NMR and NMR and SR in 2D frustrated vanadatesSR in 2D frustrated vanadates

P. Carretta (Pavia)N. PapinuttoR. Melzi (Bruker BioSpin)S. AldrovandiE. PavariniC.B. AzzoniM.C. Mozzati

R.G.M. Caciuffo (Ancona)A. Bombardi (ESRF)

P. Millet (Toulouse)S. Gonthier

NMR, NMR, SR, Cv, SR, Cv, M(T)M(T)

Band structureBand structure

EPREPR

Neutron scatteringNeutron scattering

Sample preparation, chemical and Sample preparation, chemical and structural characterizationstructural characterization

[V4+]=[Ar] 3d1 S=1/2

c

b a

Magnetic ions V4+

a

b

J2

J1

LiLi22VO(Si,Ge)OVO(Si,Ge)O4 4

structurestructure

J2 J1

0 10.5

?

Néel Order

I)

II)

Collinear Order

2D Vanadates

JJ11-J-J22 at T=0 at T=0

P ?

Li2VOSiO4 J1+J28.5 K

Li2VOGeO4 J1+J26.0 K

VOMoO4 J1+J2155 K

P. Chandra and B. Doucot PRB 38, 9335 (1988)H. J. Schulz and T.A.L.Ziman Europhys. Lett. 18 355 (1992) V.N. Kotov et al. PRB 60, 14613 (1999)L. Capriotti. and S. Sorella PRL 84, 3173 (2000) ; L. Capriotti, Int. J. Mod. Phys. B 14, 3386 (2000)V.N. Kotov et al. PRB 63, 064412 (2000);

J2/J1

0.0 0.5 1.0 1.5 2.00.00

0.05

0.10

0.15

0.0 0.4 0.8 1.2 1.6 2.00.0

0.2

0.4

0.6

a)

VOMoO4

Li2VOSiO

4

Li2VOGeO

4 spin.(

J 1+J2)

(em

u.K

/mol

e)

T/(J1+J2)

b)

Li2VOSiO

4

Li2VOGeO

4

CV/

R

T/(J1+J2)

How much frustated are the vanadates ?How much frustated are the vanadates ?

 

Compound J1+J2 Tm(K) Tc(K) J2/J1 J2/J1 from

band structure

VOMoO4 15510 100 420.5 - 0.15

Li2VOSiO4 91 5.35 2.850.05 1 - 5 12

Li2VOGeO4 6.00.3 3.5 1.950.03 0.9 - 4 5

 •R. Melzi et al. PRL 85, 1318 (2000)•R. Melzi et al. PRB 64, 024409 (2001)•H. Rosner et al. PRB 67, 014416 (2003)•G. Misguich et al. cond-mat/0302583

0 1 2 3 40

2

4

6

8

10

12

14

16

18

Ordered Phase

H (

Tes

la)

T (K)

LiLi22VOSiOVOSiO44 H vs. T phase diagram H vs. T phase diagram

• NMR and NS showed that the magnetic order is collinear• If x is the direction of V4+ magnetic moments then Q=(/a,0)

The collinear phaseThe collinear phase

• Critical exponent for the magnetization 0.24, both for Li2VOSiO4 and VOMoO4.

• At low-T the reduction of the order parameter is consistent with SWT with J10-3-10-4 (J1+J2)

• NS results in Li2VOSiO4 confirm that the order is collinear and yield (T0)0.6B

0 5 10 15 20 25 30 35 40 450

25

50

75

100

125

150

175

200

225SWT with J

1+J

2=155 K, =J

/J

//= 0.00027

B(T)=B(0) (1-T/TC)0.2435

B (g

auss

)T (K)

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.2

0.4

0.6

0.8

1.0

1.2

B(

T)/

B (0)

T/TC

Li2VOGeO

4

Li2VOSiO

4

VOMoO4

L. Siurakshina et al., PRB64, 104406 (2001)

H.J. Schulz et al., J.Phys. I 6, 675 (1996)

L. Capriotti cond-mat/0306679

4 5 6 7 81.2

1.4

1.6

1.8

0.000 0.005 0.010 0.015 0.0200

1000

2000

3000

4000

5000

VOMoO4

T=53 K

T=112 K

T=293 K

-103 95

K

104 (emu/mole)

Li2VOSiO

4

T=5 K

T=100 K7 K

(ppm

)

(emu/mole)

K=AK=A++

•At Tdist a modification in the hyperfine coupling A is evidenced•The ratio Tdist/(J1+J2) is similar for both compounds, despite of the huge difference in J1+J2. .

Evidences of frustration driven distortionsEvidences of frustration driven distortions

Tdist/(J1+J2)0.5

Tdist/(J1+J2)0.67

I. Shiozaki et al. Physica B 284-288, 1621 (2000)

Distortion in VOMoODistortion in VOMoO44 from Raman scattering from Raman scattering

15.0 15.2 15.40

1000

2000

3000AB

2.03 K

2.79 K

2.84 K

3.46 K

29Si NMR

Am

plitu

de (A

rb. U

nits

)

(MHz)

2.4 2.6 2.8 3.0 3.2 3.4 3.60.01

0.1

1~exp( /T), = 44 K

Are

a A

/Are

a B

T (K)

Diffusive lattice distortion from Diffusive lattice distortion from 2929Si NMR in Si NMR in LiLi22VOSiOVOSiO44

0.2 0.4 0.6 0.80.0

0.5

1.0

1.5

2.0

Li2VOSiO

4 , = J

1+J

2

La2CuO

4 , = J

T1(T

>>

)/T1(

T)

T/

1.0 1.5 2.0 2.5 3.01

10

100

TN

La2CuO

4

From 63Cu(2) NQR From neutron scattering

/a

1000/T (K-1)

1/T1/T11= (= (22/2N) /2N) qq|A|Aqq||22S(S(qq,,RR) ) e e22s/Ts/T

Nuclear spin-lattice relaxation for TNuclear spin-lattice relaxation for Tcc<T<J<T<J11+J+J22

-1

0

1

0

5

10

15

20

25

30

-1

0

1

29Si Form factor

|Aq|2 (k

Gau

ss2 )

qy ( /a)

qx ( /a) 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

10

20

30

40

50

60

70

Sample A Sample B

29Si NMRin Li

2VOSiO

4

1/T1 (s

-1)

T (K)

40 80 120 160 2000

1

2

3

4

5

6

95Mo NMR in VOMO4

1/T 1

(s-1

)

T (K)

2 3 4 5 6 7 8 90.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Li2VOSiO

4

Li2VOGeO

4

c (s)

T (K)

Very-low-frequency excitations for kVery-low-frequency excitations for kBBT< JT< J11+J+J22

c(1/Heis)

P. Chandra , P. Coleman and A.I. Larkin PRL 64, 88 (1990)

P. Carretta et al. PRL 88, 047601 (2002)

Collinear I

Collinear II

Domain wall motions

•From the analysis of NMR shift one concludes that P sizeably reduces J2/J1•The transition temperature to the collinear phase is weakly affected, by about 0.2 K, by P

Effect of pressure (P) on JEffect of pressure (P) on J22/J/J11

1 10 100

77.7

77.8

77.9

78.0

0 10 20 30

0.2

0.4

0.6

(M

Hz)

T (K)

1 bar 10 Kbar 20 Kbar

T m

/(J1+J

2)

P (kbar)

Peierls-like transition induced by frustration in a 2D AFPeierls-like transition induced by frustration in a 2D AF F. Becca and F. Mila, PRL89, 037204(2002)F. Becca and F. Mila, PRL89, 037204(2002)

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,40

1

2

3

4

5

6

7

8

9

10

1

J2/J

1

SP

O

T

J1,2(dij)(dij)-1

•These vanadates are characterized by a collinear ground state and JThese vanadates are characterized by a collinear ground state and J22/J/J11 >1. >1.

•Sublattice magnetization is close to the one of a pure 2DQHAF.Sublattice magnetization is close to the one of a pure 2DQHAF.•Frustration driven lattice distortions are observed by means of NMR.Frustration driven lattice distortions are observed by means of NMR.•77Li 1/T1 is surprisingly nearly T independent down to TLi 1/T1 is surprisingly nearly T independent down to TTc.Tc.•An anomalous very-low-frequency dynamics is observed by means of An anomalous very-low-frequency dynamics is observed by means of SR.SR.•Pressure reduces JPressure reduces J22/J/J11 but the GS is little affected. What happens to the but the GS is little affected. What happens to the

lattice distortion as Jlattice distortion as J22/J/J11 is reduced? is reduced?

•What happens to the lattice distortion when impurities or dilution (e.g. VWhat happens to the lattice distortion when impurities or dilution (e.g. V4+4+ for Tifor Ti4+4+ ) is present? ) is present?

Conclusions and open questionsConclusions and open questions