Vacuum Tunneling Research Articles

Reentrant transition from an incipient charge-ordered state to a ferromagnetic metallic state in a rare-earth manganate

A reentrant transition from an incipient charge-ordered (CO) state to a charge-delocalized ferromagnetic (CDFM) state has been established in the manganate ${\mathrm{Nd}}_{0.25}{\mathrm{La}}_{0.25}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3},$ in which the average $A$-site ionic radius is 1.19 \AA{}. The reentrant CDFM phase is associated with a first-order phase transition that reduces the orthorhombic distortion of the lattice, in contrast to the CO transition in other manganates where the orthorhombically distorted CO state is stabilized at low temperatures. At the CO-CDFM transition, there is a collapse of the charge-ordering gap as measured by vacuum tunneling spectroscopy.

Nonperturbative real time propagation at finite temperature

A new formalism will be presented in order to study real time evolution of quantum systems at finite temperature. Probability distributions for time-correlated observables will be studied non-perturbatively and fully quantized. This works for various systems, including ones with tunneling phenomena. We have obtained good results with some computational methods which can be used on models with several degrees of freedom. Thus it looks feasible to study vacuum tunneling in real time for relevant field theories at finite T.

Temperature-dependent vacuum tunneling spectroscopy of rare-earth manganates showing colossal magnetoresistance and charge ordering

) opens up in the density of states (DOS) near EF, below the charge-ordering transition. A gap comparable to the transport gap opens up in the paramagnetic state also. There is an absence of gap only in the ferromagnetic metallic state. We have measured the temperature dependence of these quantities.

Unusual Strong-Coupling Effects in the Tunneling Spectroscopy of Optimally Doped and OverdopedBi2Sr2CaCu2O8+δ

Tunneling spectroscopy measurements are reported on single crystals of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ using vacuum tunneling and point-contact methods. A reproducible dip feature in the tunneling conductance is found near $|\mathrm{eV}|\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2\ensuremath{\Delta}$, observed for both voltage polarities in the best resolved spectra. With overdoping the position of the dip continues to scale with $\ensuremath{\Delta}$, and its magnitude decreases as $\ensuremath{\Delta}$ decreases. These results indicate that the dip feature arises from a strong-coupling effect whereby the quasiparticle lifetime is decreased at a characteristic energy of $\mathrm{\ensuremath{\sim}}2\ensuremath{\Delta}$, consistent with an electron-electron pairing interaction.

Vacuum tunneling by cosmic strings

We consider vacuum tunneling of a new kind where the false vacua are not translationally invariant, but have topological defects that break some of their translational symmetries. In the particular case where the topological defects are cosmic strings, we show the existence of an O(2) × O(2) symmetric bounce configuration that allows a semiclassical estimate of the rate of cosmic string induced tunneling. A method of reduction is then suggested for simplifying the computation of the bounce action. Some phenomenological applications are described.

Surface etching of 1T-TaS2 with UHV-STM

Surface etching of the layered compound 1T-TaS2 with a STM tip was observed in an ultrahigh vacuum where the true vacuum tunneling gap was achieved. The etching did not always progress in a layer-by-layer manner, but etch-pits often had the depths of multiples of the unit layer. The etching rate on a clean surface was much slower than that on a surface once exposed to the ambient air, which showed a rough and irregular nano-scale morphology, implying that the surface of 1T-TaS2 is chemically degraded by the water vapor in air. On a clean surface, the STM etching started from the defect-like sites seen on the STM image, which possibly reflect the substrate impurities located on the Ta planes or van der Waals gaps, and progressed preferably in the scanning direction. The etching rate tended to increase with the bias voltage, regardless of its sign. This suggests that the electrostatic attractive force between tip atoms and surface atoms and/or the current-induced local heating contribute significantly to the STM etching of 1T-TaS2 in ultrahigh vacuum.

Break-junction experiments on single crystals: from bulk transport to vacuum tunnelling

Mechanically controllable break junctions of the heavy-fermion superconductor have been investigated. Josephson-like I - V characteristics could be found only for low-ohmic contacts, but without the oscillatory pattern of the critical current in a magnetic field. High-ohmic contacts always had a finite residual resistance as if there was a normal layer at the contact. Some of these contacts showed Andreev-reflection-like features in the differential resistance , while resistive transport usually affected them. We have also realized true vacuum tunnelling: the exponential dependence of the current on the width of the vacuum gap and the transition from tunnelling to metallic contact indicate a nearly free-electron mass of the tunnelling charge carriers, as well as a typical metallic carrier density of . The temperature dependence of the tunnelling current yields direct information about the thermal-expansion coefficient of the samples and its anisotropy. The superconducting features of the I - V characteristic vanish when the contact resistance exceeds about . They could not be resolved by vacuum tunnelling, pointing to a pair-breaking effect of the heavy-fermion interfaces.

Anomalous vacuum tunneling in antimony break junctions

Results of break junction experiments are presented, measuring the resistance as a function of inter-electrode spacing in vacuum tunnel junctions of Sb at a temperature of 1.3 K. A pronounced deviation from exponential tunneling is observed, showing a maximum in the resistance at a spacing of ∼ 1 A ̊ . A tentative explanation of the anomalous barrier-width dependence of the current in terms of Sb surface states is given.

Generalized derivative expansion and one-loop corrections to the vacuum energy of static background fields

The derivative expansion of the effective action for field theories with spontaneous symmetry breaking has a variable expansion parameter which may become locally infinite. To circumvent this difficulty, I propose an alternative expansion series in which the series expansion is simultaneously developed in order of the number of derivatives of the field and in powers of the deviation of the field from its ground-state value. As an example, I have applied this new method to calculate the quantum correction to the energy of the (1+1)-dimensional soliton in various models which have been well investigated previously. The expansion series are calculated using MATHEMATICA to 14 terms. For the models in which exact solutions have been found, such as the sine-Gordon soliton, ${\ensuremath{\varphi}}^{4}$ soliton, and ${\ensuremath{\varphi}}^{4}$ soliton with a fermion loop, the new improved series can be recognized as well-known analytically summable series. The complete results of exact solutions are recovered. More importantly, for the cases where exact solutions may not be available, Pad\'e approximants or the Borel summation can be used as an efficient method to provide an excellent approximation, in contrast with cumbersome numerical calculations. The Christ-Lee soliton and the ${\ensuremath{\varphi}}^{4}$ bag are used to illustrate this approximation. We also derive a compact hybrid formula in closed form to estimate the quantum correction to the static energy of the (1+1)-dimensional field. This new method can be easily extended to higher dimensions as well as other important applications such as vacuum tunneling, Skyrmion physics, etc.

Estimating vacuum tunneling rates

In Euclidean field theories, a rigorous upper bound on the action of the bounce with the least action can be obtained by finite numerical methods because the bounce with the least action is the global minimum of the action in an open space. The corresponding lower bound on the tunneling rate is easily computed by a new method of “reduction”.

Application of a micromachine scanning tunnelling microscope ( -STM) for vacuum tunnelling gap observation

Application of a micromachine scanning tunnelling microscope ( -STM) for vacuum tunnelling gap observation

Phase transitions and vacuum tunneling into charge- and color-breaking minima in the MSSM.

The scalar potential of the MSSM may have local and global minima characterized by non-zero expectation values of charged and colored bosons. Even if the true vacuum is not color and charge conserving, the early Universe is likely to occupy the minimum of the potential in which only the neutral Higgs fields have non-zero vev's. The stability of this false vacuum with respect to quantum tunneling imposes important constraints on the values of the MSSM parameters. We analyze these constraints using some novel methods for calculating the false vacuum decay rate. Some regions of the MSSM parameter space are ruled out because the lifetime of the corresponding physically acceptable false vacuum is small in comparison to the present age of the Universe. However, there is a significant fraction of the parameter space that is consistent with the hypothesis that the Universe rests in the false vacuum that is stable on a cosmological time scale.

Vacuum tunneling spectroscopy of high-temperature superconductors: A critical study.

Using scanning tunneling spectroscopy (STS) the local density of states (LDOS) of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{1}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ [Bi(2212)] and ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBaCuO) monocrystals were studied at low temperature (down to 4.2 K). Due to the high quality of the vacuum tunneling junctions the pair-breaking effects are consequently reduced, and rather fine features of the superconducting LDOS could be analyzed. For Bi(2212) it is shown that, in order to fit properly the experimental data, ${\mathit{d}}_{{\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2}}$ pairing (or a very anisotropic s pairing) as well as a strong in-plane anisotropy of the Fermi surface are required. Although YBaCuO STS spectra look more ``d like,'' no precise analysis was possible because of the high natural surface contamination of the samples. Both the fundamental problem of pairing symmetry and the influence of experimental conditions on vacuum STS measurements are discussed. \textcopyright{} 1996 The American Physical Society.

Scanning tunneling spectroscopy of a superlattice superconductor: Bi 2Sr 2CaCu 2O 8

It is clear that many of the cuprate superconductors are truly superlattices, composed of sheets whose individual superconducting critical temperatures may approach bulk values. Such a cuprate is Bi 2Sr 2CaCu 2O 8, often referred to as BSCCO-2212. Scanning tunneling spectroscopy (vacuum tunneling) applied to a- bBiO cleavage planes of T c ≈90 K BSCCO single crystals under liquid helium simultaneously provides topography and local d I/d Vspectra (superconducting density of states: DOS). The spectra, which are similar to those obtained from angle-resolved photoemission spectroscopy, confirm a large gap parameter Δ( x, y) on the uppermost layer. The d I/d Vspectra do not unequivocally select order parameter symmetry, but are probably consistent with d-wave or anisotropic s-wave states. Spatial variations of Δ on a 100 Å scale are attributed to variation in BiO metallicity, originating in oxygen stoichiometry variations. A model is presented to explain the different d I/d Vspectra which are seen, and associated with different local oxygen concentrations. This model, based on the superconducting proximity effect, assumes that in some regions the BiO uppermost layer is insulating and in other regions it is metallic, in the latter case induced superconductivity by proximity to the CuO 2planes. Our STM measurements appear to sample only the uppermost half cell of the crystal, and contain no obvious superlattice features. Recent measurements have confirmed Josephson radiation from voltage biased c-axis pillars of BSCCO. From the point of view of the present work, the superconducting systems which weakly couple along the c-direction to create Josephson junctions are probably half-cell slabs of height 15.4 Å, each containing two CuO 2and two BiO layers, which act as single composite electrodes for the Josephson junctions.

Instanton moduli for T3 × ℝ

We review the specific problems that arise when studying instantons on a torus. We discuss how the Nahm transformation shows that no exact charge one instanton on T**4 can exist. However, taking one of the directions (the time) to infinity, it can be shown that vacuum to vacuum tunnelling solutions exist. A precise description of the moduli space for T**3xR, studied numerically using lattice techniques, remains an interesting open problem. New is an explicit application of the Nahm transformation to (anti-)selfdual constant curvature solutions on T**4 and a discussion of its properties relevant to instantons on T**3xR.

Study of the proximity effect in Nb/Au, Bi2Sr2CaCu2O8+y/Au, and HgBa2CuO4/Ag using a scanning tunneling microscope

Proximity effect gaps were observed consistently in Nb/Au, Bi2Sr2CaCu2O8+y/Au, and HgBa2CuO4/Ag bilayers using a low temperature scanning tunneling microscope. The Nb/Au conductance curves [dI(V)/dV vs V] were fit by the Arnold model, using the accepted energy gap value of Nb, 1.55 meV. The application of gold and silver overlayers allowed vacuum tunneling and topographic images of BSCCO single crystals and the HgBa2CuO4 polycrystalline sample. Cleavage steps of varying sizes (up to 1500 Å) in the BSCCO/Au sample were observed, and gaplike features were found on such steps. Consistent, reproducible energy gaps were also found in the Hg-compound sample.

Atom Electronics: A proposal of nano-scale devices based on atom/molecule switching

This paper describes the possibility of atom/molecule switching devices, which are candidates to supersede the present metal-oxide-semiconductor (MOS) devices and to establish the new era of Atom Electronics. They are Atom Relay Transistors (ART) and Molecular Single Electron Switching (MOSES) devices, with total dimensions of a few nm and an operation speed of more than Tera (1012) Hz. Using these devices, it is estimated that a supercomputer, with 107 gates of logic circuit and 109 bits of memory, would be integrated in an area 200 μm square and operating at more than Tera (1012) Hz. ART and MOSES devices are evaluated on the basis of the characteristics necessary for information processing integrated circuit devices, together with other nano scale devices, and are found to be the most promising candidates that supersede the present metal oxide semiconductor (MOS) devices for information processing in the next decade. This paper also describes several technologies which could contribute to the establishment of Atom Electronics, including beam assisted scanning tunneling microscope (BASTM) which makes it possible to observe insulator surfaces, micromachine scanning tunneling microscope (μ-STM) by which the vacuum tunneling gap is successfully observed and the Needle Formation and Tip Imaging (NFTI) method which makes possible the in-situ evaluation of the tip apex of an STM.

Thermovoltages in vacuum tunneling investigated by scanning tunneling microscopy

By heating the tunneling tip of a scanning tunneling microscope the thermoelectric properties of a variable vacuum barrier have been investigated. The lateral variation of the observed thermovoltage will be discussed for polycrystalline gold, stepped surfaces of silver, as well as for copper islands on silver.

Vacuum tunneling spectroscopy and asymmetric density of states of Bi2Sr2CaCu2O8+ delta.

This paper reports very detailed low-temperature vacuum tunneling spectroscopy investigations of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (BSCCO) single crystals. For clean vacuum junctions formed between a cleaved BSCCO single crystal and the normal-metal tip of a scanning tunneling microscope, we obtain stable c-axis vacuum tunneling conditions that allow very reproducible low-temperature electron tunneling spectroscopy. A vacuum junction is identified by tunneling spectra which neither depend on tip/sample spacing nor change as a function of time and position on the sample in the Meissner state. In contrast to the frequently reported linear or parabolic increase with increasing bias voltage, the background conductance of such spectra is largely constant with a slight decrease up to \ifmmode\pm\else\textpm\fi{}300 meV bias. The normal-state conductance inferred from this background has a local maximum at negative sample bias, indicating a pileup below the Fermi level in the (ab)-plane normal-state density of states of BSCCO. Vacuum junctions at 4.8 K show a well developed superconducting gap with large peaks at the gap edges and a finite density of quasiparticle excitations filling the gap. These characteristics are not consistent with an isotropic BCS-like gap parameter. Outside the superconducting gap, the differential conductance curves are very asymmetric, with a striking dip which appears at negative sample bias only. This dip contributes a substantial amount of states to satisfy the conservation of states we find between normal and superconducting BSCCO.

Tunneling spectroscopy on correlated electron systems

Different current methods for tunneling spectroscopy are discussed. Spectroscopic measurements at low temperatures involving either vacuum tunneling or ‘small-junction’ tunneling are presented on different systems which show gaps in their density of states. In the Kondo insulators FeSi and CeNiSn, we found gaps 2 Δ of about 100 meV and 10 meV, respectively. In the heavy fermion materials UPd 2Al 3 and URu 2Si 2, we found gaps of about 10 meV below the antiferromagnetic ordering when tunneling along the ab planes. No gap was found along the c-axis, so that the normal state of these superconductors appears strongly anisotropic.