Magnetic field induced charge-density-wave transitions: the role of orbital and Pauli effects Mark Kartsovnik Walther-Meißner-Institut, BADW, Garching, Germany

Outline

a-(BEDT-TTF)2KHg(SCN)4: basic features r||(300 K)  30 – 100 Wcm; r^/r|| ~ 104 - 105; r||(300 K)/ r||(1.4 K)  102 T. Mori et al., 1990 2D Fermi surface Nesting instability of the Fermi surface CDW formation at  8 K very low!! Q

Pauli effect (isotropic) Spin splitting deteriorates the nesting conditions for a CDW TCDW/TCDW(0), exp Phase diagram of a-(BEDT-TTF)2KHg(SCN)4 P. Christ et al., JETP Lett. 2000 ~ 23 T Q+ Q- TCDW/TCDW(0) Theory: D. Zanchi et al., PRB 1996; P. Grigoriev&D.Lyubshin , PRB 2005 B suppresses CDW Q- < Q+

Orbital effect (B || z) Dy ~ 1/Bz Real space orbit: electrons become effectively more 1D Real space orbit:

Orbital effect (B || z) FICDW at t^’ > t^’ * Theory a-(BEDT-TTF)2KHg(SCN)4 D. Andres et al., PRB 2001 Theory FICDW at t^’ > t^’ * due to Landau quantization of the unnested FS pocket D. Zanchi et al., PRB 1996

FICDW: experiment; B || z SdHo The “slow oscillations” approximately periodic with 1/B appear at P  Pc  2.5 kbar display a weak hysteresis P = 3 kbar

FICDW: experiment; B || z The “slow oscillations” approximately periodic with 1/B appear at P  Pc  2.5 kbar display weak hysteresis slightly shift with temperature  behaviour consistent with the FICDW scenario!!

FICDW: experiment; B || z

FICDW: experiment; B || z in a-(BEDT-TTF)2KHg(SCN)4 FISDW in (TMTSF)2PF6 A. Kornilov et al., PRB 2002 FICDW is weaker than FISDW due to the Pauli effect! A. Lebed, JETP Lett. 2003

FICDW: role of the Pauli effect N = 5 4 3 2 1 4 N = 3 2 1 no Pauli effect (FISDW) Pauli effect on (FICDW) Qx = 2kF + NG Qx = 2kF  QP + NG G = 2eayBz/h QP = 2mBB/hvF

FICDW: role of the Pauli effect 4 N = 3 2 1 A. Lebed, JETP Lett. 2003 no Pauli effect (FISDW) Pauli effect on (FICDW) Qx = 2kF + NG Qx = 2kF  QP + NG G = 2eayBz/h QP = 2mBB/hvF

FICDW in a tilted field Commensurate splitting “Spin-zero” 2QP = MG (A. Bjelis et al., 1999; A. Lebed, 2003) “Spin-zero” N = 3,4 2,3 1,2 0,1 2QP = MG 2QP = (M + 1/2)G with M - integer A. Lebed, JETP Lett. 2003

FICDW in a tilted field: experiment T = 0.4 K Spin-zero condition:

FICDW in a tilted field: experiment 1st CS angle Spin-zero condition:

FICDW in a tilted field: experiment 1st CS angle Spin-zero condition:  vF  1.2105 m/s

Summary The orbital effect causes FICDW transitions in a-(BEDT-TTF)2KHg(SCN)4 at pressures above Pc = 2.5 kbar The Pauli effect, in general, weakens the FICDW instability The interplay between the orbital and Pauli effects can be controlled by changing the field orientation: - the FICDW is enhanced at commensurate splitting angles - the FICDW is suppressed at „spin-zero“ angles