External-fields induced novel phenomena in Mott insulator Ca 2 RuO 4 - PowerPoint PPT Presentation

External-fields induced novel phenomena in “Mott insulator Ca 2 RuO 4 ” Fumihiko NAKAMURA ADSM, Hiroshima University Ⅰ . Pressure induced Superconductivity Ⅱ . E induced Ins.-Metal transition

>> Collaborators << >> Pressure works << Hiroshima: Y.Senoo,Y.Nakai, T.Suzuki, T.Fujita Cambridge: P.L.Alireza, S.K.Goh, Y.T.C.Ko, M.Sutherland, G.G.Lonzarich, S.R.Julian (Toronto) Kyoto: Y.Maeno, S.Nakatsuji (ISSP), H.Fukazawa (Chiba) >> Dielectric breakdown << Hiroshima: T.Takemoto, M.Sakaki, Y.Yamauchi Kyoto: Y.Maeno, S.Yonezawa,T.Yamagishi

Ⅰ . Quantum Critical natures in the vicinity of magnetic ordered state 3D system POSSIBLE NEW STATES from P. L. Alireza, G.G.Lonzerich

How about “2D system” ? 2D system AFM 2D system FM ? 300 La 2- x Sr x CuO 4 TEMPERATURE (K) Theoretical prediction Tetragonal 200 only Antiferro Insulator Hatatani and Moriya, JPSJ, 64 (1995) 3434 100 Orthorhombic “Ca 2 RuO 4 ” was a candidate. T d S.C. 0 0.1 0.2 0.3 x : Sr concentration

Rich variety of pressure phase diagram from Mott insulator to Q2D FM metal Mixed phase Mott transition Q2D Metal (Ins./Metal) Mott Ins. -1 10 1000 -2 10  c 8.0 GPa 3.0 GPa  (m  cm) -3 2.0 GPa 10  ab 100 Q2D Metal Temperature (K) B type -4 10 2.0 GPa -5 A type 10 3.0 GPa Ca 2 RuO 4 10 -6 10 0 100 200 300 AF itinerant Temperature (K) FM 1 Quantum critical nature 0.1 0.1 1 10 Pressure (GPa) F.Nakamura, PRB65 (2002)220402, JPSJ (2007).

Itinerant and Anisotropic FM in Ca 2 RuO 4 Generalized Rhodes-Wohlfarth Plot Strongly anisotropic FM the scale for itinerant FM due to Spin-orbit coupling  0 H A ~ 9.5T 40 0.6 10  0 H || a p eff  M rem 1 M (  B /Ru-ion) 1.2 1.5 0.4 5 30 Sc 3 In  0 H || b (GPa) 1.8 1.9 2 p eff  M rem 0.2 0.1 0.15 2 K  0 H || c T C / T 0 1.8GPa 20 Ni 3 Al 0 Sr 2 CaRu 2 O 7 1.0  0 H || c ZrZn 1.9 10 ZrZn 2   /   Ca 2 RuO 4 0.5 Ca 2 RuO 4 (1 ~ 2GPa) CrB 12  0 H || a MnSi CeF 2 Au 4 V 0 0 0.1 0.2 0.3 0 4 8 12 T C / T 0   H (T) itinerant localize (up to 1) Ni Fe Co  0 H A 0.14T 0.049T 1.04T Y.Yamauchi, et al., Physica C (2010).

To explore superconductivity in a Mott ins. Ca 2 RuO 4 Our project started in 1999. >>> One decade after Over 10GPa: Very hard work !! Resistance Mott Ins. 1000 100 Q2D Metal Temperature (K) B type 250  m A type 10 anisotropic ac susceptibility AF itinerant FM 1 S C ? 0.1 0.1 1 10 Pressure (GPa) Alireza. Rev.Sci.Ins. 74 , 4728 (2003).

We found pressure induced SC at ~10 GPa Resistance (2-terminal) ac susceptibility P. L. Alireza, et al., Journal of Physics: Condensed Matter 22 (2010) 052202. arXiv:0912.1513 [cond-mat.supr-con]

From Mott insulator to “ SC ” via itinerant FM T c ~ 0.4 K at P ~14 GPa P. L. Alireza, et al., Journal of Physics: Condensed Matter 22 (2010) 052202. arXiv:0912.1513 [cond-mat.supr-con]

New SC phase in pressurised CRO Pressurisation above ~8 GPa turns CRO from FM metal to SC ( T c ~ 0.4 K and ~14 GPa). 1. How about relation between FM and SC ? 2. How about difference in SC between SRO and CRO ? ( p or s -wave SC ? ) 3. Ru214 is 2D Fermi liquid metal but what is difference ? Quantum oscillation data is required.

Ⅱ. “Electric-field” induced Mott transition “Electric field” has higher potential than P Reported breakdown in Mott insulator E th (kV/cm) E gap (eV) La 2-x Sr x NiO 4 1~10 0.26 1) Sr 2 CuO 3 1~3 2) SrCuO 2 0.3~1 2) 1) M.Imada, Rev.Mod.Phys. 70 4 (1998). 2) Y.Taguchi., PRB. 62 11 (1999). 3) Y. Iwasa . , APL. 39, 10441 (1989). 0.3~1.2 (TTeC1TTF)-TCNQ 3) Ca 2 RuO 4 ? 0.2 / 0.05 (@RT) We expect “ E th ~3kV/cm” @RT for Ca 2 RuO 4 based on Zener breakdown model. 2 ε gap E th = e 2 ε 0 a

Breakdown in 4d Mott insulator Ca 2 RuO 4 occurs at “Surprisingly weak E th ~40V/cm” 40V/cm 50V/cm 1.0 Ca 2 RuO 4 Ca 2 RuO 4 0.8 0.8 E // ab E // c Current (A) Current (A) 0.6 295K 0.6 295K 0.4 0.4 0.2 0.2 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Voltage (V) Zener Breakdown No ! Why is E th ~ 40V/cm so weak in Ca 2 RuO 4 ? Avalanche Breakdown ?

Metal-Insulator transition in Ca 2 RuO 4 accompanied by structural change 1. Substitution ( (Sr/Ca) 2 RuO 4 ) Mott Insulator 2. Heating (temperature ) U/W 3.Pressure 4.Electric field La 2 CuO 4 Insulator ( S-Pbca flatted ) Ca 2 RuO 4 O2 (Ru-O2 < Ru-O1) Filling O1 d xz d yz Sr 2 RuO 4 Band d xy Ru ～ Metal ( L-Pbca ) d Ca/Sr O2 (Ru-O1 <Ru-O2 ) O O1 d xy d xz d yz

The breakdown accompanied by structural transition from S- to L-Pbca Breakdown in CRO is “Bulk transition”. Avalanche Breakdown NO ! 1 2 .4 (006) 100 Ca 2 RuO 4 Ca 2 RuO 4 Lattice parameter c ( Å ) Volume fraction (%) 1 2 .3 67V/cm S-Pbca 75 E // c E // c 1 2 .2 L-Pbca L-Pbca 295K 295K (Metal) (Metal) 50 Intensity (arb. units) 1 2 .1 42V/cm (Ins.) (Insulator) 25 1 2 .0 S-Pbca S-Pbca L-Pbca 0 40V/cm 1 1 .9 0.4 Ca 2 RuO 4 20V/cm I (A) 0.2 E // c 0 V/cm Ca 2 RuO 4 295K 0.0 44 45 46 0 20 40 60 2  (degree) E (V/cm)

Summary Dielectric Breakdown in Mott insulator Ca 2 RuO 4 occurs at “Superisingly weak E th ~40V/cm” accompanying with structural transition c axis: 11.9 Å (insulator) → 12.3 Å (metal) DB in CRO → Bulk transition 1. Zener Breakdown No 2. Joule heating No 3. Avalanche Breakdown No What is the possible mechanism for weak E th ?

How about possible mechanism for Dielectric breakdown in Ca 2 RuO 4 ? Change of the internal charge distribution. Enough amount of charge for the metalisation is internally stored in the apical oxygen (O2) of CRO, and then it can be poured into the RuO 2 plane only by quite weak field of E th ~40V/cm. E th ~ 40V/cm O2 O2 O1 O1 Insulator ( S-Pbca ) Metal ( L-Pbca )

Other possible mechanism 1 2 .4 We observed NDR ! Insulator Ca 2 RuO 4 Ca 2 RuO 4 100 Lattice parameter c ( Å ) Volume fraction (%) 1 2 .3 (Negative Differential Resistance) S-Pbca E // c E // c 75 1 2 .2 L-Pbca L-Pbca 295K 295K (Metal) (Metal) 50 nonequilibrium current sweep 1 2 .1 e.g. filamentary (Ins.) (Ins.) 25 S-Pbca S-Pbca 1 2 .0 L-Pbca 0.3 0 1 1 .9 0.2 YES 0.4 I (A) Ca 2 RuO 4 0.1 Structural I (A) change 0.2 E // c 0.0 0.0 0.5 1.0 1.5 295K V (V) 0.0 bulk Metal NO 0 20 40 60 E (V/cm)