Energy gaps measured by scanning tunneling microscopy

Abstract

A scanning tunneling microscope {STM) has been used to measure energy gaps in the chargedensity-wave (CDW) phases of the layer-structure dichalcogenides and in the high-temperature superconductor Bi&Sr,CaCu&08. Measured values of Ac& at 4.2 K for 2H-TaSe"2H-TaS"and 2H-NbSe2 are 80, 50, and 34 meV giving values of 26C&/k& T, equal to 15.2, 15.4, and 23.9, indicating strong coupling in these CDW systems. Measured values of 6&. & at 4.2 K in 1T-TaSe. and 1T-TaS, are -150 meV for both materials giving 2hcp~/k~r, . =5.8. STM scans of Bi,Sr,CaCu. O, at 4.2 K resolve atoms on the BiO"layer and show possible variations in electronic structure. The energy gap determined from I versus V and dI/d V versus V curves is in the range 30 -35 meV giving values of 2A/kBT, =8. Spectroscopy measurements with the STM can exhibit large zero-bias anomalies which complicate the analysis of the energy-gap structure, but adequate separation has been accomplished.

Femtosecond snapshots of gap-forming charge-density-wave correlations in quasi-two-dimensional dichalcogenides1T−TaS2and2HDemšar1, Forró2, Berger3 et al. 2002Phys. Rev. B1359991View full textAdd to dashboardCite

“…The theoretical fit for S(T ) and τ s ∝ 1/∆(T ) using the model by Kabanov et al [3] using a BCS gap function is superim-posed. The value of the gap from the fit 2∆(0) = 70 ± 10 meV is somewhat smaller than the maximum gap obtained from tunneling [17], but in good agreement with the maximum gap value of 65 meV from ARPES [8]. The divergence of τ s and the drop in S(T ) as T → T i−n ≈ 120 K from below are unmistakable signs of a T -dependent gap which is closing near T i−n [3].…”

Gaps and kinks in the electronic structure of the superconductor2H-NbSe2from angle-resolved photoemission at 1 K

“…Also note that tunneling spectroscopy indicates a k-averaged CDW gap which is asymmetric and has its minimum located slightly above E F . 10,11 For wavevectors at which the CDW contributes significantly to the opening of the spectral gap, the stated ∆ values may therefore underestimate the total gap size.…”

“…In addition to generally allowing for a better identification of truly unconventional effects in high-temperature superconductors, the answers to (i)-(iii) would specifically help to understand why the T c of 2H-NbSe 2 is the highest within the family of transition-metal dichalcogenides. 1 At first glance it seems surprising that the above questions are still largely open although many different sorts of experiments have been performed on 2H-NbSe 2 , including specific heat, 6 thermal conductivity, 7 magnetization, 8 and penetration depth 9 measurements, as well as tunneling spectroscopy [10][11][12][13] and angle-resolved photoemission spectroscopy (ARPES). [14][15][16][17] But much of the discord can indeed be traced back either to the lack of k resolution of the experimental probes or to the relatively high temperatures ( > ∼ 0.7 T c ) at which the ARPES measurements [14][15][16][17] were done.…”

Charge density waves in the graphene sheets of the superconductor CaC6

“…Many such materials, including the transition-metal dichalcogenides [7][8][9] , metalcontaining organic compounds 10 , and cuprate high-temperature superconductors 11 , exhibit superconductivity as well as CDWs 12,13 . It is, therefore, very natural to ask whether graphene, the simplest 2D material, can host a CDW/striped state 1,2,14 .…”