A Superlattice model for superconductivity in the Borocarbides Thereza Paiva M. El Massalami Raimundo R. dos Santos UFRJ

Borocarbides Model Transport properties Phase diagrams Conclusions

Borocarbides

RT 2 B 2 C  1 RC layer T=Ni R=Sc, Y, Ce, Dy, Ho, Er, Tm, Lu, U, Th  SUC coexistence SUC and MAG (all above but Lu) R= Yb  Heavy fermion RTBC  2 RC layers T=Ni  no SUC, no HF T=Co  single layer R=Lu, Tm, Er, Ho Dy, Gd, Ce  no SUC R=La  single layer T=Ni  no SUC no MAG T=Pd, Pt  SUC

U<0 U=0 U<0 U=0 U<0 U=0       RT 2 B 2 C       RTBC U<0 U=0 U=0 Model attractive sites T 2 B 2 RC  no f electrons

 Layering  L 0 =1 and L 0 =2  Chemical Composition  ,  and U SUC Lanczos Method  Exact Finite-sized sistems  no spontaneous symmetry breaking One-dimensional system  no true LRO quasi-ordered states  power law decay of “SUC” correlations with distance Extrapolations towards thermodynamic limit

Transport properties Charge gap  single particle excitations  C = E(N c,N e +1)+E(N c,N e 1) - 2E(N c,N e )  C D C I  0 = 0 S  0  0 M = 0  0 Drude weight   (  )=D C  (  )+g(  )

Charge Gap Extrapolation with 1/N S  C = 0   <    C  0      Gaussian fit to  2  C /  2    =2.7±0.6 L 0 =1  =5/3 U=-4

Drude Weight Extrapolation with 1/N S 2 ( 1/N S, ln N S ) D C = 0     D D C  0   <  D D SL /D H =   D =18±1 L 0 =1  =5/3 U=-4 Exponential decay

S-wave singlet correlation function  i  l  i+l C(i,l) =½ i  attractive site C( i,l = 2) =1   =2±1  D =7.0 ±0.5  =11/6 N S =24 U=-4

Phase Diagram  fixed |U|  C =1 L 0 =1  C =1.33 L 0 =2 Strong coupling  >> |U| >> 1 CC

Repeat the procedure L 0 =2 other 

Phase Diagram  fixed   =5/3 >  C Reentrant SUC

Conclusions Balance between layering, chemical composition and SUC SUC  larger region for L 0 =1 than L 0 =2 single layer material  SUC double layer material  no SUC Reentrant SUC