A Superlattice model for superconductivity in the Borocarbides

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

Borocarbides • RT2B2C  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 singlelayer R=Lu, Tm, Er, Ho Dy, Gd, Ce no SUC • R=La singlelayer T=Nino SUC no MAG T=Pd, Pt SUC

Model U<0 U=0 U<0 U=0 U<0 U=0         RT2B2C     RTBC U<0 U=0 U=0 U<0 U=0 U=0  T2B2 RC no f electrons  attractive sites

Layering L0=1 and L0=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 Drude weight  ()=DC()+g() Charge gap  single particle excitations C= E(Nc,Ne+1)+E(Nc,Ne1) - 2E(Nc,Ne) C DC I 0 = 0 S 0  0 M = 0 0

Charge Gap L0=1 =5/3 U=-4 Extrapolation with 1/NS C = 0  <  C  0     Gaussian fit to 2C/2 =2.7±0.6

Drude Weight L0=1 =5/3 U=-4 Extrapolation with 1/NS2 (1/NS,ln NS) DC = 0   D DC  0   < D DSL/DH= 10-3 D=18±1 Exponential decay

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

Phase Diagramfixed |U| Strong coupling  >> |U| >> 1 C=1 L0=1 C=1.33 L0=2 C

Repeat the procedure L0=2 other 

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

Conclusions • Balance between layering,chemical composition andSUC • SUClarger region for L0=1 than L0=2 • single layer material SUC • double layer material no SUC • Reentrant SUC

A Superlattice model for superconductivity in the Borocarbides