Kondo temperature T(K) from approximately 3.5?K for CeNi(9)Ge(4) to about 70?K for CeNi(9)Si(4). This increase of the Kondo temperature is attended by a change of the CF level scheme of the Ce ions. The interplay of the different energy scales results in an incipient reduction of the ground state degeneracy from an effectively fourfold degenerate non-magnetic Kondo ground state with unusual non-Fermi-liquid features of CeNi(9)Ge(4) to a lower one, followed by an increase towards a sixfold, fully degenerate ground state multiplet in CeNi(9)Si(4) (T(K)?～?Δ(CF)).     

Manipulation of the Kondo effect via two-dimensional molecular assembly

We report the manipulation of a Kondo resonance originating from the spin-electron interactions between a two-dimensional molecular assembly of TBrPP-Co molecules and a Cu(111) surface at 4.6 K. By manipulating nearest-neighbor molecules with a scanning tunneling microscope tip we are able to tune the spin-electron coupling of the center molecule inside a hexagonal molecular assembly in a controlled step-by-step manner. The Kondo temperature increases from 105 to 170 K with decreasing the number of nearest neighbor molecules from six to zero. The scattering of surface electrons by the molecules located at edges of the molecular layer reduces the spin-electron coupling strength for the molecules inside the layer. Investigations of different molecular arrangements indicate that the observed Kondo resonance is independent on the molecular lattice.     

Tuning the Kondo effect with a mechanically controllable break junction

We study electron transport through C(60) molecules in the Kondo regime using a mechanically controllable break junction. By varying the electrode spacing, we are able to change both the width and the height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. The linear conductance as a function of T/T(K) agrees with the scaling function expected for the spin-1/2 Kondo problem. We are also able to tune finite-bias Kondo features which appear at the energy of the first C(60) intracage vibrational mode.     

Mesoscopic Kondo screening effect in a single-electron transistor embedded in a metallic ring.

We study the Kondo screening effect generated by a single-electron transistor or quantum dot embedded in a small metallic ring. When the ring circumference L becomes comparable to the fundamental length scale xi(0)(K) = Planck's constant over upsilon(F)/T(0)(K) associated with the bulk Kondo temperature, the Kondo resonance is strongly affected, depending on the total number of electrons (mod4) and magnetic flux threading the ring. The resulting Kondo-assisted persistent currents are also calculated in both Kondo and mixed-valence regimes, and the maximum values are found in the crossover region.     

Temperature dependence of a single Kondo impurity

Recent advances in scanning tunneling microscopy have allowed the observation of the Kondo effect for individual magnetic atoms. One hallmark of the Kondo effect is a strong temperature-induced broadening of the Kondo resonance. In order to test this prediction for individual impurities, we have investigated the temperature dependent electronic structure of isolated Ti atoms on Ag(100). We find that the Kondo resonance is strongly broadened in the temperature range T = 6.8 K to T = 49.0 K. These results are in good agreement with theoretical predictions for Kondo impurities in the Fermi liquid regime, and confirm the role of electron-electron scattering as the main thermal broadening mechanism.     

Intersite coupling effects in a kondo lattice

The La dilution of the Kondo lattice CeCoIn5 is studied. The scaling laws found for the magnetic susceptibility and the specific heat reveal two well-separated energy scales, corresponding to the single-impurity Kondo temperature T(K) and an intersite spin-liquid temperature T(*). The Ce-dilute alloy has the expected Fermi liquid ground state, while the specific heat and resistivity in the dense Kondo regime exhibit non-Fermi-liquid behavior, which scales with T(*). These observations indicate that the screening of the magnetic moments in the lattice involves antiferromagnetic intersite correlations with a larger energy scale in comparison with the Kondo impurity case.     

Evidence for charge Kondo effect in superconducting Tl-doped PbTe

We report results of low-temperature thermodynamic and transport measurements of Pb1-xTlxTe single crystals for Tl concentrations up to the solubility limit of approximately x=1.5%. For all doped samples, we observe a low-temperature resistivity upturn that scales in magnitude with the Tl concentration. The temperature and field dependence of this upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature T(K) approximately 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high T(c) values.     

Universality of the Kondo effect in quantum dots with ferromagnetic leads

We investigate quantum dots in clean single-wall carbon nanotubes with ferromagnetic PdNi-leads in the Kondo regime. Most of the Kondo resonances exhibit a splitting, which depends on the tunnel coupling to the leads and an external magnetic field B, but only weakly on the gate voltage. Using numerical renormalization group calculations, we demonstrate that all salient features of the data can be understood using a simple model for the magnetic properties of the leads. The magnetoconductance at zero bias and low temperature depends in a universal way on gμ(B)(B-B(c))/k(B)T(K), where T(K) is the Kondo temperature and B(c) the external field compensating the splitting.     

Persistence of Li induced Kondo moments in the superconducting state of cuprates

Using 7Li NMR shift data, the anomalous local moment induced by spinless Li impurities persists below T(c) in YBa 2Cu 3O6+y. In the underdoped regime, the moments retain their Curie law below Tc. In contrast, near optimal doping, the large Kondo screening observed above Tc (TK = 135 K) is strongly reduced below Tc as expected theoretically when the superconducting gap develops. The limited spatial extent of the induced moment (on first near neighbor Cu) is not drastically modified below Tc, which allows a comparison with STM determination of the local density of states. Our results constrain theoretical models of the impurity electronic properties.