Electron pairing in the vast majority of superconductors follows the Bardeen-Cooper-Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel ...spins in a singlet state with an s-wave symmetry. Unconventional superconductivity was predicted in single-layer graphene (SLG), with the electrons pairing with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in SLG. The realization of unconventional superconductivity in SLG offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.
Considerable evidence for proximity-induced triplet superconductivity on the ferromagnetic side of a superconductor-ferromagnet (S-F) interface now exists; however, the corresponding effect on the ...superconductor side has hardly been addressed. We have performed scanning tunneling spectroscopy measurements on NbN superconducting thin films proximity coupled to the half-metallic ferromagnet La sub(2/3)Ca sub(1/3) Mn O sub(3) (LCMO) as a function of magnetic field. We have found that at zero and low applied magnetic fields the tunneling spectra on NbN typically show an anomalous gap structure with suppressed coherence peaks and, in some cases, a zero-bias conductance peak. As the field increases to the magnetic saturation of LCMO where the magnetization is homogeneous, the spectra become more BCS-like and the critical temperature of the NbN increases, implying a reduced proximity effect. Our results therefore suggest that triplet-pairing correlations are also induced in the S side of an S-F bilayer.
We have applied scanning tunneling spectroscopy in studies of the electronic level structure of surface-functionalized colloidal Si nanocrystals (Si-NCs) as a function of their size for various ...capping ligands. The energy gaps extracted from the tunneling spectra increase with decreasing NC size, manifesting the effect of quantum confinement. This is consistent with the blueshift revealed by photoluminescence (PL) from dodecene functionalized Si-NCs. The tunneling spectra measured on NCs functionalized with NH4Br or allylamine show band-edge shifts toward higher energies, akin to p-type doping. This behavior can be accounted for by the combined contributions of the ligands’ dipole moments and charge transfer between a Si-NC and its surface groups. Concomitantly, size-independent PL spectra, which cannot be associated with NC band gap variations, were observed for the latter Si-NCs.
Motivated by recent observations of chiral-induced magnetization and spin-selective transport we studied the effect of chiral molecules on conventional BCS superconductors. By applying scanning ...tunneling spectroscopy, we demonstrate that the singlet-pairing s-wave order parameter of Nb is significantly altered upon adsorption of chiral polyalanine alpha-helix molecules on its surface. The tunneling spectra exhibit zero-bias conductance peaks embedded inside gaps or gap-like features, suggesting the emergence of unconventional triplet-pairing components with either d-wave or p-wave symmetry at the Nb organic-molecules interface, as corroborated by simulations. These results may open a way for realizing simple superconducting spintronics devices.
A new approach for doping of Cu2S nanocrystal arrays using thermal treatment at moderate temperatures (T < 400 K) is presented. This thermal doping process yields conductance enhancement by 6 orders ...of magnitude. Local probe measurements prove this doping is an intraparticle effect and, moreover, tunneling spectroscopy data signify p-type doping. The doping mechanism is attributed to Cu vacancy formation, resulting in free holes. Thermal-doping temperature dependence exhibits an Arrhenius-like behavior, providing the vacancy formation energy of 1.6 eV. The moderate temperature conditions for thermal doping unique to these nanocrystals allow patterned doping of nanocrystal films through local heating by a focused laser beam, toward fabrication of nanocrystal-based electronic devices.
Abstract
We investigate the voltage–current characteristics of a superconductor–insulator–ferromagnet heterostructure, where the insulating layer contains pinhole-defects. The superconducting layer ...exhibits multiple voltage jumps that are hysteretic with the current sweep direction. This characteristic of the resistive state is due to pinholes that induce local, distinct, coupling regions between the superconducting and ferromagnetic layers which may generate phase-slip lines or vortex channeling. These findings point to a magnetically driven design of a superconductor memristor. Concomitantly, the junctions display both absolute and differential negative resistances below the superconducting critical temperature and current. This anomalous behavior is analyzed using a circuit approach and is attributed to current passing through pinholes within the insulating layer. These two unique effects, which stem from the special topology of the pinholes-governed interface can be applied in superconductor-based switches and memory devices.
The electronic properties of metal-semiconductor nanojunctions are investigated by scanning tunneling spectroscopy of gold-tipped CdSe rods. A gap similar to that in bare CdSe nanorods is observed ...near the nanodumbbell center, while subgap structure emerges near the metal-semiconductor nanocontact. This behavior is attributed to the formation of subgap interface states that vanish rapidly towards the center of the rod, consistent with theoretical predictions. These states lead also to modified Coulomb staircase, and in some cases to negative differential conductance, on the gold tips.
We studied the temperature dependence of the photocurrent spectra of a Ge-SiO2 composite thin film. We found that the spectral position of the photocurrent peak is determined by the competition ...between absorption and non-radiative recombination and that its temperature dependence is associated with the population variation of the energetically deep levels in the system under “thermal quenching” conditions. Combining these results with our previous deep-level transient spectroscopy data enables the association of these levels with the quantum confinement effect. We thus identify here a non-radiative recombination process associated with deep-level sensitization that stems from quantum confinement.
•Photocurrent spectra (PCS) temperature dependence measured on Ge/SiO2 thin films.•Absorption (Abs.) vs. non-radiative-recombination (NRR) competition discussed.•Trend in PCS peak vs. temperature attributed to Abs. vs. NRR competition.•Results were compared with previous deep-level transient spectroscopy study.•Our integrated studies suggest NRR related to quantum confinement.
The combination of a superconductor with a magnetically inhomogeneous material has been established as an efficient mechanism for the generation of long-ranged spin-polarized (spin-triplet) Cooper ...pairs. Evidence for this mechanism, however, has been demonstrated based on studies done on thin-film multilayers, where the strong bonds existing at the interface between the superconductor and the magnetic material should in principle enhance proximity effects and strengthen any electronic correlations. Here, we fabricate devices based on van der Waals (vdW) stacks of flakes of the NbS_{2} combined with flakes of Cr_{1/3}NbS_{2}, which has a built-in magnetic inhomogeneity due to its helimagnetic spin texture at low temperatures. We find that the critical temperature of these vdW heterostructures is strongly dependent on the magnetic state of Cr_{1/3}NbS_{2}, whose degree of magnetic inhomogeneity can be controlled via an applied magnetic field. Our results demonstrate evidence for the generation of long-ranged spin-triplet pairs across the Cr_{1/3}NbS_{2}/NbS_{2} vdW interface.
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