Magnetic Quantum Oscillations in the Organic Antiferromagnetic Superconductor -(BETS 2016. 5....

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Transcript of Magnetic Quantum Oscillations in the Organic Antiferromagnetic Superconductor -(BETS 2016. 5....

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    Walther-Meißner-Institut Technische Universität Physik Department

    für München Lehrstuhl E23 Tieftemperaturforschung

    Bachelorthesis

    Magnetic Quantum Oscillations in the Organic Antiferromagnetic

    Superconductor κ-(BETS)2FeBr4 Florian Michael Kollmannsberger

    Aufgabensteller: Prof. Dr. Rudolf Gross

    Abgabetermin: 25.4.2016

    http://www.wmi.badw.de/ http://www.tum.de/ http://www.ph.tum.de/

  • Contents

    1 Introduction 1

    2 The organic metal κ-(BETS)2FeBr4 3 2.1 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.3 Band structure and electronic properties . . . . . . . . . . . . . . . . . . 5

    3 Theoretical background 9 3.1 Electrodynamics in a magnetic field . . . . . . . . . . . . . . . . . . . . . 9 3.2 Shubnikov-de Haas oscillations . . . . . . . . . . . . . . . . . . . . . . . 10

    3.2.1 Temperature damping RT . . . . . . . . . . . . . . . . . . . . . . 11 3.2.2 Impurity damping RD . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2.3 Spin damping RS . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

    4 Experimental setup 15 4.1 Sample preparation and sample holder . . . . . . . . . . . . . . . . . . . 15 4.2 3He-system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.3 Superconducting magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    5 Results and discussion 17 5.1 Temperature and field dependent resistance . . . . . . . . . . . . . . . . 17

    5.1.1 Superconducting transition . . . . . . . . . . . . . . . . . . . . . 17 5.1.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . 17

    5.2 The antiferromagnetic state: Shubnikov-de Haas oscillations . . . . . . . 18 5.2.1 δ frequency orbit . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.2.2 η frequency orbit . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

    5.3 The normal metallic state . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.3.1 Field induced superconductivity . . . . . . . . . . . . . . . . . . . 23 5.3.2 β Shubnikov-de Haas oscillations . . . . . . . . . . . . . . . . . . 26 5.3.3 α Shubnikov-de Haas oscillations . . . . . . . . . . . . . . . . . . 28

    6 Summary 31

    Bibliography 31

    iii

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  • 1 Introduction

    In 1911 Heike Kamerlingh Onnes [1] discovered superconductivity by measuring the resistance of mercury down to 4.2 K. With time more and more materials showing su- perconductivity were discovered, and in 1958 Bardeen, Cooper and Shrieffer developed the microscopic theory for superconductors (SC). They explained superconductivity by pairs of electrons, which are called Cooper pairs, forming a bosonic state and are held together by an attractive exchange interaction. They showed that phonons mediate this positive exchange interaction in conventional superconductors. Today many un- conventional superconductors like cuprat, heavy fermion and iron pnictides are known, where the pairing mechanism is not yet understood. In 1964 W. A. Little [2] proposed possible superconductivity in organic superconductors, where a long organic aromatic main chain with polarisable side chains forms a superconductor. The oscillation in the side chains could generate an attractive interaction between the electrons in the main molecule, and provide the attractive interaction giving rise to SC pairing at high criti- cal temperatures. However Little’s theory could so far not be realized, but lead to the discovery of organic superconductors. These superconductors known today are quasi 1D or quasi 2D organic charge transfer salts consisting of an inorganic anion and an organic aromatic molecule, for examples see [3]. In this thesis we will focus on κ-(BETS)2FeBr4. κ-(BETS)2FeBr4 is part of the family of (BETS)2FeX4 where X stands for Cl or Br and the greek letter labels the packing modification of the BETS molecules in the salt. The salts of this family are alternating layers of BETS and FeX4, where the π orbitals of the BETS molecules overlap and form conducting bands. The layered structure results in strongly quasi 2D conduction bands. The interest to this family started when superconductivity induced by magnetic field was discovered in λ-(BETS)2FeCl4 [4, 5]. A strong interaction between the π electrons and the 3d spins of the Fe ions in λ-(BETS)2FeCl4 leads to an antiferromagnetic (AFM) type of ordering of the 3d spins and the π electrons at low temperatures [6] and even to an insulating ground state. As this insulating ground state is broken by magnetic field λ-(BETS)2FeCl4 shows an insulator-metal-superconductor transition at low tem- peratures with rising magnetic field. The main focus in this thesis is on the role of π-d interactions on the conducting layers more precisely on the effect of internal magnetic fields on the Shubnikov-de Haas quantum oscillations as well as the impact of the AFM ordering on the Fermi surfaces. To investigate these questions we measured Shubnikov- de Haas oscillations in the antiferromagnetic state as well as in the paramagnetic state of the salt, as the salt transfers to a paramagnetic state when the antiferromagnetic ordering is broken by magnetic field. In the AFM state, a slow oscillation has been reported and which has been interpreted as a result of a superstructure due to the AFM ordering. Since only one low frequency Shubnikov-de Haas oscillation has been detected, more experimental evidence is desirable to support this theory and to further

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  • 1 Introduction

    clarify the structure of the Fermi surface in the AFM state. Another issue addressed in this thesis is related to the effect of AFM ordering on the different spin subbands of the π-electron conduction layers. By theory it was proposed that there is no Zeeman split- ting for the carriers in antiferromagnetic conductors with small carrier pockets [7, 8]. In the paramagnetic state all 3d iron spins are ordered and form an internal field, so we expect Zeeman splitting for the energy levels of the carrier bands in the conducting layers. We investigated the influence of Zeeman splitting on the Shubnikov-de Haas oscillations, and compared it to the behavior predicted by theory.

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  • 2 The organic metal κ-(BETS)2FeBr4

    2.1 Sample preparation

    The single crystals of κ-(BETS)2FeBr4, where BETS stands for the organic molecule Bis(ethylenedithio)tetraselenafulvalene, were prepared electrochemically as described in [9]. Therefore, the sections about the crystal structure and band structure, follows closely [9]. BETS and tetraethylammonium iron(III) tetrabromide were dissolved in a 10% ethanol/chlorobenzene solution and for 2-4 weeks a constant current of 0.7µA was applied between the electrodes at room temperature or at 40◦C. The grown crystals were thin black plates with typical dimensions of 0.7x0.7x0.05 mm3. The κ-(BETS)2FeBr4 samples used in this experiment were provided by H.Fujiwara.

    2.2 Crystal structure

    The κ-(BETS)2FeBr4 crystals consist of conducting and insulating layers arranged al- ternating along the b-axis. The crystal has an orthorombic structure, with the unit cell parameters a = 11.5Å, b = 36.4Å and c = 8.5Å. The unit cell is large along b direction, as it contains two BETS and anion layers. The isolating layers consist of FeBr−4 an- ions that carry a magnetic moment and act as an acceptor, while the BETS molecules, which form the conducting layers, act as donor cations. Due to this transfer we call κ- (BETS)2FeBr4 a charge transfer salt. Fig. 2.1 shows the BETS molecule and the FeBr4 anion as well as atom labeling, and Fig. 2.2 shows the crystal structure. Fig 2.2(a) shows the layered structure of the compound and in Fig 2.2 we see the structure of the BETS layers which are aliened in the typical way of κ-structure, where the BETS molecules are strongly dimerised. Compared to the λ-(BETS)2FeCl4 salt, the interaction between the π-conducting electrons of the BETS layer and the Fe3+ 3d localized spins are weaker in the κ-(BETS)2FeBr4 salt. In the conducting layer short S-S and Se-S contacts provide the inter molecular overlap that forms the metallic bands.

    FeCl4.9b,11 The analyses of the BETS intermolecular distances show that there is only one Se-Se type short contact (

  • 2 The organic metal κ-(BETS)2FeBr4

    interaction betweenπ metal electrons of the donor molecules and localized magnetic moments of the anions has received much attention in creating new types of conducting systems.7,8

    The largest target among these systems may be an antiferro- magnetic or ferromagnetic organic superconductor. We have investigated BETS salts involving transition metal halides such as λ-(BETS)2MCl4 [BETS ) bis(ethylenedithio)tetraselena- fulvalene; M) Ga and Fe] with a superconducting (M) Ga) or insulating (M) Fe) ground state.9 Furthermore, we have recently found an exotic organic conductor,λ-(BETS)2FexGa1-x- BryCl4-y (0 < x < 1, 0< y < 0.5) exhibiting a superconductor- to-insulator or superconductor-to-metal transition. In these systems, the strongπ-d coupling between theπ-metal electron systems of the BETS molecules and the localized Fe3+ magnetic moments of the anions plays an important role in breaking the superconducting state of these systems and causes coopera- tively an antiferromagnetic insulating state.10 Contrary to the λ-type (BETS)2MCl4 system, the other main modifications, κ-(BETS)2MX4 (M ) Ga, Fe, and In; X) Br and Cl), retain their metallic states down to 2 K.9b,11 A crystal structure and an extended Hu¨ckel tight-binding band calc