Tunneling Spectroscopy and Vortex Imaging in Boron-doped Diamond

Tunneling Spectroscopy and Vortex Imaging in Boron-doped Diamond Kitaoka Lab. Toshiyuki Tsuchida Ref.)B.Sacepe et al. cond-mat/0510541

Contents • Introduction • Boron-doped Diamond • STM • Measurement of Superconductor by STM • Experiments • Summary

Superconductivity in B-doped diamond onset 4K offset 2.3K Ekimov, et al., Nature 428, 542 (2004) Superconductivity takes place in the diamond at the interface between graphite and B4C Synthesis under High Pressure (8～9GPa) and High Temperature(2,800K) [HPHT Method]

SC in B-doped diamond film MPCVD method Polycrystalline onset 7K GAS: H2 + CH4 + B(CH)3 Advantages of this method It is easy to control thickness of diamond film growth facet Boron density offset 4K Y.Takano Appl .Phys.Lett., 85,4 2004

Boron-doped diamond 3 B-density vs Tc B-density vs resistivity(room temperature) Superconductivity J.-P. Lagrange et al. D.R.M 7 (1998) 1390–1393 Umezawa et al. condmat-05503303 (2005) Superconductivity takes place in vicinity of metal-insulator boundary Tc of (111) film > Tc of (100) film

STM (Scanning Tunneling Microscope) 1 ・Observations of microscopic geometry on the surface Atomic-scale resolution ・Measurements of local property

N N N N STM (Scanning Tunneling Microscope) 2 V=0 DOS probe (normal metal) EF I sample (normal metal) V=V1 DOS V EF eV1

STM (Scanning Tunneling Microscope) 3 I probe (normal metal) EF N N V sample (normal metal) ↓ (superconductor) I 2Δ EF S V N Δ/e

S.C.measurement by STM D(E) Tunneling conductance EF E ΔI/ΔV V 2Δ：energy gap

Sample information1 ・synthesized by MPCVD (Microwave Plasma-assisted chemical vapor deposition ) ・boron density ～ nB=1.9×1021cm-3 (≒1.1%) ・ thickness ～150 nm Umezawa et al. condmat-05503303 (2005)

Sample information2 Hc2(T=0K)~1.5T →ξ~150Å (ξ: coherence length) Tconset=2.5K ΔTc is very narrow

600 nm substrate Surface topography by STM ・The parallel strips reflect the vicinal surface structure of the substrate . ・the average of steps is ~1.8 nm 1.8nm step 600nm (1.5×1.5μm2)

Tunneling conductance at Superconducting state 2Δ Δ=285μeV (In the BCS theory,Δ/kBTc～1.76)

Temperature dependence of the BCS gap BCS fitting curve indicates Tc=1.85K susceptibility and transport  Tc ~ 1.9K Tc=1.85K

Superconducting state observed by STM Δ/kBTc～1.74 Tc=1.85K B-doped diamond is conventional BCS superconductor

Vortex Type-I diamagnetic Type-II Hc2 Applied magnetic field Hc1 Hc S.C. state Vortex state Normal state Superconducting state Normal state ξ Δ

NbSe2 H. F. Hess et al ,Phys.Rev.Lett. 62,214(1989) Vortex images 1.5μm White・・・・・S.C. state Black・・・・・normal state In the higher field, more vortice exist. H=1200Oe H=1900Oe d~140nm d~110nm Comparing with the ideal triangular vortex lattice, the vortex lattice of B-doped diamond is inhomogeneous. →because of random substitution of Boron

Density of states around the vortex The diameter of vortex is estimated about 15nm. (c.f. ξGL=15nm) Even within the BCS gap, DOS around EF exists. H=1800Oe

Summary • First observation of SC state by STM • B-doped diamond is the conventional BCS superconductor • Under magnetic field,the vortices are arranged in disordered triangular lattice

Tunneling Spectroscopy and Vortex Imaging in Boron-doped Diamond