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Chapter 4 Electron Transport Near the 2D Mott Transition
Published in Sergey Kravchenko, Strongly Correlated Electrons in Two Dimensions, 2017
Tetsuya Furukawa, Kazushi Kanoda
High compressibility due to the van der Walls nature of molecular packing in κ - (ET)2X and Y[Pd(dmit)2]2 allows us to con- trol bandwidth W of a system by applying pressure; indeed, these organic systems exhibit the pressure-induced bandwidth-controlled Mott transition. In particular the Mott transition in κ - (ET)2Cu[N(CN)2]Cl and κ - (ET)2Cu2(CN)3, which are abbreviated by κ - C1 and κ - Cu2(CN)3, respectively have been studied intensively so far Figure 4.6 shows the temperature-pressure phase diagrams for κ - C1 (Lefebvre et al., 2000; Limelette et al., 2003a; Kagawa et al., 2004) and κ - Cu2(CN)3 (Kurosaki et al., 2005; Kobashi, 2007). For both phase diagrams, the Mott insulating phase and the metallic phase are separated by the first-order Mott transition line, which ends at a finite-temperature critical end point. Above the critical point, the Mott transition becomes a metal-insulator crossover The critical temperatures are 40 K for κ - C1 and approximately 20 K for κ - Cu2(CN)3. In the vicinity of the critical point of κ - (ET)2X, many physical quantities, such as conductivity (Kagawa et a1., 2005), sound velocity (Fournier et al., 2003), thermal expansion (Bartosch et al., 2010; De Souza et al., 2007), and nuclear magnetic resonance (Kagawa et al., 2009), show anomalies. At low temperatures in the insulating phase, the antiferromagnetic long-range order with an ordered moment of 0.45 μB (μB: Bohr magneton) appears in κ - C1 (Miyagawa et al., 1995). On the other hand, for κ - Cu2(CN)3, magnetic order is absent, and quantum spin liquid phase emerges owing to the strong geometrical frustration coming from the nearly isotropic triangular lattice (Shimizu et al., 2003; Shimizu et al., 2006) with t’/t ≈ 0.81.0 (Kandpal et al., 2009; Nakamura et al., 2009; Koretsune and Hotta, 2014). In the quantum spin liquid phase, several exotic properties are observed, such as the (marginally) gapless nature of low-temperature excitations (Shimizu et al., 2003; Yamashita, S., et al., 2008; Yamashita, M., et al., 2009) and emergent inhomogeneity under a magnetic field (Shimizu et al., 2006). It is noted that the slope of the first-order transition line is determined by the difference of entropy between the insulating and metallic phases, obeying the Clausius-Clapeyron relation. The bend of the transition line above the Néel temperatures for κ - C1 indicates a rapid decrease in spin entropy of the insulating phase due to antiferromagnetic short-range order before the long-range order.
Two-dimensional quantum-spin-1/2 XXZ magnet in zero magnetic field: Global thermodynamics from renormalisation group theory
Published in Philosophical Magazine, 2019
Thermodynamics at low temperatures show distinct characters in XY-like () and Ising-like () regimes. In the XY-like regime at low temperatures, system is in algebraically ordered KT phase, where magnetisation vanishes in all directions: . However, vortex-antivortex pairs are bound together in KT phase, giving rise to non-zero correlations . The low-lying excitations are gapless, and can be understood by linear spin-wave theory or by vortex theory [9]. In this quantum spin-liquid phase at low-temperatures, due to a large density of low-energy states, specific heat is expected to be linear in temperature [129,130]:Here, is the Sommerfeld coefficient. Such linear behaviour of specific heat at low-temperatures has been observed for the two-dimensional quantum spin-liquid ZnCu(OH)Cl [131,132] and for the quasi-two-dimensional easy-plane-type XXZ ferromagnet KCuF [130]. Our results are in agreement with the expected linear form, as shown in Figure 13(a), where we plot specific heat as a function of temperature in XY-like regime at low-temperatures.
Optical control of layered nanomaterial generation by pulsed-laser ablation in liquids
Published in Journal of Modern Optics, 2020
Likewise, we were able to generate layered metastable basic zinc(II)-containing nanocrystals by pulsed-laser ablation in liquids (14). They were layered hydroxide double salts that are also known as anionic clays (35). Relative stabilities of these hydroxy-hydrated zinc minerals are pH dependent; all are soluble in strongly acidic solution (36). The most stable zinc hydroxide chloride hydrate polymorphs at room temperature are β-Zn(OH)Cl, ZnO·ZnCl2·2H2O, and 4Zn(OH)2·ZnCl2·H2O (= Zn5(OH)8Cl2·H2O, simonkolleite) (37), all of which convert into ZnO and ZnCl2 upon heating (35). We observed solely generation of layered crystalline nanosimonkolleite of stiochiometry Zn5(OH)8Cl2·H2O during pulsed-laser ablation of zinc metal in aqueous ZnCl2 + NH4OH solution (14), although very high electron temperatures of ca. (8,400 ± 1,300) K were reached during preparation (Figure 4); we added NH4OH to the aqueous ZnCl2 ablation liquid to raise the pH to neutral and be able to obtain a non-dissolved solid material. We also laser-prepared the nitrate analogue, the layered anionic clay zinc hydroxide nitrate hydrate of the stoichiometry Zn5(OH)8(NO3)2·2H2O (14). Zinc hydroxide nitrate hydrate is monoclinic, exhibits a layered structure that is closely related to that of zinc hydroxide chloride hydrate (38), and decomposes above 300°C via an anhydrous zinc nitrate phase to ZnO (39). Despite the high temperatures in the laser synthesis we did not observe any ZnO or Zn(NO3)2 in the generated nanomaterial; instead we obtained phase-pure crystalline zinc hydroxide nitrate hydrate. Further, we used pulsed-laser ablation in liquids to generate a mixed-metal phase of stoichiometry Cu3(Cu, Zn)Cl2(OH)6 (14), called zincian paratacamite, which occurs in nature as herbertsmithite (40, 41); the material has been extensively studied as a structurally perfect S = 1/2 kagome antiferromagnet in search of a quantum spin liquid (42, 43).
Looking for a quantum spin liquid in the BaNi2(V1−xPx)2O8 spin 1 honeycomb system
Published in Philosophical Magazine, 2019
Mimi Chung, Tai Kong, R. J. Cava
Motivated by these concepts, here we report the general magnetic properties of the BaNi2(V1−xPx)2O8 solid solution system, a series of tunable spin-1 honeycomb oxides, in a search for a quantum spin liquid. Whereas the phase space for a spin ½ honeycomb lattice has been theoretically mapped out as mentioned above, possible ground states in a spin 1 honeycomb lattice are not well established. BaNi2X2O8 (X = As, P, V) type materials have rhombohedral crystal structures [10–12] in the space group and are based on layers of edge-sharing spin-1 NiO6 octahedra arranged on the vertices of a honeycomb lattice (Figure 1). These layers are stacked between layers of nonmagnetic Ba2+ and P5+, As5+ or V5+ ions along the c axis of the rhombohedral cells (there are three layers per unit cell). Although sharing a similar crystal structure, the magnetic properties of BaNi2X2O8 are rather different. BaNi2V2O8 displays 2D honeycomb short range ordering at high temperatures, locking into an in-plane-tilted Neel ordered state at about 50 K, whereas BaNi2P2O8 is reported to order 3D into a Neel state at around 24 K [10,13]. Previous studies involving chemical substitutions have characterised the Ba(Ni1-xCux)2P2O8 system [10,14]. But to best determine whether a QSL exists in this family of materials, off-magnetic-site doping is preferred, to minimise the effects of disorder. In light of the phase diagram for the spin ½ honeycomb lattice where a QSL can be found in between different magnetic ground states, the current experimental work aims to scan through the ground state phase space between magnetically 2D-like BaNi2V2O8 and 3D-like BaNi2P2O8. Determining the basic magnetic properties of one such system, where P5+ systematically replaces V5+ in the MO4 tetrahedra between Ni honeycomb layers in BaNi2V2O8 is the focus of the current work. The magnetic interactions and ground states are clearly tuned by the substitution, but no clear evidence for the occurrence of a QSL is seen.