Download (C 5 H 12 N) 2 CuBr 4 Spin ½ AFM chain (C 5 H 12 N)CuBr 3

Single crystal growth of Heisenberg spin ladder and spin
chain
Bingying Pan, Weinan Dong, Xiaochen Hong and Shiyan Li
Department of Physics, Fudan University, Shanghai, China
Low dimensional magnets have attracted great attention because of their simplicity in theoretical models, novel quantum phenomena and relation to high
temperature superconductivity. Among them, quasi-1D systems such as spin ladders and spin chains have found their realization in several materials, especially in
some organic-metal compounds which can sustain good 1D dimensionality down to very low temperature. We have grown Heisenberg spin ½ antiferromagnetic
spin ladder (C5H12N)2CuBr4 by evaporation method and obtained (C5H12N)CuBr3 in a similar way. We identify (C5H12N)CuBr3 as a Heisenberg spin chain via its
magnetic property and crystal structure. No antiferromagnetic transition was observed down to 1.8K. Thermal conductivity measurement down to millikelvin will be
measured soon, to detect the low-energy excitations in them.
Low dimensional spin structure
Spinons in quasi-1D spin ½ AFM systems
J
a spin chain
Jleg
J
Jrung
J
a two-leg spin ladder
a two dimensional square lattice
(a)
Spin ½ HAFM ladder (C5H12N)2CuBr4
H  J leg
S
i , j 1, 2
2i , j
 S 2i 1, j  J rung  Si ,1  Si , 2  h
i
S
i , j 1, 2
(b)
Detection of low-energy excitations in field-induce
Luttinger liquid state of (C5H12N)2CuBr4 by (a) inelastic
neutron scattering [2] and (b) specific heat measurement [1].
Note the comparison of experiment and theory of spinon
continuum in (a) and linear relation of specific heat and
temperature in the Luttinger liquid state regime in (b).
Specific heat measurement of Cu benzoate [3],
it is a good 1D spin ½ AFM chain material down
to very low temperature when H=0T with spinon
excitations, the linear relation of magnetic specific
heat to temperature is a feature of spinons.
Z
i, j
Spin ½ AFM chain (C5H12N)CuBr3
H  J  S 2i  S 2i 1  h S
i
Magnetic susceptibility
Z
i
i
T-H phase diagram of (C5H12N)2CuBr4 [1]
Features of this system
Antiferromagnetic interaction is through Cu-Br-----Br-Cu superexchange pathway
Quantum disorder ground state with a gap between singlet and triplet state
(a)
(b)
Crystal Structure of (C5H12N)CuBr3. (a) And (b) are
planes parallel and perpendicular to the chains.
The exchange coupling on the chain is through
the Cu-Br-Cu pathway.
Magnetic susceptibility of powder
(C5H12N)CuBr3, Booner-Fisher fit
(solid line) gives J/kB=17.51K
Jleg, Jrung is relatively small by two Br- exchange pathway (here, Jleg=3.3K,
Jrung=12.9K) than by oxide ion which is of the amplitude of 103 K as in cuprate.
Laboratory magnetic field can suppress this gap by Zeeman effect when gμBB=∆.
Two quantum critical points occurs at Bc=6.96T, Bs=13.85T. Bc: Quantum
disordered state to Lutting liquid state; Bs:Luttinger liquid state to fully polarized
state
 Magnon BEC state regime between Bc and Bs below 110 mK
summery
Heisenberg two-leg spin ladder (C5H12N)2CuBr4 single crystals were synthesized.
(C5H12N)CuBr3 single crystals were synthesized, after measuring its susceptibility and resolving
its structure, we identify it to be an antiferromagnetic spin chain.
The low-energy excitations of a Heisenberg antiferromagnetic spin chain as well as the fieldinduced Luttinger liquid state of an antiferromagnetic spin ladder are spinons, we hope to detect
these excitations by thermal conductivity down to millikelvin. As for other fermions, spinons would
contribute a linear term to thermal conductivity. The exotic magnon condensate and quantum
critical points in (C5H12N)2CuBr4 are also interested, we will see how they affect the thermal
conductivity.
Comparison of susceptibility along three different
directions and powder sample, the upturn of the H//x
data (blue triangle) should be due to the free spins
induced by a small fraction of the sample solving
in the glue during measurement.
References
[1] Ch. Ruegg et al, Phys. Rev. Lett. 101, 247202 (2008)
[2] B. Thielemann et al, Phys. Rev. Lett. 102, 107204 (2009)
[3] D. C. Dender et al, Phys. Rev. Lett. 79, 1750 (1997)
[4] B. R. Patyal et al, Phys. Rev. B 41, 1657 (1990)