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Peter Dowben Publications Research Papers in and Astronomy

2013 Grand challenges in condensed physics: from knowledge to innovation Evgeny Y. Tsymbal University of Nebraska-Lincoln, [email protected]

Peter A. Dowben University of Nebraska-Lincoln, [email protected]

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Tsymbal, Evgeny Y. and Dowben, Peter A., "Grand challenges in : from knowledge to innovation" (2013). Peter Dowben Publications. 259. http://digitalcommons.unl.edu/physicsdowben/259

This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Peter Dowben Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. SPECIALTY GRAND CHALLENGE ARTICLE published: 27 December 2013 PHYSICS doi: 10.3389/fphy.2013.00032 Grand challenges in condensed matter physics: from knowledge to innovation

Evgeny Y. Tsymbal* and Peter A. Dowben

Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE, USA *Correspondence: [email protected]

Edited by: Alex Hansen, Norwegian University of Science and Technology, Norway

Keywords: condensed matter physics, grand challenges, strongly correlated systems, topological insulators, skyrmions, nanotechnology, spintronics

Condensed Matter Physics (CMP) while also mentioning some emerging High- explores the fundamental properties of functional properties of materials where leads directly to one of the immense matter and their origins resulting from the the associated potential applications could problems in CMP—understanding the interactions of a large number of foster the technological Innovation. properties of strongly correlated elec- and . The intricate of these tronic systems. The key feature of the interactions results in properties and asso- FROM KNOWLEDGE strongly correlated systems is that their ciated phenomena that often hint at a rich In condensed matter, striking phenom- electronic behavior cannot be described vein of underlying physics. Although the ena emerge from interactions between adequately in terms of the non-interacting perspective is changing constantly with the constituent particles and the inter- particles picture. For example, due to new discoveries, the basic challenges in play between coupled degrees of free- poor screening, the interaction CMP are to predict and observe new phe- dom. The quantum-mechanical nature between valence electrons in doped com- nomena and elucidate novel properties of of these interactions makes condensed plex oxide materials often overcomes their materials often pushing at the frontiers of matter phenomena non-trivial and often , resulting in a strongly cou- quantum mechanics [1]. counterintuitive. Superconductivity is one pled many-body . Similarly, CMP is also a field which stimulates of the extraordinary examples of such a actinides and lanthanides are charac- technological innovation that revolution- behavior. terized by the localized f levels, which izes modern society. For more than five Superconductivity is the property of a are often hybridized with s, p,andd decades, the engine of CMP has largely material to carry an electrical current with states, leading to the strong on-site and been driven by industry. no dissipation of energy. First discovered inter-site Coulomb interactions where a Probably the most notable example is the by Onnes in 1911, superconductivity had single-particle wave function is a poor invention of the transistor which was rec- no explanation for nearly half a century. approximation. Properties of the strongly ognized by the 1956 Nobel Prize in Physics Only in 1957 did Bardeen, Cooper, and correlated systems are controlled by given to William Shockley, , Schrieffer (BCS) elucidate superconduc- the competition between different elec- and Walter Brattain. The transistor—a tivity as an effect caused by tronic phases, often characterized by basic building block of modern elec- of Cooper pairs into a boson-type state various types of charge and order- tronic devices—was a result of innovative [4]. The BCS theory provided a consis- ing and involving different length and research in the field of . tent understanding of this phenomenon energy scales. This competition leads to The transistor and the invention of the in metals where the superconducting intrinsic inhomogeneities (e.g., sep- integrated circuit in 1958 was the starting transition occurs at cryogenic tempera- aration) in these materials and intricate point for exponential increase in the com- tures. However, 20 years later Karl Müller phase diagrams. As a result of the strong putational power known as Moore’s law and Johannes Bednorz discovered cuprate -electron correlations, these mate- [2]. There is a persistent interplay between superconductors which had much higher rials are extremely sensitive to external the fundamental science and technologi- transition [5]. Properties of perturbations and display a variety of cal applications which provides breadth to these high-temperature superconductors interesting properties, such as high- CMP [3]. did not match the BCS theory based on temperature superconductivity, colossal One cannot possibly give full justice electron pairing due to electron- magnetoresistance, metal- tran- to the entire range of CMP problems interaction. So far, no generally accepted sitions, etc. Condensed matter systems that now command the attention of the theory of high-temperature superconduc- are intrinsically many body, and while the condensed matter and materials physics tivity exists, while the recent discovery quantum mechanical single particle pic- community. Therefore, rather than even of superconductivity in pnictides ture (plus a whole host of perturbations try, in this short essay we point out may also indicate a non-conventional and corrections) works surprisingly well a few fundamental problems of major mechanism. Microscopic understanding in explaining much of the phenomena importance whose would further of superconductivity in these compounds observed, we have to admit that the solu- expand our understanding and Knowledge, remains a challenge for CMP. tions constructed are often ad hoc,and

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only vaguely address the many body elec- thermal or mechanical means. The soft and has been expanded now to other struc- tronic structure. Reformulating -state matter involves a variety of organic mate- turally similar two-dimensional materials, theory to adequately describe strongly cor- rials such as , , and such as hexagonal boron nitride, MoS2, related systems is another grand challenge . An important common feature and WSe2, opening a vista of reduced in CMP. of these materials is that the most inter- dimensional systems with a range of spin- The emergence of non-trivial cooper- esting properties of emerge orbit coupling effects from barely none at ative phenomena in CMP is often driven directly from its atomic or molecular con- all to quite significant. by the interplay between well-known con- stituents. The complexity and diversity of Thereareanumberofsystemswhere stituents; yet the collective behavior may physical behaviors of soft matter is due to key properties (conductivity, spin-orbit be strikingly dissimilar and often unex- the fact that the macroscopic properties of coupling, spin current) are protected by pected. This is the case for the new these materials is determined by interac- topology. Among them are the topolog- quantum states of matter recently discov- tions at the mesoscopic scale which, on one ical insulators—electronic materials that ered. For example, the fractional quantum hand, involves a large number of atoms have a bulk band gap like ordinary insu- Hall state represents a peculiar electronic and , but, on the other hand, is lators, but exhibit conducting states on liquid, where an added electron breaks much smaller than the macroscopic scale. theiredgeorsurface[10]. Unlike band up into new particles, each carrying an Although soft materials emerge in dif- insulators that can also support conduc- exact fraction of the electron charge [6, 7]. ferent forms, many of their physical and tive surface states [11, 12], the surface Perpendicular-to-the-plane magnetic field chemical properties have common ori- states of topological insulators are special applied to a two-dimensional electron sys- gins, such as multiple degrees of freedom, due to being symmetry protected by time tem breaks up the otherwise continu- weak interactions between structural con- reversal symmetry. The two-dimensional ous distribution of electron into stituents, and a subtle balance between can be considered as discrete states known as Landau lev- contributions to the free energy from a quantum spin Hall state, where helical els. At a sufficiently high magnetic field, and . All these materi- edge states interconnect spin and momen- all electrons lie on the lowest Landau als are sensitive to external conditions and tum of the carriers and lead to quan- level precisely at the same kinetic energy. characterized by sizable thermal fluctua- tized conductance of charge and spin Coulomb interaction between electrons tions, emergence of metastable states, and [13]. Indeed, the quantum spin dominates their behavior, representing a a wide variety of forms. in HgTe/(Hg,Cd)Te quantum wells may strongly interacting quantum system: a Several significant efforts in CMP are point to fundamentally new quantum state new strongly-correlated state of matter. related to the exploration of materi- of matter [14]. Other topological objects Critical phenomena and phase tran- als properties at the nanometer scale. in condensed matter (though extensively sitions are an important constituent of Materials properties at distances of 100 discussed outside condensed matter for modern CMP. refers to nanometers or less deviate significantly decades) are skyrmions [15]—a twisted the transformation of thermodynamic sys- from the bulk properties leading to new vertex configuration of a two-dimensional tem from one phase or state of matter phenomena and functional characteristics. ferromagnet. Skyrmions may form spon- to another. Systems undergoing a phase As dimensions shrink to the nanoscale, taneously in magnetic materials though transition exhibit critical behavior, where surface and interface properties domi- a Dzyaloshinskii-Moriya interaction with- several properties such as specific and nate the material behavior: according to out the assistance of external fields or susceptibility diverge. Some transitions, Kroemer (a 2000 Nobel Prize Laureate) the proliferation of defects [16]. Magnetic such as the paramagnetic-ferromagnetic “The interface is the device.” The old skyrmions are typically as small as a 1 nm phase transition, occur “continuously” so view: “God made the bulk; the surface in size and arrange in two-dimensional that close to the critical temperature it was invented by the devil,” articulated by lattices [17]. They can be created and becomes impossible to assign the state to Wolfgang Pauli (the 1945 physics Nobel erased in a controlled way using local either of the two phases involved, and Prize Laureate), must be somehow over- spin-polarized currents from a scanning the system exhibits a new critical behav- come in the culture of CMP. tunneling microscope [18], which opens ior. Continuous phase transitions are An important class of nanoscale the potential for their use in information- described by the Ginzburg-Landau theory objects is based on an intrinsically two- storage concepts. [8], which exploits the so-called mean field dimensional material—graphene—a sheet approximation. However, a number of of carbon atoms bonded in a honeycomb TO INNOVATION phase transitions, such as metal-insulator pattern [9]. Groundbreaking experiments Discoveries of novel phenomena and transitions, do not follow the Ginzburg- on graphene were acknowledged by the materials properties driven by fundamen- Landau approach. Phase transitions, espe- 2010 Nobel Prize in Physics given to Geim tal studies in CMP often are realized in cially in strongly correlated systems, is an and Novoselov. The growth of single crys- industrial innovations. We have already active area of research. talline graphene with the controllable mentioned the discovery of the transistor Thecomplexityofphysicalphenom- number of layers still remains a challenge, which has revolutionized the semiconduc- ena is also evident in soft matter—a sub- although a significant progress has been tor industry. Another prominent example field of condensed matter which involves achieved in the recent years. The intensity (though not of the same scope as semi- physical states that are easily deformed by of research on graphene is now enormous, conductor technology) is spintronics where

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both electron charge and spin contribute The emergent phenomena in CMP can be used for conversion between ther- to the functional properties of electronic will likely be decisive in extending or mal and electrical energy. and data storage devices [19]. altering the shape of the International Obviously the above examples are not Thespinnatureofelectrontransport Technology Roadmap for Semiconductors, exhaustive but provide a flavor of the lies in the heart of modern magnetic data which could help in maintaining the breadth of the CMP research. The future storage technologies, such as computer Moore’s law [2]. Reducing CMOS tran- looks bright for the development of new hard drives and magnetic random access sistor dimensions to cater for higher materials to obtain more and greater memories (MRAMs), and illustrates the speed and higher packing density leads functionality and enhanced performance. progress from Mott’s efforts in 1936 to to the increasing source-drain leakage and Concerning the fundamental challenges, understand the importance of the elec- hence to the escalating power dissipation. such as an adequate description of strongly tron spin in electrical conductivities of the Overcoming this problem requires novel correlated electronic systems, there is the transition metals and their alloys [20, 30]. materials and approaches for device opera- glimmer of new knowledge that now Current spintronics technology employs a tion. Various schemes have been proposed beckon us toward a much better under- magnetoresistive device, a metallic spin- involving voltage control of magnetiza- standing of not only CMP itself, but valve or a magnetic tunnel junction, that tion, graphene field effect transistors, also other disciplines, such as quantum consists of two ferromagnetic electrodes non-local spin valves, ferroelectric tun- mechanics generally. Moreover, there are separated by a thin spacer layer of non- nel junctions, and others. Special empha- analogies between phenomena occurring magnetic material. The flow of carriers sis is made on non-volatile operation of in condensed matter, such as formation of through the device is determined by the these devices which puts forward again an topological defects or symmetry-breaking direction of their spin relative to the mag- emphasis on and ferro- phase transitions, and processes intrin- netization of the device’s electrodes result- electricity, long standing important sub- sic to early [29]. This ing in giant magnetoresistance (GMR) fields of CMP. The challenges are vast: the relevance of CMP to other disciplines [21, 22] if the spacer is metallic and in tun- stated goals are non-volatile memory and emphasizes the stature of the field and the neling magnetoresistance (TMR) [23, 24] logic with switching energy less that 10−2 significance of its grand challenges. if the spacer is insulating. The discov- fJ,switchingspeedsof1GHzorbetterand ery of GMR was recognized by the 2007 stability over 1016 switches. ACKNOWLEDGMENTS Nobel Prize in Physics given to Fert and CMP plays an important role in meet- The authors acknowledge the NSF funded Grünberg, for the impact of GMR on the ing energy demands of the modern society. Nebraska Materials Research Science and technology that is used to read data on Affordable energy, mainly from fossil fuels, Engineering Center (Grant No. DMR- hard disks. has enabled technological and human 0820521) for supporting fruitful research Spintronics has evolved into a vigor- development. However, the deepening and collaboration of the authors during ous field of research involving, e.g., the energy crisis requires a coordinated effort the last 11 years and the NRI funded search for magnetic dilute semiconduc- to harness alternative resources of renew- Center for NanoFerroic Devices for stim- tors with the above the able energy to create more energy-efficient ulating engagement of the authors in the room temperature, spin injection in met- and environmentally friendly technolo- technologically challenging research. The als and semiconductors, spin-dependent gies. The exploration of new materials is a authors are grateful to Verona Skomski thermal effects, and current driven spin- necessary condition to effectively address for her assistance in preparation of the transfer torques. 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