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2 Science Drivers - Condensed Matter and Materials Physics
Pages 20-66

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From page 20...
... Although applied magnetic fields in many cases have a relatively small effect on the overall electronic structure, they enable experimental techniques that can reveal properties of the underlying electronic structure that would be otherwise inaccessible. In other cases, magnetic field effects can be strong enough to drastically change the nature of the electronic state 20
From page 21...
... Magnetic fields couple most strongly to the electrons in a material, either by acting on electrical currents generated by the electrons' quantum mechanical motion, or by coupling to the magnetic moments arising from the electrons' intrin sic spin. These coupling mechanisms are indeed the basis for the majority of experi ments using high magnetic fields to study the electronic structure of materials.
From page 22...
... Applications of high magnetic fields in these experiments make use of the torque exerted by a magnetic field on an object with an anisotropic diamagnetic susceptibility or the force exerted on any object by a strong gradient in field strength. In the remainder of this chapter, a number of examples are discussed where high magnetic fields play a critical role in condensed matter research.
From page 23...
... . Quantum criticality, when ordering is prohibited at any nonzero temperature and occurs only at zero temperature, is increasingly believed to be a central fea ture of the phase behaviors of virtually every class of correlated electron system from the f-electron-based heavy fermions (Gegenwart, 2008; Von Lohneysen and
From page 24...
... When magnetic fields suppress magnetic order in heavy fermions, or superconductivity in YBCO (Sebastian et al., 2010) to expose bare QCPs, the electrical resistivity ρ becomes linear in temperature, although the quadratic temperature dependence of a normal metal is regained when the system is tuned sufficiently far from the QCP (Custers et al., 2003)
From page 25...
... . High magnetic fields have been instrumental for delineating the full phase behaviors for these systems, most notably in URu2Si2 (Figure 2.1)
From page 26...
... The explo ration of the phase behaviors of these new compounds is crucial, and there is an underlying expectation that there is an overarching phase diagram, with individual compounds representing various regimes of this master phase diagram. Magnetic fields, especially if they can be combined with other variables like pressure, are important not only for tuning the strength of order but also as a thermodynami cally relevant scaling variable.
From page 27...
... Making an analogy with a black hole, the passage from noncritical to critical quantum matter involves crossing a "material event horizon." Experiments that tune a material from the normal metal past a QCP force electrons through the "horizon" in the phase diagram into the interior of the quantum critical matter, from which they ultimately reemerge through a second horizon on the other side into a new universe of magnetically ordered matter. SOURCE: Reprinted by permission from Macmillan Publishers Ltd.: Nature.
From page 28...
... Implementation of FIB processing may mean that much higher pulsed fields could be available for user science soon, with the promise of expansion to even higher fields as improved magnet conductors become available. FIGURE 2.3  Four-probe resistance bars for simultaneous c-axis and ab-plane resistivity measure ments carved out of a SmFeAsO0.7F0.25 single crystal using the FIB.
From page 29...
... S c i e n c e D r i v e r s -- C o n d e n s e d M at t e r and M at e r i a l s P h ys i c s 29
From page 30...
... Applied magnetic fields offer a way to tune these effects, either by stabilizing them (as for ferromagnetic fluctuations) or destabilizing them (as for antiferromagnetic fluctuations)
From page 31...
... . They showed that for 1D spin chains with strong quantum fluctuations, the magnetization is topologically quantized as a function of magnetic field, a phenomenon that bears similarity to the quantum Hall effect observed in 2D electron metals in a magnetic field.
From page 32...
... 32 High Magnetic Field Science and I t s A pp l i c a t i o n in the US FIGURE 2.4  (Top) A pair of solitons of opposite sign in a 1D chain, where the pendula represents spins.
From page 33...
... 2(OH) 2, Physical Review Letters 94:227201, Figure 1(b)
From page 34...
... Another example of the importance of high magnetic fields for quasi-1D mag nets is the spin chain system CoNb2O6. In this compound, the cobalt spins are of the Ising type, meaning that the spin direction is constrained by local atomic forces to point along a single direction, the Ising axis.
From page 35...
... Smeibidl, and K Kiefer, 2010, Quantum criticality in an Ising chain: Experimental evidence for emergent E8 symmetry, Science 327:177-180.
From page 36...
... It is conceivable that ordi nary magnetism in organics is a common mask for a superconducting state that is revealed only in very high magnetic fields. Superconductors uncovered in this manner would provide intriguing examples motivating possible synthetic routes for new superconductors in low fields.
From page 37...
... Despite possessing Tcs that are much lower than the mean field predic tion, frustrated magnets are affected by magnetic fields on the scale of the spin spin interactions, reflecting the local nature of the frustration phenomenon. For example, even though ZnCr2O­4 orders at 12 K, suggesting that fields of order 10 tesla would be able to modulate Tc, such fields have almost no effect.
From page 38...
... Harris, 1998, Frustration in Ising-type spin models on the pyrochlore lattice, Journal of Physics: Condensed Matter 10:L215-L220. © IOP Publishing.
From page 39...
... Kojima et al., 2010, Magnetic orders of highly frustrated spinel, ZnCr 2O4 in magnetic fields up to 400 T, Journal of Low Temperature Physics 159:3-6, Figure 1. Quantum Matter Probed by High Magnetic Fields In the example described earlier of high-field studies of the quasi 1D Ising system CoNb2O6, the effect of the magnetic field was to offset the effect of the mean ordering field.
From page 40...
... Zhitomirsky and Tsunetsugu predicted this structure to exhibit a quantum spin nematic phase at high fields by a magnetic analogy to the orientational ordering of needlelike molecules in liquid crystals. Figure 2.11 shows the magnetization of LiCuVO4 using a pulsed-field technique.
From page 41...
... S c i e n c e D r i v e r s -- C o n d e n s e d M at t e r and M at e r i a l s P h ys i c s 41 FIGURE 2.11  (Upper panel) dM/dH of LiCuVO4 measured using a pulsed-field technique.
From page 42...
... The ability to create magnetic fields over a wide range of energies allows us to continu ously challenge and expand our understanding of matter. Superconductors in High Magnetic Fields: An Expanding Frontier Early in the history of superconductivity (see Box 2.1)
From page 43...
... Consequently, the ability to perform measurements in very high magnetic fields is an essential component of frontier research in HTS. In effect, the "mixed" state between Hc1 and Hc2, where superconductivity coexists with magnetic field, became the dominant region in the phase diagram, while the Meissner state is found in only a tiny sliver at very low fields, below Hc1.
From page 44...
... and Hc2(T) , relying on magnetic fields as high as tens or even hundreds of tesla, has been one of the richest and intellectually most rewarding areas of research in physics during the past 10 to 15 years and is bound to remain so in the coming decade.
From page 45...
... Very importantly, as one looks for deviations from the mean-field BCS descrip tion, these low-energy quasiparticles and their interactions with superconduct ing vortices, through thermal and quantum fluctuations, become a fundamental problem in reconstructing the phase diagram of HTS. Various elements of this problem are the subject of intense ongoing research, much of which involves very high magnetic fields (some recent examples are in Riggs et al., 2011; Sebastian et al., 2008; Wu et al., 2011)
From page 46...
... High fields allow access to the low-temperature regime of cuprates and pnictides, where quantum fluctuations away from the BCS theory dominate. Thus, entirely new quantum states of matter, not just uncovered by but induced by high magnetic fields, become a realistic possibility.
From page 47...
... While these materials nominally are not HTS, they are extreme type II systems, and their special anisotropic properties allow one to use high magnetic fields to enter into regimes that relate to the most interesting issues in cuprates and pnictides. In particular, the competition of superconductivity with spin and charge density-waves, quantum oscillations in the superconducting state indicating gapless excitations, along with other phenomena was first observed in these materials.
From page 48...
... • The problem of unconventional superconductivity is perhaps the most remarkable but not the only example of phase transitions in correlated metals. Ultrahigh magnetic fields will aid deeper understanding of these phase transitions, provided it becomes possible to apply spectroscopic
From page 49...
... Semiconductors and Semimetals Magnetic fields can have a particularly large effect in materials with a low density of charge carriers. In these systems, Planck's constant times the cyclotron frequency induced by a strong magnetic field can become comparable to the Fermi energies arising from quantum mechanical motion of the carriers, or to the energy scale of Coulomb interactions between the carriers.
From page 50...
... ,1 a collec tion of peculiar phenomena that occur in 2D electron systems, at low temperatures, in strong magnetic fields. Under these conditions, electrons' orbitals coalesce into Landau levels (LLs)
From page 51...
... However, higher magnetic fields become necessary if one wants to explore QHE in materials with lower mobility. One specialized facility that has played a significant role in exploring aspects of the QHE is the NHMFL high B/T facility at the University of Florida.
From page 52...
... Thus, compared to traditional 2DEG, higher magnetic fields were required to measure the Landau level gaps, break the fourfold degeneracy of the lowest Landau levels, and demonstrate the quantum Hall ferromagnetic state in graphene. Interestingly, the energetic separation between graphene's lowest Landau levels is unusually large, enabling FIGURE 2.13  Atomic lattice and a portion of the electronic band structure of graphene.
From page 53...
... The knowledge and insights gained from the early quantum Hall measure ments on graphene have been invaluable in elucidating the peculiar properties of that material, and high field studies of graphene continue to be indispensible today. The current outstanding questions include the nature of the QH states at f = 0, or zero charge density, which have diverging Hall and longitudinal resistance in both single-layer and bilayer graphene; quantum phase transitions among the symmetry-broken QH states; topologically nontrivial phases; presence of sky rmions (spin textures)
From page 54...
... High magnetic fields have been employed to probe the spin-orbit interactions, to close band gaps in semiconducting nanotubes, and to induce spin polarization and Aharonov-Bohm quantum interference effects. These high field studies are expected to continue to yield important information on fundamental interactions in 1D wires.
From page 55...
... Topological Phases During the last few years, a new class of materials, "topological phases," has emerged as an exciting frontier of science. These materials have a number of remarkable properties that were not previously envisaged, and strong magnetic fields have played an important role in their exploration.
From page 56...
... However, transport studies of the surface states have proved a challenge because of the coexistence of bulk transport channels due to doping by defects in available topological insulator samples. Nevertheless, experi ments on thin samples in high magnetic fields, carried out at NHFML, were able to distinguish the surface from the bulk contributions.
From page 57...
... Using those samples, we may anticipate that novel quantum transport phenomena, including the fractional quantum Hall effect in topological surface states, will become an exciting realm of high magnetic field science. The Dirac nature of the surface states of topological insulators has also been observed in STM and optical experiments involving high magnetic fields.
From page 58...
... It is expected that more topological superconductors will be discovered in natural compounds or in artificially constructed hybrid systems. Considering the important roles that high magnetic fields have played in supercon ductivity research, we may expect that topological superconductors will naturally provide new exciting opportunities for high magnetic field science.
From page 59...
... Similarly, nanoscale chemical aggregates or biological cells can be oriented using high magnetic fields and studied in situ or ex situ. Since the orientational torque is quadratic in B, high magnetic fields are required to study nanometer-sized aggregates (Maret and Dransfield, 1985)
From page 60...
... Although it is easily possible to deform many objects with an electric field, the large values of typical dielectric constants mean that the local electric field may be very different from the externally applied field, which renders any quantitative analysis very difficult in that case. Apart from aligning aggregates in solution with magnetic fields and studying them in situ, one may also use the fields to order matter in solution and then fix it in some way and study the ordered material ex situ afterwards.
From page 61...
... . In summary, there are many as yet unexplored areas in research on soft matter in high magnetic fields since it is not widely recognized that at high magnetic fields diamagnetic energies may become important.
From page 62...
... Copy right 2007, American Institute of Physics. Concluding Comments High magnetic fields are a critical research tool in many areas of condensed matter and materials physics.
From page 63...
... 2010. AC measurement of heat capacity and magnetocaloric effect for pulsed magnetic fields.
From page 64...
... Superconductors in High Magnetic Fields Ando, Y., G.S. Boebinger, A
From page 65...
... 2009. Bismuth in strong magnetic fields: Unconventional Zeeman coupling and correlation effects.
From page 66...
... High Magnetic Fields in Soft Matter Research Beaugnon, E., and R Tournier.


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