Spin dependent tunneling in junctions involving normal and superconducting CDW metals

1. Spin dependent tunneling in junctions involving normal and superconducting CDW metals A.M. Gabovich and A.I. Voitenko (Institute of Physics, Kyiv, Ukraine) T. Ekino (Hiroshima University, Japan) Mai Suan Li and H. Szymczak (Institute of Physics, Warsaw, Poland) M. Pękała (Warsaw University, Poland)

2. Electronics vs. Spintronics: Ferromagnets: Magnetization is linked to the difference between spin sub-band populations in the conduction band Objective: To estimate the polarization of the tunnel current Introduction

3. Tunnel conductances G(V) for metal/gapped material junction at temperature T0: the Fermi distribution of metal electrons serves as a probe of the electron density of states (DOS) of the gapped material electrode Factors in the integrand of G(V) are caused by metal electrons Starting points of Tedrow and Meservey (1973):

4. TM’s original idea for FM—BCS junction: • If the gapped material = BCS superconductor, its peak-possessing DOS may also serve as a probe of the metal DOS in the vicinity of the Fermi surface (FS) ! • Warning: absence of an electron spin-flipping while tunneling Problem: To segregate the spin-polarized components of the tunnel current

5. Splitting of spin sub-bands in the BCS superconductor Solution: Spin sub-bands in a BCS s-wave superconductor can be split in an external magnetic filed, *B is the effective Bohr magneton • To apply H Requirement: Availability of a gapped FS section on one side and a non-gapped FS section on the other side of the junction

6. New problems and their solutions • Meissner effect: • Thin films. • Temperature smearing: • Use as low T as possible. • In any case, T < Tc. • Spin-orbit interaction ~Z4: • Use constituting elements as light as possible. The effect was measured for Al: Z = 13, Δ=0.4 meV, Tc = 1.19 K Counter-electrodes: Fe, Ni, Co.

7. CDW metal: FS comprises both gapped (d) and non-gapped (nd) sections. The DOS structure: at the d-sections is similar to that of BCS superconductor (the dielectric order parameter Σ), at the nd-section to that of ordinary metal (no gap). Advantages: No Meissner effect Less stringent requirements to sample geometry Bigger range of the dielectric gaps |Σ|: critical temperatures Td is in the range 1 K 1000 K Spin-splitting is observable in the “symmetrical" (CDWM/CDWM') setup Possibility to use the effect in studying CDWMs themselves. For example: 2H-NbSe2: ZNb=41 (ZSe=34), Σ=34 meV, Td=33.5 K Our idea: To use CDW metals

8. Green’s function method of the tunnel current calculation • FM—CDWM junction

9. Drastic distinctions from the FM—BCS case: peaks on one CVC branch and cusps on the other one, h = *BH/0, 0= (T=0) Strong dependence on the parameter μ FM—CDWM junction

10. Sensitivity to the parameter P and to the sign of Σ

11. CVCs for the FM —I—superconducting CDWM junction - superconducting gap for T = 0 in the absence of CDWs

12. ( Symmetrical CDWM—CDWM junction Distinction from FM—BCS case: Different disposition (+ - - +) of spin-polarized peaks BCS = (- + - +) CDWM′—CDWM setup no effect in BCS′—BCS setup) Energy scheme, processes ——— with spin splitting - - - - - without spin splitting

13. Sensitivity to gapping level μ temperature

14. CVCs for the CDWM —I—CDWM junction • CDWMs are normal • The phase of the left electrode equals to zero

15. Conclusions CDW metals (CDW superconductors) • Can be used in tunnel experiments to detect spin splitting • Possess advantages over BCS superconductors: • no Meissner effect • bigger range of gap amplitudes • Can be observed in symmetrical junctions, since there are both degenerate and non-degenerate FS sections • are perspective objects for investigation in spintronics