M1 Colloquium Presentation Arora Varun 29A13106 (Shimizu Lab) High Pressure Study of Na x TiNCl and CeFe 2

Contents Introduction Motivation Experimental Method Results Summary

Intercalated MNX Compound (Na x TiNCl)

Introduction MNX M : Group IV metal ( Ti, Zr, Hf ) N : Nitrogen X : Halogen ( Cl, Br, I ) Two types of layer-structured polymorphs of MNX ： – α-form ( Orthorhombic ) – β-form ( Rhombohedra ) Superconductivity upto T c = 25.5 K in β-form :- Lithium doped ZrNCl : Superconductivity at 13 K (1996) Lithium doped HfNCl : Superconductivity at 25.5 K (1998) Superconducting Nitride Halides (MNX), Christian M. Schurz et al., Zeitschrift for Kristallographie 226, (2011)

TiNCl α-MNX type structure Layer Structure [MN] + layer is sandwiched between two sheets of halide anions [X] - Orthorhombic Structure Sensitive to moisture Energy Band = 1.7 eV (Semi-conductor) Basal Spacing Ti Cl N Superconducting Nitride Halides (MNX), Christian M. Schurz et al., Zeitschrift for Kristallographie 226, (2011)

What is Inter-calation? To insert something between elements or layers. Dopants can be introduced through the interlayer space without substituting or disturbing the original network for electron transport. Two types of layer structured crystals : Molecular - Van der Walls force ( eg : Graphite ) Ion – exchangeable - Redox reaction ( eg : clay minerals ) Superconducting Nitride Halides (MNX), Christian M. Schurz et al., Zeitschrift for Kristallographie 226, (2011)

Previous Research on Na 0.16 TiNCl Superconducting Nitride Halides (MNX), Christian M. Schurz et al. Zeitschrift for Kristallographie 226, (2011)

Motivation On applying pressure, distance between the layers “d” decreases, and so Tc should also increase ? Superconducting Nitride Halides (MNX), Christian M. Schurz et al., Zeitschrift for Kristallographie 226, (2011)

Check for Superconductivity in Na 0.16 TiNCl at High Pressure Resistivity Measurement Sharp Drop in Resistivity Meisner Effect Sharp change in AC Susceptibility ( χ ) Superconducting Nitride Halides (MNX), Christian M. Schurz et al., Zeitschrift for Kristallographie 226, (2011)

AC - Susceptibility Experiment

AC Susceptibility and Superconductivity Sample can be considered as a solenoid B in = μ 0 nI = μ 0 M B out = 0 If such a sample is placed in a external magnetic field, H a, then B a = μ 0 H a Thus, B in(total) = μ 0 (H a + M) In case of paramagnetic material, in the absence of any external magnetic field, M = 0. M is directly proportional to the external magnetic field. Thus, M = χH a (χ : AC Susceptibility ) Thus, B in(total) = μ 0 (1+χ) H a In Case of Superconductivity, B in(total) = 0, therefore, χ = -1

Principle of AC Susceptibility Measurement B (Magnetic field by primary coil) : Kept small, on the order of few Oe, so as not to avoid local heating f (Frequency of the field) : Increasing frequency increases not only the signal size but also the noise. n, r (No. of turns, radius of pick up coil) : Signal size is directly proportional to number of turns and radius of coil, but there is a limitation to both of these parameter when we are dealing with high pressure experiment. Vs, Vc (Volume of sample and coil) : Increasing the ratio of Vs/Vc will make the signal stronger

Setting (AC Susceptibility Measurement) Pick up Coil Cu wire (diameter = 16 μm) Diameter of coil = 300 μm Primary Coil Cu wire (diameter = 100 μm) Number of turns = 350

Results

Setting 1 Pressure Medium : Fluorinert (1:1) Sample : Na 0.16 TiNCl and Pb Pressure : 0.1GPa Pb Fluctuations due to change in the heater range!

Setting 2 Pressure Medium : Fluorinert (1:1) Sample : Na 0.16 TiNCl and Pb Pressure : 0.1GPa Pb

Setting 3 ( No pressure medium ) Sample : Na 0.16 TiNCl and Pb Pressure : 0.1GPa Pb

Heating (1V)Heating (0.5V) Pb

Resistivity Measurement ( 4‐probe Method)

Sample 500  m Ruby Gasket Insulating Layer 1GPa 1.5 GPa

Summary Na 0.16 TiNCl shows superconductivity at T c = 18.1 K at ambient pressure. Exact reason for no superconductivity observed when pressure is applied is not known, but possible reasons which can be given are : – Sample is sensitive to air/moisture, so there is a chance that sample was contaminated while installing it between the diamonds. – Maybe on applying pressure, it undergoes some structural change, and Na 0.16 TiNCl does not show superconductivity.

Next Plan Try to load sample inside the cell with even more care, so that is there is absolutely no contamination with air/moisture. Next time see the crystal structure of Na 0.16 TiNCl under pressure and see if there is any change in structure.

CeFe 2

Introduction ・ Lanthanide LaCePrNdSmEuGdTbDyHoTmYb Lu 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 4f 1 5s 2 5p 6 5d 1 6s 2 n(r) r/r B Ce Pm Localized or delocalized Cubic C15 Laves Structure (MgCu 2 ) Strongly hybridized compound between Fe 3d electrons and Ce 4f electrons. Ce Fe

CeFe 2 v/s other REFe 2 compounds CeFe 2 has lower Tc (around 230K) than other REFe 2 compounds(around 600K). The ferromagnetic ordering temperature is higher in the REFe 2 compounds because of strong Fe-Fe interaction. In case of CeFe 2, the Fe3d - Fe3d interactions compete with Fe3d - Ce4f interactions, and thus has lower Tc. Temperature Dependence of the Electrical Resistivity of REFe 2, Gratz et al., Solid State Communication, Vol.69,No. 10 (2011)

Competing Ferro-Antiferromagnetic interaction in CeFe 2 Ferromagnetism in CeFe 2 is unstable because of competition between Fe3d- Fe3d interaction (Ferromagnetic) and Fe3d - Ce4f interaction (Anti- ferromagnetic). Presence of such a delicate balance between Ferromagnetism and Anti-ferromagnetism in CeFe 2 has been proved by two methods : 1.Doping CeFe 2 with Co or Al 2.High Pressure Antiferromagnetic order in pure CeFe 2 under pressure, Braithwaite et al, Phys Rev B 76 (2007)

Previous Research (High Pressure) On applying pressure, distance between Ce and Fe decreases 3d - 4f interactions increases Anti-ferromagnetic becomes prominent Antiferromagnetic order in pure CeFe 2 under pressure, Braithwaite et al, Phys Rev B 76 (2007)

Motivation To know how this system behave under pressure when the 4f-3d hybridization is enhanced in CeFe 2

Experiment Setting 100  m Gasket

My Results Tc Tc ( My Experiment 0.25 GPa

Summary CeFe 2 is a strongly hybridized in which Anti-ferromagnetic state is unstable. Anti-ferromagnetic stability can be achieved by applying pressure or by doping pure CeFe 2 with Co or Al

Next Plan Apply higher pressure and study the effect of increased 3d-4f interactions in CeFe 2