Pressure-induced Quantum Critical Point Research Articles

Magnetotransport of Sm2Ir2O7 across the pressure-induced quantum-critical phase boundary

Rare-earth pyrochlore iridates host two interlocking magnetic sublattices of corner-sharing tetrahedra and can harbour a unique combination of frustrated moments, exotic excitations and highly correlated electrons. They are also the first systems predicted to display both topological Weyl semimetal and axion insulator phases. We have measured the transport and magnetotransport properties of single-crystal Sm2Ir2O7 up to and beyond the pressure-induced quantum critical point for all-in-all-out (AIAO) Ir order at pc = 63 kbar previously identified by resonant X-ray scattering and close to which Weyl semimetallic behavior has been previously predicted. Our findings overturn the accepted expectation that the suppression of AIAO order should lead to metallic conduction persisting down to zero temperature. Instead, the resistivity-minimum temperature, which tracks the decrease in the AIAO ordering temperature for pressures up to 30 kbar, begins to increase under further application of pressure, pointing to the presence of a second as-yet unidentified mechanism leading to non-metallic behavior. The magnetotransport does track the suppression of Ir magnetism, however, with a strong hysteresis observed only within the AIAO phase boundary, similar to that found for Ho2Ir2O7 and attributed to plastic deformation of Ir domains. Around pc we find the emergence of a new type of electronic phase, characterized by a negative magnetoresistance with small hysteresis at the lowest temperatures, and hysteresis-free positive magnetoresistance above approximately 5 K. The temperature dependence of our low-temperature transport data are found to be best described by a model consistent with a Weyl semimetal across the entire pressure range.

The breakdown of both strange metal and superconducting states at a pressure-induced quantum critical point in iron-pnictide superconductors

Here we report the first observation of the concurrent breakdown of the strange metal (SM) normal state and superconductivity at a pressure-induced quantum critical point in Ca10(Pt4As8)((Fe0.97Pt0.03)2As2)5 superconductor. We find that, upon suppressing the superconducting state, the power exponent (α) changes from 1 to 2, and the slope of the temperature-linear resistivity per FeAs layer (A□) gradually diminishes. At a critical pressure, A□ and superconducting transition temperature (Tc) go to zero concurrently, where a quantum phase transition from a superconducting state with a SM normal state to a non-superconducting Fermi liquid state occurs. Scaling analysis reveals that the change of A□ with Tc obeys the relation of Tc ~ (A□)0.5, similar to what is seen in other chemically doped unconventional superconductors. These results suggest that there is a simple but powerful organizational principle of connecting the SM normal state with the high-Tc superconductivity.

Charge density wave and superconductivity in the kagome metal CsV3Sb5 around a pressure-induced quantum critical point

Using first-principles density functional theory calculations, we investigate the pressure-induced quantum phase transition (QPT) from the charge density wave (CDW) to the pristine phase in the layered kagome metal CsV$_3$Sb$_5$ consisting of three-atom-thick Sb$-$V$_3$Sb$-$Sb and one-atom-thick Cs layers. The CDW structure having the formation of trimeric and hexameric V atoms with buckled Sb honeycomb layers features an increase in the lattice parameter along the $c$ axis, compared to its counterpart pristine structure having the ideal V$_3$Sb kagome and planar Sb honeycomb layers. Consequently, as pressure increases, the relatively smaller volume of the pristine phase contributes to reducing the enthalpy difference between the CDW and pristine phases, yielding a pressure-induced QPT at a critical pressure $P_c$ of ${\sim}$2 GPa. Furthermore, we find that (i) the superconducting transition temperature $T_c$ increases around $P_c$ due to a phonon softening associated with the periodic lattice distortion of V trimers and hexamers and that (ii) above $P_c$, optical phonon modes are hardened with increasing pressure, leading to monotonous decreases in the electron-phonon coupling constant and $T_c$. Our findings not only demonstrate that the uniaxial strain along the $c$ axis plays an important role in the QPT observed in CsV$_3$Sb$_5$, but also provide an explanation for the observed superconductivity around $P_c$ in terms of a phonon-mediated superconducting mechanism.

Strange-metal behaviour in a pure ferromagnetic Kondo lattice.

A wide range of metals exhibit anomalous electrical and thermodynamic properties when tuned to a quantum critical point (QCP), although the origins of such strange metals have posed a long-standing mystery. The frequent association of strange metals with unconventional superconductivity and antiferromagnetic QCPs1-4 has led to the belief that they are highly entangled quantum states5. By contrast, ferromagnets are regarded as an unlikely setting for strange metals, because they are weakly entangled and their QCPs are often interrupted by competing phases or first-order phase transitions6-8. Here we provide evidence that the pure ferromagnetic Kondo lattice9,10 CeRh6Ge4 becomes a strange metal at a pressure-induced QCP. Measurements of the specificheat and resistivity under pressure demonstrate that the ferromagnetic transition is continuously suppressed to zero temperature, revealing a strange-metal behaviour around the QCP. We argue that strong magnetic anisotropy has a key role in this process, injecting entanglement in the form of triplet resonating valence bonds into the ordered ferromagnet. We show that a singular transformation in the patterns of the entanglement between local moments and conduction electrons, from triplet resonating valence bonds to Kondo-entangled singlet pairs at the QCP, causes a jump in the Fermi surface volume-a key driver of strange-metallic behaviour. Our results open up a direction for research into ferromagnetic quantum criticality and establish an alternative setting for the strange-metal phenomenon. Most importantly, strange-metal behaviour at a ferromagnetic QCP suggests that quantum entanglement-not the destruction of antiferromagnetism-is the common driver of the varied behaviours of strange metals.

Pressure-induced quantum critical point in the heavily hydrogen-doped iron-based superconductor LaFeAsO

An iron-based superconductor ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{H}}_{x}\phantom{\rule{0.16em}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}0.6)$ undergoes two antiferromagnetic (AF) phases upon H doping. We investigated the second AF phase ($x=0.6$) using NMR techniques under pressure. At pressures up to 2 GPa, the ground state is a spin-density-wave state with a large gap; however, the gap closes at 4.0 GPa, suggesting a pressure-induced quantum critical point. Interestingly, the gapped excitation coexists with gapless magnetic fluctuations at pressures between 2 and 4 GPa. This coexistence is attributable to the lift up of the ${d}_{xy}$ orbital to the Fermi level, a Lifshitz transition under pressure.

Pressure-induced quantum critical behavior in LaFeAsO1-xHx studied via NMR

LaFeAsO1-xHx is a prototypical iron-based superconductor. However, its phase diagram presents unique properties compared to conventional iron-based superconductors: (1) the superconducting phase is sandwiched between two antiferromagnetic (AF) phases appearing in lightly (x ≤ 0.05) and heavily (0.49 ≤ x) H-doped regimes. (2) The AF phase (0.49 ≤ x) is strongly suppressed when applying pressure. We measured the relaxation time T1 at 75As sites for x=0.6 under pressures up to 3.7 GPa to investigate the behavior near the quantum critical point (QCP). The relaxation rate divided by temperature (T) 1/T1T exhibited a peak at the AF transition temperature (TN) followed by Curie-Weiss behavior at low temperatures below TN.. The result demonstrates that the gapped excitation originating from the spin-density-wave state and the gapless excitation characteristic to the QCP coexist near the pressure-induced QCP.

Pressure effects in the itinerant antiferromagnetic metal TiAu

We report the pressure dependence of the N\'eel temperature ${T}_{N}$ up to $P\ensuremath{\approx}27$ GPa for the recently discovered itinerant antiferromagnet (IAFM) TiAu. The ${T}_{N}(P)$ phase boundary exhibits unconventional behavior in which the N\'eel temperature is enhanced from ${T}_{N}\ensuremath{\approx}33$ K at ambient pressure to a maximum of ${T}_{N}\ensuremath{\approx}35$ K occurring at $P\ensuremath{\approx}5.5$ GPa. Upon a further increase in pressure, ${T}_{N}$ is monotonically suppressed to $\ensuremath{\sim}22$ K at $P\ensuremath{\approx}27$ GPa. We also find a crossover in the temperature dependence of the electrical resistivity $\ensuremath{\rho}$ in the antiferromagnetic (AFM) phase that is coincident with the peak in ${T}_{N}(P)$, such that the temperature dependence of $\ensuremath{\rho}={\ensuremath{\rho}}_{0}+{A}_{n}{T}^{n}$ changes from $n\ensuremath{\approx}3$ during the enhancement of ${T}_{N}$ to $n\ensuremath{\approx}2$ during the suppression of ${T}_{N}$. Based on an extrapolation of the ${T}_{N}(P)$ data to a possible pressure-induced quantum critical point, we estimate the critical pressure to be ${P}_{c}\ensuremath{\approx}45$ GPa.

Magnetic field-induced Fermi surface reconstruction and quantum criticality in

We present detailed results of the field evolution of the de Haas–van Alphen (dHvA) effect in . A magnetic field-induced reconstruction of the Fermi surface is clearly shown to occur inside the antiferromagnetic state, in an applied field of around T, which is evidenced by the appearance of several new dHvA branches. The angular dependence of the dHvA frequencies reveals that the Fermi surfaces of at and are similar. The results suggest that the Ce-4f electrons in become itinerant at due to the Kondo effect, prior to the field-induced quantum critical point (QCP) at T. The electronic states at the field-induced QCP are therefore different from that of the pressure-induced QCP where a dramatic Fermi surface reconstruction occurs exactly at the critical pressure, indicating that multiple types of QCP may exist in .

MnSi薄膜における静水圧効果

Effects of hydrostatic pressure on a thin film of helimagnet MnSi were reviewed. The MnSi film exhibits similar non-Fermi liquid (NFL) behavior near the pressure-induced quantum critical point to that observed in bulk MnSi, although its temperature range is extended to higher temperature side compared with the bulk sample. Since the NFL behavior of MnSi is related with presence of “Skyrmion”, the extended NFL region observed in the thin film may come from stabilization of the Skyrmions due to uniaxial anisotropy. Furthermore, we discussed a general role of hydrostatic pressure in a thin-film sample grown by molecular beam epitaxy.

Pressure–temperature phase diagram of CeAg

We have performed electrical-resistivity measurements under hydrostatic pressures ranging up to 4.47GPa on the cubic compound CeAg. A pressure–temperature phase diagram has been revised in a high-pressure range above ∼2.1GPa, where the system is known to enter into an unidentified phase through a first-order phase transition. We have found that the electrical resistivity shows a clear kink anomaly below ∼ 4K in this unknown phase. The anomaly shifts to lower temperature with increasing pressure, and shows a tendency to reach absolute zero at ∼6GPa, suggesting the presence of a pressure-induced quantum critical point around this pressure. We have also observed that a first-order phase boundary line, which separates the phase diagram into two regions of a tetragonal phase and the unidentified phase, seems to terminate in a critical end point located at (PCEP, TCEP) ∼(3.2GPa,200K). This suggests that the crystal symmetry of the unidentified phase above ∼2.1GPa is also tetragonal.

Electrical resistivity study of CeZn11: Magnetic field and pressure phase diagram up to 5 GPa

Thorough resistivity measurements on single crystals of CeZn${}_{11}$ under pressure $p$ and magnetic field $H$ are presented. At ambient pressure, CeZn${}_{11}$ orders antiferromagnetically at ${T}_{N}=2$ K. The pressure dependence of the resistivity reveals an increase of the Kondo effect. We determine the pressure evolution of the magnetic exchange interaction between conduction and localized $4f$ electrons. It qualitatively reproduces the pressure evolution of the magnetic ordering temperature ${T}_{{O}_{1}}$ (with ${T}_{{O}_{1}}={T}_{N}$ at ambient pressure). In addition to ${T}_{{O}_{1}}$, a new anomaly ${T}_{{O}_{2}}$ appears under pressure. Both anomalies are found to increase with applied pressure up to $4.9$ GPa, indicating that CeZn${}_{11}$ is far from a pressure induced quantum critical point. Complex $T$-$H$ phase diagrams are obtained under pressure which reveal the instability of the ground state in this compound.

Quantum Magnets under Pressure: Controlling Elementary Excitations inTlCuCl3

We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl3 through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point.

Pressure-induced quantum critical point in the heavy fermion compound [formula omitted

The electrical resistivity of single crystalline CeAl 2 has been measured at high pressure up to 6 GPa. It is found that the heavy fermion state becomes unstable at around 2.8 GPa, where the coefficient of T 2 dependence diverges and the Kondo temperature T K is 0. This fact indicates the occurrence of quantum phase transition near 2.8 GPa. The result will be discussed in connection with the disappearance of the antiferromagnetic ordering temperature near 3 GPa.