On The Quest of Superconductivity at Room Temperature

Authors: Christian E. Precker¹, Pablo D Esquinazi¹, Ana Champi², José Barzola-Quiquia¹, Mahsa Zoraghi¹, Santiago Muiños-Landin¹, Annette Setzer¹, Winfried Böhlmann¹, Daniel Spemann^3,6, Jan Meijer³, Tom Muenster⁴, Oliver Baehre⁴, Gert Kloess⁴, Henning Beth⁵

Affiliation:
¹Division of Superconductivity and Magnetism, Institut für Experimentelle Physik II, Universität Leipzig, Germany,
²Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, São Paulo, Brazil,
³Division of Nuclear Solid State Physics, Institut für Experimentelle Physik II, Universität Leipzig, Germany,
⁴Institut für Mineralogie, Kristallographie und Materialwissenschaft, Fakultät für Chemie und Mineralogie, Universität Leipzig, Germany,
⁵Golden Bowerbird Pty Ltd., Mullumbimby, NSW, Australia,
⁶Present address: Leibniz Institute of Surface Modification, Physical Department, Leipzig, Germany.

Superconductivity is the phenomenon in nature where the electrical resistance of a conducting sample vanishes completely below a certain temperature which is known as “critical temperature (T_c)”. For low enough applied magnetic fields upon sample geometry, the phenomenon of flux expulsion (the Meissner effect) is observable, an effect of special importance for the physics of superconductivity. Due to its interesting characteristics, the phenomenon of superconductivity discovered by Kammerlingh Onnes in Leiden in 1911, is one of the most studied phenomena in experimental and theoretical solid state physics. It has important applications, like the generation of high magnetic fields using superconducting solenoids cooled at liquid He (4K) up to liquid nitrogen (77K) temperatures, or the use of extremely sensitive magnetic field sensors via the so-called Josephson effect. The higher the critical temperature, the easier is the use of superconducting devices, especially in microelectronic. The critical temperature of superconducting materials ranges between a few tens of mK to ~200K, this last critical temperature found recently in SxHy under very high pressures [1].

Among experts in low-temperature physics, in particular those with solid backgrounds on superconductivity, there exists a kind of unproven law regarding the (im)possibility to have superconductivity at room temperature, which means having a material with a critical temperature above 300K. In short, for most of the experts it is extremely difficult to accept that a room temperature superconductor would be possible at all, although there is actually no clear theoretical upper limit for T_c. This general (over)skepticism is probably the reason why, for more than 35 years, the work of Kazimierz Antonowicz [2] (on the superconducting-like behavior he observed on annealed graphite/amorphous carbon powders at room temperature [3]) was not taken seriously by the scientific community. Probably, the lack of easy reproducibility of the observed superconducting-like behavior and the vanishing of the amplitude of the signals within a few days [3] (added to the (over)skepticism of scientists) did not encourage them to look more carefully at those results. The work of Antonowicz on the room temperature superconductivity in carbon powders [3] was not cited in reviews discussing the possibility to reach superconductivity at room temperature, see, e.g., [4].

In the last 16 years, however, different measurements done in highly oriented pyrolytic graphite samples and graphite powders, see [5,6] for reviews, suggest that some kind of interfaces in the graphite structure may quite possibly be the origin for some of the measured signals. This may explain several aspects of this hidden superconductivity, like low reproducibility, time instability, small amount of superconducting mass and the difficulty to localize the superconducting phase(s).

Assuming that somewhere in graphite samples the room temperature superconductivity exists, the question arises: which is actually the critical temperature? This was the main question the work of Precker et al. [7] wanted to answer. For that purpose, the authors took natural crystals from Brazil and Sri Lanka mines. A reader would perhaps be surprised that in these days someone selects natural graphite crystals instead of highly pure and ordered pyrolytic graphite, so called HOPG, for research. The main reason to start with ordered natural crystals is that their several microns long interfaces are very well defined, see Fig. 1. The team in [7] also performed measurements with HOPG samples, whose results support those found in natural graphite crystals. Highly ordered natural graphite crystals of good quality were created during the earth's early evolution at temperature and pressure conditions unreachable in laboratories nowadays. Therefore, the well-defined stacking order phases (hexagonal and rhombohedral) and their interfaces shown in Fig.1 may contribute substantially to the metallic-like behavior of graphite [5,6].

Fig.1: (Click on the image to view with higher resolution) Scanning Transmission electron microscopy (STEM) pictures taken from three ~100nm thick lamellae from three different regions of a natural crystal from Brazil. The e-beam points always parallel to the graphene layers. The different colors mean different stacking ordered regions or regions with the same stacking order but rotated a certain angle around the c-axis. The c-axis is always normal to the graphene planes and interfaces. The picture (c) shows that there are regions in the same sample with no or much less interfaces density. The scale bars at the right bottom denotes 1 µm.

Detailed X-ray diffraction studies done in Ref.[7] show that in all samples a mixture of hexagonal (ABAB…, the majority phase) and rhombohedral (ABCABCA…) stacking orders exist in bulk graphite samples, independently of the sample origin. These two phases as well as their twist around the c-axis are the reason for the different colors in the STEM pictures of Fig.1. There are experimental [5,6] as well as theoretical reasons [8] that indicate that the origin for the metallic, and also most probably the superconducting behavior of graphite, is localized at some of those interfaces. One of the reasons why one expects superconductivity at certain interfaces, e.g. between rhombohedral and hexagonal stacking order, is that the relation between energy and wave-vector for conduction electrons becomes dispersionless. In this case and following the common BCS theory of superconductivity, the superconducting critical temperature is proportional to the Cooper pairs interaction strength. Therefore, it is expected that T_c is much higher than in the case of a quadratic dispersion relation [8].

Coming back to the main question, i.e. the critical temperature of the hidden superconductivity in graphite samples with interfaces, two results obtained in Ref.[7] and shown in Fig.2 resume the main evidence suggesting the existence of granular superconductivity below 350K in the measured crystal.

Figure 2(a) shows the temperature dependence of the resistance (a linear in temperature background is subtracted from the original data) around the transition. It is accompanied by the difference between the field cooled and zero field cooled magnetic moment that starts to increase at the lowest temperature onset of the transition in the resistance. Figure 2(b) shows the change in resistance for the same sample at 325K and after cooling it from 390K at zero field. The relatively large response of the resistance with field and the irreversibility are compatible with granular superconductivity; see also other results in [5,6].

Fig.2: (Click on the image to view with higher resolution) (a) The difference (left y-axis, red points) between the measured field cooled magnetic moment m_FC and the zero field cooled m_ZFC vs. temperature at a field of 50 mT applied at 250K for a natural graphite crystal from Brazil. Right y-axis: Difference between the measured resistance and a linear in temperature background vs. temperature -- for a sample from the same batch at zero field. (b) Change of the resistance with field at a temperature of 325K after cooling it from 390K at zero field. The field was applied normal to the interfaces.

The observed remanence in the resistance indicates that magnetic flux remains trapped within certain regions of the graphite samples. The origin and characteristics of this trapped flux and its non-monotonous temperature behavior [7] have to be clarified in the future and using other experimental techniques. One should also clarify to what extent a magnetically ordered state could have some influence on the observed phenomena. The observed phenomenology in Ref.[7] (see Fig.2) as well as in different studies done on graphite in the past [5,6] strongly suggest the existence of superconductivity. Although several details of the phenomenology, especially the large magnetic anisotropy of the effects in resistance, do not support magnetic order as a possible origin, one should not rule out yet the existence of unusual magnetic states at the graphite embedded interfaces, which are partially being studied theoretically nowadays, see, e.g., Ref.[9].

References:
[1] A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov, S. I. Shylin, "Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system", Nature, 525, 73–6. Abstract.
[2] Kazimierz Antonowicz (1914–2003) started in the 60s the carbon research at Nicolas Copernicus University (Torum, Poland) investigating the structural and electronic properties of different forms of carbon.
[3] K. Antonowicz, "Possible superconductivity at room temperature", Nature, 247, 358–60 (1974). Abstract;   "The effect of microwaves on DC current in an Al–carbon–Al sandwich", Physica Status Solidi (a), 28, 497–502 (1975). Abstract.
[4] Arthur W. Sleight, "Room Temperature Superconductors", Accounts of Chemical Research, 28, 103-108 (1995). Abstract.
[5] Pablo Esquinazi, "Invited review: Graphite and its hidden superconductivity", Papers in Physics, 5, 050007 (2013). Abstract.
[6] P. Esquinazi, Y.V. Lysogorsky, "Experimental evidence for the existence of interfaces in graphite and their relation to the observed metallic and superconducting behavior", ed. P Esquinazi (Switzerland: Springer) pp 145-179 (2016), and refs. therein.
[7] Christian E Precker, Pablo D Esquinazi, Ana Champi, José Barzola-Quiquia, Mahsa Zoraghi, Santiago Muiños-Landin, Annette Setzer, Winfried Böhlmann, Daniel Spemann, Jan Meijer, Tom Muenster, Oliver Baehre, Gert Kloess, Henning Beth, "Identification of a possible superconducting transition above room temperature in natural graphite crystals", New Journal of Physics, 18, 113041 (2016). Abstract.
[8] T.T Heikkilä, G.E. Volovik, "Flat bands as a route to high-temperature superconductivity in graphite", ed. P Esquinazi (Switzerland: Springer) pp 123-143 (2016), and refs. therein.
[9] Betül Pamuk, Jacopo Baima, Francesco Mauri, Matteo Calandra, "Magnetic gap opening in rhombohedral stacked multilayer graphene from first principles", arXiv:1610.03445 [cond-mat.mtrl-sci].

Two dimensional Superconducting Quantum Interference Filter (SQIF) arrays using 20,000 YBCO Josephson Junctions

From Left to Right: (top row) Emma Mitchell, Jeina Lazar, Keith Leslie, Chris Lewis,
(bottom row) Alex Grancea, Shane Keenan, Simon Lam, Cathy Foley.

Authors: 
Emma Mitchell¹, Kirsty Hannam¹, Jeina Lazar¹, Keith Leslie¹, Chris Lewis¹, 
Alex Grancea², Shane Keenan¹, Simon Lam¹, Cathy Foley¹

Affiliation: 
¹CSIRO Manufacturing, Lindfield, NSW, Australia,
²CSIRO Data61, Epping, NSW, Australia.

Josephson junctions form the essential magnetic sensing element at the heart of most superconducting electronics. A Josephson junction consists of two superconducting electrodes separated by a thin barrier [1]. Provided the barrier width is less than the superconducting coherence length, Cooper pairs can tunnel quantum mechanically from one electrode to the other coherently when the temperature is below the critical temperature of the two electrodes. Due to the macroscopic quantum coherence of the Cooper pairs in the superconducting state, Josephson junctions not only detect magnetic fields and RF radiation over an extremely wide frequency band, but can also emit radiation. The highly sensitive response of the Josephson junction current to magnetic fields is the key to many of its applications, including magnetometers, absolute magnetic field detectors and low noise amplifiers. More recent applications also include small RF antennas which utilize the Josephson junction’s broadband (dc- THz) detection abilities.

The dc SQUID, or Superconducting Quantum Interference Device, consists of two Josephson junctions connected together in parallel via a superconducting loop. The SQUID is an extremely sensitive flux-to voltage transducer, but despite this simplicity, an exact solution of this problem can only be given in the case of negligible inductance of the loop containing the two junctions. When the SQUID is biased with a current and an external magnetic field is applied, the voltage response oscillates periodically with applied magnetic field (Figure 1a). The period of the oscillation is inversely proportional to the loop area. SQUIDs have been connected together into arrays of increasing size and complexity to improve device sensitivity.

Figure 1: Voltage responses for a (a) dc SQUID with two step-edge junctions (b) two dimensional SQIF with 20,000 step-edge junctions.

Feynman et al. (1966) first predicted [2] an enhancement in the SQUID interference effect by having multiple (identical) junctions in parallel, analogous to a multi-slit diffraction grating. This enhancement was originally observed using superconducting point contact junctions [3] and has been further developed using series arrays of low temperature superconducting (LTS) Nb SQUIDs. More recently 1D arrays of SQUIDs with incommensurate loop areas (non-identical and variable spread) with a non-periodic voltage response were suggested [4]. The voltage response of these superconducting quantum interference filters (SQIFs) is then analogous to a non-conventional optical grating where different periodic responses from individual SQUIDs with different loop areas are summed. This results in a voltage response to a magnetic field in which a dominant anti-peak develops at zero applied field due to constructive interference of the individual SQUID responses. Weaker non-periodic oscillations occur at non-zero fields where the individual SQUID responses destructively interfere. The magnitude and width of the anti-peak for a SQIF is governed by the range and distribution of SQUID loop areas and inductances.

In our recent work [5] we demonstrate high temperature superconducting (HTS) two dimensional SQIF arrays based on 20,000 YBCO step-edge Josephson junctions connected together in series and parallel (Figure 1b). The maximum SQIF response we measured had a peak-to-peak voltage of ~ 1mV and a sensitivity of (1530 V/T) using a SQIF design with twenty sub-arrays connected in series with each sub-array consisting of 50 junctions in parallel connected to 20 such rows in series. The variation in loop areas within each subarray had a pseudo- random distribution with a mean loop area designed to have an inductance factor β_L = 2LI_c/Φ₀ ~1 [6]. Figure 2a shows part of our array with four whole sub-arrays visible. At higher magnification the variation of individual loop areas is evident (Figure 2b) with the rows of step-edge junctions indicated by arrows.

Figure 2: (a) Part of the 20,000 YBCO step-edge junction SQIF array showing four complete sub-arrays of 1,000 junctions each (b) one sub-array at higher magnification showing rows of junctions (arrows) and variable loop areas (darker material is the YBCO).

The Josephson junctions in our samples are step-edge junctions formed when a grain boundary develops between the YBCO electrodes that grow epitaxially when a thin film is deposited over a small step approximately 400nm high with an angle of ~38^o, etched into the supporting MgO substrate [7, 8]. It is well documented that HTS Josephson junctions are difficult to fabricate in large numbers across a substrate. However, step-edge junctions have the advantage of being relatively simple and inexpensive to fabricate and can be placed, at high surface density almost anywhere on a substrate. To date, we have made 2D arrays showing a SQIF response with 20,000 up to 67,000 step-edge junctions on a 1cm² substrate.

Two dimensional SQIF arrays allow for large numbers of junctions to be placed in high density across a chip, enabling increases in the output voltage and sensitivity of the device. 2D arrays also allow for impedance matching of the array to external electronics by varying the ratio of junctions in parallel to those in series, by virtue of the junction normal resistance, R_n.

In addition, we demonstrated that the sensitivity of the SQIF depends strongly on the mean junction critical current, I_c, in the array, and the inductance (area) of the average loop in the array. In both cases keeping these parameters small such that β_L < 1 is necessary for improving the SQIF sensitivity, but can be difficult to achieve with HTS junctions in which the typical spread in I_c can be 30%. The SQIF response also depends on the number of junctions; a linear increase in the SQIF sensitivity with junction number was measured for our SQIF designs.

We were also able to demonstrate RF detection at 30 MHz using our HTS SQIFs at 77 K [5]. More recently a broadband SQIF response from DC to 140 MHz was demonstrated following improvements to our SQIF sensitivity (unpublished). This follows on from reports of near field RF detection to 180 MHz using 1000 low temperature superconducting (LTS) junctions [9], where more complex and expensive cryogenic requirements limit the LTS array applications outside the laboratory.

References:
[1] B.D. Josephson,  "Possible new effects in superconductive tunneling". Physics Letters, 1, 251 (1962). Abstract.
[2] Richard P. Feynman, Robert B. Leighton, Matthew Sands, “The Feynman lectures on Physics, Vol III” (Addison-Wesley, 1966).
[3] J.E. Zimmerman, A.H. Silver, "Macroscopic quantum interference effects through superconducting point contacts", Physical Review, 141, 367 (1966). Abstract.
[4] J. Oppenländer, Ch. Häussler, N. Schopohl, "Non-Φ_o periodic macroscopic quantum interference in one-dimensional parallel Josephson junction arrays with unconventional grating structures", Physical Review B, 63, 024511 (2000). Abstract.
[5] E.E. Mitchell, K.E. Hannam, J. Lazar, K.E. Leslie, C.J. Lewis, A. Grancea, S.T. Keenan, S.K.H. Lam, C.P. Foley, “2D SQIF arrays using 20,000 YBCO high RN Josephson junctions”, Superconductor Science and Technology, 29, 06LT01 (2016). Abstract.
[6] “The SQUID Handbook, Vol. I Fundamentals and technology of SQUIDs and SQUID systems", eds. John Clarke and Alex I. Braginski (Wiley, 2004).
[7] C.P. Foley, E.E. Mitchell, S.K.H. Lam, B. Sankrithyan, Y.M. Wilson, D.L. Tilbrook, S.J. Morris, "Fabrication and characterisation of YBCO single grain boundary step edge junctions", IEEE Transactions on Applied Superconductivity, 9, 4281 (1999). Abstract.
[8] E.E. Mitchell, C.P. Foley, “YBCO step-edge junctions with high IcRn”, Superconductivity Science and Technology, 23, 065007 (2010). Abstract.
[9] G.V. Prokopenko, O.A. Mukhanov, A. Leese de Escobar, B. Taylor, M.C. de Andrade, S. Berggren, P. Longhini, A. Palacios, M. Nisenoff, R. L. Fagaly, “DC and RF measurements of serial bi-SQUID arrays”, IEEE Transactions on Applied Superconductivity, 23, 1400607 (2013). Abstract.

A Magnetic Wormhole

(From left to right) Carles Navau, Alvaro Sanchez, Jordi Prat-Camps

Authors: Jordi Prat-Camps, Carles Navau, Alvaro Sanchez 

Affiliation: Departament de Física, Universitat Autònoma de Barcelona, Spain.

Link to Superconductivity Group UAB >>

Is it possible to build a wormhole in a lab? Taking into account that large amounts of gravitional energy would be required [1], this seems an impossible task. However, redefining a wormhole into a path between two points in space that is completely undetectable, Greenleaf and colleagues [2] suggested in 2007 a (theoretical) way of realizing an electromagnetic wormhole capable of guiding light through an invisible path. They demonstrated that this is topologically equivalent as if the light had been sent through another spatial dimension. However, such a wormhole required metamaterials with extreme properties, which prevented its construction.

In our work, we have constructed an actual 3D wormhole working for magnetostatic fields. It allows the passage of magnetic field between distant regions while the region of propagation remains magnetically invisible. Our wormhole takes advantage of the possibilities that magnetic metamaterials offer for shaping static magnetic fields [3]. These metamaterials can be constructed using existing magnetic materials that can provide extreme magnetic permeability values ranging from zero - superconductors - to effectively infinity - ferromagnets.

Figure 1: (Left) 3D sketch of the magnetic wormhole, showing how the magnetic field lines (in red) of a small magnet at the right are transferred through it. (Right) From a magnetic point of view the wormhole is magnetically undetectable so that the field of the magnet seems to disappear at the right and reappear at the left in the form of a magnetic monopole. (Image credit: Jordi Prat-Camps and Universitat Autònoma de Barcelona).

The magnetic wormhole requires three properties: (i) to magnetically decouple a given volume from the surrounding 3D space, (ii) to have the whole object magnetically undetectable, and (iii) to have magnetic fields propagating through its interior. The first two properties are achieved by constructing a 3D magnetic cloak. Based on previous ideas [4] such a cloak could be made by surrounding a superconducting sphere with a specially created ferromagnetic (meta)surface, such that the magnetic signature of the superconductor was cancelled by the ferromagnet. For the third property, we use magnetic hoses, also made by magnetic metamaterials, as developed in [5].

The parts composing the magnetic wormhole are shown in fig. 2: a central magnetic hose to guide the magnetic field from one end of the hose to the opposite one, and a magnetic cloak composed of a superconducting-ferromagnetic bilayer to make the hose magnetically invisible.

Figure 2: (a) 3D image of the magnetic wormhole, formed by concentric shells: from outside inwards, an external metasurface made of ferromagnetic pieces (b), an internal superconducting shell made of coated conductor pieces (c), and a magnetic hose made of ferromagnetic foil (d). (e) Cross-section view of the wormhole, including the plastic formers (in green and red) used to hold the different parts. (Image credit: Jordi Prat-Camps and Universitat Autònoma de Barcelona).

Experimental results clearly demonstrate [6] the two desired properties for the wormhole: (i) magnetic field from a source at one end of the wormhole appear at the opposite end (actually as a kind of isolated magnetic monopole), (ii) the overall device is magnetically undetectable (it does not noticeably distort an applied magnetic field, even a non-uniform one).

Besides the scientific interest per se in the realization of an object with properties of a wormhole, our device may have applications in practical situations where magnetic fields have to be transferred without distorting a given field distribution, as in magnetic resonance imaging.

Acknowledgements: We thank Spanish project MAT2012-35370 and Catalan 2014-SGR-150 for financial support. A.S. acknowledges a grant from ICREA Academia, funded by the Generalitat de Catalunya. J. P.-C. acknowledges a FPU grant form Spanish Government (AP2010-2556).

References:
[1] Michael S. Morris, Kip S. Thorne, Ulvi Yurtsever, "Wormholes, Time Machines, and the Weak Energy Condition", Physical Review Letters, 61, 1446 (1998). Abstract.
[2] Allan Greenleaf, Yaroslav Kurylev, Matti Lassas, Gunther Uhlmann, "Electromagnetic Wormholes and Virtual Magnetic Monopoles from Metamaterials", Physical Review Letters, 99, 183901 (2007). Abstract.
[3] Steven M. Anlage, "Magnetic Hose Keeps Fields from Spreading", Physics, 7, 67 (2014). Full Article.
[4] Fedor Gömöry, Mykola Solovyov, Ján Šouc, Carles Navau, Jordi Prat-Camps, Alvaro Sanchez, "Experimental realization of a magnetic cloak", Science, 335, 1466 (2012). Abstract.
[5] C. Navau, J. Prat-Camps, O. Romero-Isart, J. I. Cirac, A. Sanchez. "Long-Distance Transfer and Routing of Static Magnetic Fields", Physical Review Letters, 112, 253901 (2014). Abstract.
[6] Jordi Prat-Camps, Carles Navau, Alvaro Sanchez. "A Magnetic Wormhole", Scientific Reports 5, 12488 (2015). Full Article.

Tuning Superconductivity in a Molecular System: The Key Role of the Jahn-Teller Metallic State

Authors of the paper in Science Advances (reference [1]). Left to Right: (top row) R. H. Zadik, Y. Takabayashi, G. Klupp, R. H. Colman, A. Y. Ganin, A. Potočnik, (middle row) P. Jeglič, D. Arčon, P. Matus, K. Kamarás, Y. Kasahara, Y. Iwasa, (bottom row) A. N. Fitch, Y. Ohishi, G. Garbarino, K. Kato, M. J. Rosseinsky, K. Prassides.

Authors: Gyöngyi Klupp and Ruth H. Zadik

Affiliation: Department of Chemistry, Durham University, UK

In our recent work, published in the journal Science Advances [1], we have addressed the relationship between the parent insulating phase, the normal metallic state and the superconducting pairing mechanism, a key challenge for all unconventional superconductors, in a series of chemically-pressurized fulleride superconductors. This work has revealed a new state of matter straddling the Mott insulating and Fermi liquid states at the two extremes of the phase diagram: the Jahn-Teller metal, where localized electrons on the fullerene molecules coexist with metallicity.

Figure 1: Crystal structure of face-centred-cubic A₃C₆₀ (A = alkali metal) with C₆₀^3- anions in grey, and the alkali metal cations in colour; reproduced from ref. [1].

Alkali metal intercalated fullerene compounds with the stoichiometry A₃C₆₀ (Fig. 1) are superconducting, with the highest superconducting T_c found in a molecular material being 38 K in pressurised Cs₃C₆₀ [2]. This material is an unconventional superconductor; however, its Rb analogue, Rb₃C₆₀ exhibits conventional superconductivity. The question arises how the two types of behaviours are related both in the normal and in the superconducting state. The nature of the normal state from which the highest T_c emerges is also of key importance.

In order to address these questions, mixed bulk superconducting salts of C₆₀ were prepared with compositions Rb_xCs_3-xC₆₀ (0 < x < 3) [1]. In the isostructural face-centred-cubic- structured phases of this compositional series, tuning the ratio of cations with different diameters controls the distance between C₆₀^3- anions, without the need to apply external pressure, thus permitting a wide range of measurements to be employed. Variable temperature high-resolution synchrotron x-ray powder diffraction, SQUID (superconducting quantum interference device) magnetometry, nuclear magnetic resonance (NMR), infrared (IR) spectroscopy and specific heat measurements were undertaken. When the intermolecular separation is large, like in Cs₃C₆₀ at ambient conditions, the electrons of the C₆₀³⁻ anions cannot hop from one site to the other, and the material is a Mott insulator (Fig. 2). Electrons localised on the C₆₀^3- anions couple to intramolecular vibrations leading to the distortion of the molecule [3]. The Jahn-Teller distortion removes electronic as well as vibrational degeneracies. The latter results in splitting of vibrational lines in the IR spectrum, which provides an excellent way of detecting the Jahn-Teller effect. Thus the parent insulating state of A3C60 superconductors is a Mott-Jahn-Teller insulator, as has previously been demonstrated through IR spectroscopy for the most expanded member Cs₃C₆₀ [3].

Figure 2: The different electronic phases encountered in Rb_xCs_3-xC₆₀ fullerides ranging from the conventional metallic (green) through the Jahn-Teller metallic (orange) to the Mott-Jahn-Teller insulating regime (cyan). The top two rows show schematics of the molecular geometry together with the molecular electronic structure determined by the Jahn-Teller effect and the most characteristic region of the infrared spectrum. The middle panel is the electronic phase diagram; symbols represent the insulator-to-metal transitions and the superconducting T_cs as a function of the volume occupied by a C₆₀^3- anion, which is increasing as the intermolecular distance increases. The lower panel shows the variation in the superconducting gap normalised by T_c. Figure reproduced from ref. [1].

Decreasing the intermolecular distance from this state allows the hopping of the electrons, yielding metallicity. Thus an insulator-to-metal transition, or crossover, is observed on increasing the proportion of the smaller Rb⁺ ion in the material (Fig. 2). Signatures of this transition from the experimental techniques deployed include anomalous shrinkage of the unit cell size, cusps (or maxima) in the magnetic susceptibility and temperature-normalised spin-lattice relaxation rates as a function of temperature, and a step-wise decrease in IR spectral background transmittance.

However, the metallic state encountered close to the metal-insulator boundary is not conventional; the electrons are not forming the conventional bands of a Fermi liquid. Electron correlation results in some persisting localised features in the electron system, like the continued presence of the Jahn-Teller effect as evidenced by IR spectroscopy. The coexistence of the molecular Jahn-Teller effect with metallicity is reflected in the designation of the newly observed phase as a Jahn-Teller metal. The further decrease of intermolecular distances leads to the gradual disappearance of the localised features, like e.g. the Jahn-Teller effect, until a conventional metal is encountered in Rb₂CsC₆₀ and Rb₃C₆₀ (Fig. 2). When the electrons are delocalised over the whole crystal to provide a conventional band, they cannot induce Jahn-Teller distortion any more [4].

A similar crossover between the unconventional and the conventional behaviour is also present in the superconducting state. The superconducting gap probed by NMR spectroscopy at large intermolecular separations is much larger than that of conventional BCS (Bardeen, Cooper and Schrieffer)-type weakly-coupled superconductors (Fig. 2). As the localised character of the electronic structure fades away gradually with decreasing intermolecular distances, the size of the gap returns to the value characteristic for conventional superconductors. Decreasing the intermolecular separation in the unconventional region leads to a rise in T_c , while it leads to the long-known decrease in the conventional region. Thus the highest T_c emerges where the molecular and extended properties of the electronic structure are balanced.

The observed behaviour shows how the superconducting T_c can be tuned in fullerides, paving the way for the preparation of other molecular superconductors with enhanced T_c. Establishing the whole phase diagram of face-centered-cubic A₃C₆₀ superconductors and tracking the transition between conventional and unconventional states can provide important clues for the understanding of high-T_c superconductivity in other materials, as well. Our results are also expected to stimulate the development of improved theoretical descriptions of the A₃C₆₀ system [5], further advancing our understanding of the origins and mechanism of superconductivity in other strongly-correlated high-T_c superconductors.

References:
[1] Ruth H. Zadik, Yasuhiro Takabayashi, Gyöngyi Klupp, Ross H. Colman, Alexey Y. Ganin, Anton Potočnik, Peter Jeglič, Denis Arčon, Péter Matus, Katalin Kamarás, Yuichi Kasahara, Yoshihiro Iwasa, Andrew N. Fitch, Yasuo Ohishi, Gaston Garbarino, Kenichi Kato, Matthew J. Rosseinsky, Kosmas Prassides, “Optimized unconventional superconductivity in a molecular Jahn-Teller metal”. Science Advances, 1, e1500059 (2015). Abstract.
[2] Alexey Y. Ganin, Yasuhiro Takabayashi, Yaroslav Z. Khimyak, Serena Margadonna, Anna Tamai, Matthew J. Rosseinsky, Kosmas Prassides, “Bulk superconductivity at 38 K in a molecular system”. Nature Materials, 7, 367 (2008). Abstract.
[3] Gyöngyi Klupp, Péter Matus, Katalin Kamarás, Alexey Y. Ganin, Alec McLennan, Matthew J. Rosseinsky, Yasuhiro Takabayashi, Martin T. McDonald, Kosmas Prassides, “Dynamic Jahn-Teller effect in the parent insulating state of the molecular superconductor Cs₃C₆₀”. Nature Communications, 3, 912 (2012). Abstract.
[4] A. Wachowiak, R. Yamachika, K. H. Khoo, Y. Wang, M. Grobis, D.-H. Lee, S. G. Louie, M. F. Crommie, “Visualization of the molecular Jahn-Teller effect in an insulating K₄C₆₀ monolayer”, Science, 310, 468 (2005). Abstract.
[5] Yusuke Nomura, Shiro Sakai, Massimo Capone, Ryotaro Arita, “Unified understanding of superconductivity and Mott transition in alkali-doped fullerides from first principles”, arXiv:1505.05849v1 [cond-mat.supr-con] (2015).