Pressure-induced superconductivity in pnictides

The recent discovery of iron-based superconductors (pnictides) has opened a new door to high-T_c superconductor studies.^{Reference Kamihara, Hiramatsu, Hirano, Kawamura, Yanagi, Kamiya and Hosono23,Reference Kamihara, Watanabe, Hirano and Hosono24} Prominent features of these superconductors include: (1) superconductivity within an iron arsenide layer, in which Fe²⁺ exists in the parent antiferromagnetic (AFM) state; (2) formation of a spin-density wave (SDW) phase from the parent state; and (3) inducing superconductivity by suppressing the SDW state via chemical doping or applied external pressure. Applied pressure has been shown to effectively suppress AFM ordering and to induce a high critical temperature T _c in the pnictide system^{Reference Chu and Lorenz25}—for example, superconductivity only occurs at high pressure in undoped “122” systems (Figure 3).

Figure 3.

Magnetic and structural transition diagram of BaFe₂As₂ (solid lines) and SrFe₂As₂ (dashed lines) pnictides at high-pressure low-temperature. T _S: tetragonal-orthorhombic transition temperature based on x-ray diffraction results; T _N: paramagnetic-antiferromagnetic transition temperature based on Mössbauer results.^{Reference Wu, Lin, Wang, Liu, Zhu, Xiao, Chow and Jin27,Reference Bi, Zhao, Lin, Jia, Hu, Jin, Ferry, Yang, Struzhkin and Alp28} Note: T _c, superconducting critical temperature.

Synchrotron-based x-ray diffraction (XRD), Mössbauer spectroscopy, and nuclear resonant inelastic x-ray scattering spectroscopy (NRIXS) under high-pressure and low-temperature conditions are ideally suited to unravel the interplay among structure, magnetism, and phonon dynamics in pnictide systems.^{Reference Wu, Lin, Wang, Liu, Zhu, Xiao, Chow and Jin26,Reference Wu, Lin, Wang, Liu, Zhu, Xiao, Chow and Jin27} High-pressure results have shown that magnetic and structural transitions are concurrently suppressed to much lower temperatures near a quantum critical pressure, where the AFM phase transforms into a bulk superconducting state (Figure 3). XRD and Mössbauer spectroscopy results have revealed that SrFe₂As₂ undergoes a magnetically ordered phase transition that is strongly coupled with the structural transition.^{Reference Wu, Lin, Wang, Liu, Zhu, Xiao, Chow and Jin26} Magnetic ordering of BaFe₂As₂ surprisingly precedes the structural transition at high pressure in the parent compound (Figure 3), in sharp contrast to the chemical-doping case.^{Reference Wu, Lin, Wang, Liu, Zhu, Xiao, Chow and Jin27} High-pressure low-temperature NRIXS techniques have been recently developed to reveal the role of phonon dynamics on the origin of superconductivity in pnictides.^{Reference Bi, Zhao, Lin, Jia, Hu, Jin, Ferry, Yang, Struzhkin and Alp28}

Lattice dynamics by inelastic x-ray scattering

Emerging at the forefront for exploring the lattice dynamics under high pressure are synchrotron-based inelastic x-ray scattering (IXS) techniques. IXS has been used to study acoustic and optical phonon dispersions and phonon density of states of materials at high pressure, unveiling very rich emergent phenomena, such as phonon anomalies,^{Reference Lin, Wu, Zhu, Mao, Said, Leu, Cheng, Uwatoko, Jin and Zhou29} acoustic phonon softening,^{Reference Lin, John, Alp, Zhao, Lerche, Sturhahn, Xiao and Chow30} charge ordering, as well as mode hardening.^{Reference Lin, Wu, Zhu, Mao, Said, Leu, Cheng, Uwatoko, Jin and Zhou29,Reference Jarrige, Rueff, Shieh, Taguchi, Ohishi, Matsumura, Wang, Ishii, Hiraoka and Cai31} Specifically, IXS studies on the lattice dynamics of single-crystal molybdenum showed the phonon anomaly near the H-point of the Brillouin zone due to strong electron–phonon coupling, which decreases upon compression due to the Fermi level shift with respect to the relevant electronic bands.^{Reference Farber, Krisch, Antonangeli, Beraud, Badro, Occelli and Orlikowski32} High-pressure studies have also shown abnormal elastic and vibrational behaviors of single-crystal magnetite, which are attributed to the occurrence of octahedrally coordinated Fe²⁺-Fe³⁺-Fe²⁺ ions charge-ordering along [110] in the inverse spinel structure,^{Reference Lin, Wu, Zhu, Mao, Said, Leu, Cheng, Uwatoko, Jin and Zhou29} revealing a new phase diagram of magnetite at high-pressure low-temperature.^{Reference Lin, Wu, Zhu, Mao, Said, Leu, Cheng, Uwatoko, Jin and Zhou29} Recent advances in IXS techniques in a cryogenically cooled DAC and laser-heated DAC^{Reference Lin, Sturhahn, Zhao, Shen, Mao and Hemley33} provide new arsenals for studying fundamental materials physics under extreme conditions.

Plastic deformation of nanomaterials

Understanding the plastic deformation of nanocrystalline materials is a long-standing challenge. Various controversial observations, mainly on the existence of dislocations and the mechanisms for a reversed Hall–Petch effect, have been reported.^{Reference Schiøtz, Di Tolla and Jacobsen34–Reference Lu, Chen, Huang and Lu36} However, in situ observations of plastic deformation in ultrafine (sub-10 nm) nanocrystals has long been difficult, precluding the direct exploration of mechanics at the nanometer scale. Chen and co-workers plastically deformed nickel and spinel nanoparticles to pressures above 35 GPa^{Reference Chen, Lutker, Raju, Yan, Kanitpanyacharoen, Lei, Yang, Wenk, Mao and Williams17} and observed texture development in situ during plastic deformation by using a radial diamond anvil cell (rDAC) XRD technique (incident x-rays are perpendicular to the compression axis) recently developed for geophysical research. The size of nanocrystals has long been viewed as limiting dislocation activity and associated deformation. Dislocation-mediated plastic deformation and texturing are expected to become inactive below a critical particle size, which is often thought to be between 10 and 30 nm, based on computer simulations^{Reference Schiøtz, Di Tolla and Jacobsen34} and transmission electron microscope analysis.^{Reference Shan, Stach, Wiezorek, Knapp, Follstaedt and Mao37} The new findings regarding the dislocation activities in ultrafine nickel^{Reference Chen, Lutker, Raju, Yan, Kanitpanyacharoen, Lei, Yang, Wenk, Mao and Williams17} and MgAl₂O₄ demand considering the role of defects in the physical behaviors of nanomaterials.

Pressure-induced polyamorphic transition and devitrification in metallic glass

The polyamorphic transitions from a low-density amorphous state to a high-density amorphous state in glasses often result from increased atomic coordination. Such coordination increases and polyamorphism were thought to be impossible in nondirectional, densely packed metallic glass.^{Reference Sheng, Luo, Alamgir, Bai and Ma38} When combining in situ high-pressure XRD pair distribution function with molecular dynamics simulations, surprising polyamorphic transitions were obtained in Ce₅₅Al₄₅ metallic glass^{Reference Sheng, Liu, Cheng, Wen, Lee, Luo, Shastri and Ma39} and in La₃₂Ce₃₂Al₁₆Ni₅Cu₁₅ bulk metallic glass.^{Reference Zeng, Li, Feng, Liermann, Somayazulu, Shen, Mao, Yang, Liu and Hu40} It was found that under high pressure, the 4f electrons of Ce in Ce-based metallic glass transform from the ambient localized state to an itinerant state, resulting in the polyamorphic transition. This new type of “electronic” polyamorphism in densely packed metallic glass is dictated by the Ce constituent and is fundamentally distinct from the polyamorphism that involves a coordination increase.^{Reference Guthrie, Tulk, Benmore, Xu, Yarger, Klug, Tse, Mao and Hemley41,Reference Meade, Hemley and Mao42} These studies have opened up a new research field on metallic glasses and have stimulated extensive studies on polyamorphic transitions in many other 4f elements-based metallic glass.

Whether a glass can have long-range structural order is a challenging question, because current experimental probes and theory are usually limited to local clusters in glasses. Using in situ high-pressure XRD, a Ce₇₅Al₂₅ metallic glass was found to crystallize into a single crystal of indefinite length at ∼25 GPa in a DAC.^{Reference Zeng, Ding, Mao, Luo, Blomqvist, Ahuja, Yang, Shu, Sinogeikin, Meng, Brewe, Jiang and Mao43} This discovery established direct linkage between single crystal and glass, two extreme end members in terms of structural order, providing a unique opportunity for observation of this otherwise invisible long-range order.^{Reference Zeng, Sheng, Ding, Wang, Yang, Jiang, Mao and Mao44}

Synchrotron in situ study of materials synthesis at high pressures

Using synchrotron XRD, the phase evolution of ZnO and GaN was monitored in reactions at pressures <9.2 GPa and temperatures up to 1500 K.^{Reference Chen, Wang, Bai, Hanson, Warren, Muckerman, Fujita and Rodriguez64} A solid solution of (Ga_1–xZn_x)(N_1–xO_x) in the wurtzite structure was produced at high pressure and temperature; this material has the potential of splitting water via visible light photocatalytic reactions.

As potentially cheaper alternatives to diamonds for cutting and shaping hard metals and ceramics, superhard noble metal nitrides and carbides were synthesized in DACs at high pressure and temperature, and the compressibility measured accordingly.^{Reference Gregoryanz, Sanloup, Somayazulu, Badro, Fiquet, Mao and Hemley65–Reference Young, Sanloup, Gregoryanz, Scandolo, Hemley and Mao67} PtN was produced at above 45–50 GPa and 2000 K, and the bulk modulus was measured to be 372 GPa.^{Reference Gregoryanz, Sanloup, Somayazulu, Badro, Fiquet, Mao and Hemley65} IrN₂ and OsN₂ were produced above 50 GPa and 2000 K, and the bulk modulus of IrN₂was measured to be 428 GPa.^{Reference Young, Sanloup, Gregoryanz, Scandolo, Hemley and Mao67} PtC was produced at 70 GPa and 2600 K, and in this case, the bulk modulus was measured to be 301 GPa.^{Reference Ono, Kikegawa and Ohishi66}

Fullerene materials at high pressure and temperature

Fullerenes provide promising applications in medicine, solar cells, hydrogen storage, sensors, and strengthening of metals due to the specific buckyball structure and spectacular physical, chemical, and mechanical properties. Time-resolved synchrotron XRD showed that amorphization of C₆₀ starts at 700–900 K at 12.5 GPa and at 700–800 K at 14.3 GPa, due to breaking of the five-membered rings of the C₆₀ molecules and collapsing of the original cage structure. The high-pressure behaviors of single-walled carbon nanotubes, polymerized fullerenes, and C₆₀ peapods have also been investigated at room temperature^{Reference Kawasaki, Matsuoka, Yao, Okino, Touhara and Suito68} or upon heating.^{Reference Kawasaki, Matsuoka, Yokomae, Nojima, Okino, Touhara and Kataura69}

The chemical nature of carbon materials under pressure arises from the interchange between the σ and π C–C bonds that are primarily oriented on or normal to a graphite layer, respectively, at ambient pressure. Synchrotron x-ray inelastic scattering revealed that at a pressure of ∼17 GPa, one-half of the π bonds between graphite layers convert to σ bonds between graphite layers, whereas the other half remain as π bonds in the high-pressure form (see Figure 5).^{Reference Mao, Mao, Eng, Trainor, Newville, Kao, Heinz, Shu, Meng and Hemley7} This change in bonding accounts for the observed formation of a new superhard phase of graphite.

Figure 5.

Change in chemical bonding in graphite under compression. (a) X-ray diffraction peaks broaden due to distortion of graphite basal planes under pressure. (b) Inelastic x-ray scattering (IXS) spectra showing one-half of the interplane carbon π bonds changed to σ bonds. Adapted with permission from Reference Reference Mao, Mao, Eng, Trainor, Newville, Kao, Heinz, Shu, Meng and Hemley7.© AAAS 2003. Note: a, lattice parameter; c, lattice parameter; θ, scattering angle; A, stacking layer; B, stacking layer; black sphere, layer-bridging carbon; gray sphere, nonbridging carbon; horizontal, IXS taken in the horizontal direction to probe carbon bonding in the a-plane; vertical, IXS taken in the vertical direction to probe carbon bonding in the c-plane.