Heavy fermion spin liquid in herbertsmithite

Abstract

We analyze recent heat capacity measurements in herbertsmithite ZnCu3(OH)6Cl2 single crystal samples subjected to strong magnetic fields. We show that the temperature dependence of specific heat Cmag formed by quantum spin liquid at different magnetic fields B resembles the electronic heat capacity C el of the HF metal YbRh2Si2. We demonstrate that the spinon effective mass M * mag ∝ Cmag/T exhibits a scaling behavior like that of C el /T . We also show that the recent measurements of Cmag are compatible with those obtained on powder samples. These observations allow us to conclude that ZnCu3(OH)6Cl2 holds a stable strongly correlated quantum spin liquid, and a possible gap in the spectra of spinon excitations is absent even under the application of very high magnetic fields of 18 T.PACS numbers: 75.40. Gb, 71.27.+a, 71.10.Hf Quantum spin liquids (QSLs) are promising new phases, where exotic quantum states of matter could be realized. Although much theoretical effort has been devoted to understand their physical nature, the question is still far from its complete clarification. Generally speaking, the QSL is a quantum state, formed with hypothetic particles like fermionic spinons carrying spin 1/2 and no charge. A number of QSLs with various types of ground states are proposed [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] but the lack of real materials possessing them obscure the underlying physical mechanism. On the other hand, one needs a real theory that plays important role in the understanding and interpreting accessible experimental facts [19][20][21][22]. Measurements on magnetic insulators with geometrical frustration produce important experimental data shedding light on the nature of spinon composing QSL. Recent measurements indicate that the insulator ZnCu 3 (OH) 6 Cl 2 (herbertsmithite) is very likely to be the first promising candidate to host a QSL in real bulk materials. In herbertsmithite, the dynamic magnetic susceptibility shows that at low temperatures quasiparticle excitations, or spinons, form a continuum, and populate an approximately flat band crossing the Fermi level [17,21]. At the same time, our analysis of herbertsmithite thermodynamic properties allows us to reveal their scaling behavior. The above results demonstrate that the properties of the herbertsmithite are similar to those of heavy-fermion (HF) metals. Thus, it can be viewed as a new type of strongly correlated HF electrical insulator exhibiting properties of HF metals but resisting electric current [20][21][22].The development of this concept, however, faces fundamental problems still remaining to be resolved. The first one is that the experimental data are taken in measurements on herbertsmithite powder samples [4][5][6][20][21][22]. As a result, both out-of-plane magnetic defects and site disorder between the Cu and Zn ions can strongly change the behavior of the bulk susceptibility or even alter its low-temperature scaling at weak magnetic fields, see e.g. Ref. [16]. The second problem is based on exper...

Thermodynamic, Dynamic, and Transport Properties of Quantum Spin Liquid in Herbertsmithite from an Experimental and Theoretical Point of View

“…In these respects, herbertsmithite is the best candidate among quantum magnets to contain QSL described above [3][4][5][6][7][8]. These assessments are supported by model calculations indicating that an antiferromagnet on kagome lattice has gapless spin liquid ground state [16][17][18][19][20][21][22][23]. At the same time, recent suggestion [9,24,25], that there can exist a small spin-gap in the kagome layers may stand in conflict to this emerging picture (see also Refs.…”

“…To analyze QSL behavior theoretically, we employ the strongly correlated quantum spin liquid (SCQSL) [16-18, 20, 23] model. A simple kagome lattice may host a dispersionless topologically protected branch of the quasiparticle spectrum with zero excita-tion energy, known as a flat band [16,18,23,[30][31][32]. In that case the topological fermion condensation quantum phase transition (FCQPT) can be considered as a quantum critical point (QCP) of the ZnCu 3 (OH) 6 Cl 2 spinoncomposed QSL.…”

“…The example of Cmag/Tmax and Tmax at B ≃ 3 T is shown. (Color online) Normalized specific heat (Cmag/T )N versus normalized temperature TN at at different magnetic fields (legend)[17,23] extracted from specific heat Cmag/T measured on powder[4,5] and single-crystal[6][7][8] herbertsmithite samples, seeFig. 6.…”

Experimental Manifestations of Fermion Condensation in Strongly Correlated Fermi Systems

“…To analyze theoretically the QSL in herbertsmithite and other geometrically frustrated magnets, we employ the strongly correlated quantum spin liquid (SC-QSL) [3,18,20,23] model. A simple kagome lattice may host a dispersionless topologically protected branch of the quasiparticle spectrum with zero excitation energy, known as a flat band [3,5,18,23].…”

“…In these respects, herbertsmithite is the best candidate among quantum magnets to contain the QSL described above [10][11][12][13][14][15]. These assessments are supported by model calculations indicating that the ground state of kagome antiferromagnet is a gapless spin liquid [3,6,[18][19][20][21][22][23].…”