Field-induced Crossover Research Articles

Chiral-fluctuations mediated helical to paramagnetic phase transition and scaling study in β-Mn type Co8Zn8Mn4 chiral magnet

We report detailed magnetization and ac susceptibility studies on a chiral cubic β-Mn type Co8Zn8Mn4 alloy near the onset of magnetic ordering temperature. The Co8Zn8Mn4 alloy undergoes a complex series of temperature-driven transition from paramagnetic (PM) to helimagnetic phase through an intermediate inhomogeneous chiral fluctuations phase. The Gaussian shaped peak in ( = 0 Oe) at 343.19 K is analogous to that of the zero-field transition to long-period modulated helical state. Moreover, a pronounced field-induced anomaly in (T, 500 Oe) rises quickly with field above like a fingerprint, evident of a field-induced crossover into a precursor region. The in-field critical behavior characterizing a ferromagnetic-PM phase transition in the vicinity of ordering temperature confirms that Co8Zn8Mn4 chiral magnet belongs to the 3D-Heisenberg universality class. The existence of precursor region and the sizable expansion in the transition temperature, can be explained by the Landau equation.

Constraining the parameter space of a quantum spin liquid candidate in applied field with iterative optimization

The quantum spin liquid (QSL) state is an exotic state of matter featuring a high degree of entanglement and lack of long-range magnetic order in the zero-temperature limit. The triangular antiferromagnet YbMgGaO4 is a candidate QSL host, and precise determination of the Hamiltonian parameters is critical to understanding the nature of the possible ground states. However, the presence of chemical disorder has made directly measuring these parameters challenging. Here we report neutron scattering and magnetic susceptibility measurements covering a broad range of applied magnetic field at low temperature. Our data shows a field-induced crossover in YbMgGaO4, which we reproduce with complementary classical Monte Carlo and Density Matrix Renormalization Group simulations. Neutron scattering data above and below the crossover reveal a shift in scattering intensity from M to K points and, collectively, our measurements provide essential characteristics of the phase crossover that we employ to strictly constrain proposed magnetic Hamiltonian parameters despite the chemical disorder. Constrained exchange parameters further suggest the material's proximity to the QSL state in the clean limit. More broadly, our approach demonstrates a means of pursuing QSL candidates where Hamiltonian parameters might otherwise be obscured by disorder.

Temporal and field evolution of spin excitations in the disorder-free triangular antiferromagnet Na2BaCo(PO4)2

The realization of quantum spin liquids on a triangular lattice magnet remains challenging due to quenched disorders present in real materials. Here, we report local-probe signatures of spin-liquid-like excitations in the structurally perfect triangular antiferromagnet ${\mathrm{Na}}_{2}\mathrm{BaCo}{({\mathrm{PO}}_{4})}_{2}$. Our zero-field and longitudinal-field muon spin relaxation measurements reveal both spatially and temporally correlated spins with no hint of static magnetism down to 80 mK, consistent with a quantum spin liquid. Comprehensive $^{23}\mathrm{Na}$ nuclear magnetic resonance experiments enable us to identify a field-induced crossover of magnetic excitations from a spinonlike to a gapped magnon at the critical field ${\ensuremath{\mu}}_{0}{H}_{C}=1.65$ T. Our findings uncover the temporal, thermal, and magnetic-field characteristics of the magnetic excitations in the triangular-lattice quantum spin liquid candidate ${\mathrm{Na}}_{2}\mathrm{BaCo}{({\mathrm{PO}}_{4})}_{2}$.

Field-induced SU(4) to SU(2) Kondo crossover in a half-filling nanotube dot: Spectral and finite-temperature properties

We study finite-temperature properties of the Kondo effect in a carbon nanotube (CNT) quantum dot using the Wilson numerical renormalization group (NRG). In the absence of magnetic fields, four degenerate energy levels of the CNT consisting of spin and orbital degrees of freedom give rise to the SU(4) Kondo effect. We revisit the universal scaling behavior of the SU(4) conductance for quarter- and half-filling in a wide temperature range. We find that the filling dependence of the universal scaling behavior at low temperatures $T$ can be explained clearly with an extended Fermi-liquid theory. This theory clarifies that a $T^{2}$ coefficient of conductance becomes zero at quarter-filling whereas the coefficient at half-filling is finite. We also study a field-induced crossover from the SU(4) to SU(2) Kondo state observed at the half-filled CNT dot. The crossover is caused by the matching of the spin and orbital Zeeman splittings, which lock two levels among the four at the Fermi level even in magnetic fields $B$. We find that the conductance shows the SU($4$) scaling behavior at $\mu_{B}B<k_{B}T_{K}^{\mathrm{SU(4)}}$ and it exhibits the SU($2$) universality at $\mu_{B}B\gg k_{B}T_{K}^{\mathrm{SU(4)}}$, where $T_{K}^{\mathrm{SU(4)}}$ is the SU($4$) Kondo temperature. To clarify how the excited states evolve along the SU(4) to SU(2) crossover, we also calculate the spectral function. The results show that the Kondo resonance width of the two states locked at the Fermi level becomes sharper with increasing fields. The spectral peaks of the other two levels moving away from the Fermi level merge with atomic limit peaks for $\mu_{B}B \gtrsim k_{B}T_{K}^{\mathrm{SU(4)}}$.

Weak ferromagnetism and possible non-Fermi-liquid behavior in the itinerant electronic material Co3SnC

We report the observation of weak ferromagnetism and possible non-Fermi-liquid behavior in itinerant electronic material ${\mathrm{Co}}_{3}\mathrm{SnC}$ via magnetic properties, electrical transport, and specific heat under various magnetic fields. The analysis of magnetic properties suggests that ${\mathrm{Co}}_{3}\mathrm{SnC}$ undergoes a noncollinear itinerant ferromagnetic-paramagnetic phase transition around 3.6 K. With this transition, non-Fermi-liquid (NFL) behavior appears at low temperature covering a wide temperature range. The features of NFL are revealed by the power-law temperature dependence of resistivity and also proved by the $--T\mathrm{log}T$ dependence upturn in specific heat and the ${T}^{4/3}$ dependence of inverse susceptibility. With increasing the fields, the temperature coefficient of the ${T}^{2}$ term in the resistivity data shows a ${(H\phantom{\rule{0.16em}{0ex}}--\phantom{\rule{0.16em}{0ex}}{H}_{\mathrm{c}})}^{\ensuremath{-}0.5}$ divergence approaching the ${H}_{\mathrm{c}}$ and reduces monotonously with increasing the field further, apart from the usual $1/(H\phantom{\rule{0.16em}{0ex}}--\phantom{\rule{0.16em}{0ex}}{H}_{\mathrm{c}})$ dependence with the whole Fermi surface under the singular scattering. The exponent $n$ in the temperature dependence of resistivity shows an increase from $n=1.0$ to 2.0 with increasing the field as the evidence for the field-induced crossover from NFL to FL behavior; it is thought that field dependence of magnetic ground states should be responsible for this crossover; the relative mass enhancement factor $\ensuremath{\lambda}(H)/\ensuremath{\lambda}(0)$ reduces monotonically to nearly 60% at 9.0 T, indicating the gradual suppression of magnetic fluctuations and electron-electron scattering. Our results indicate that ${\mathrm{Co}}_{3}\mathrm{SnC}$ is a candidate for exploring itinerant quantum materials.

Electric field-induced crossover from 3D to 2D topological defects in a nematic liquid crystal: experimental verification.

A substrate was patterned with two pairs of half-integer strength topological defects, (+1/2, +1/2) and (+1/2, -1/2). In a sufficiently thick cell, a disclination line runs in an arch above the substrate connecting the two half integer defects within each pair. The director around the disclination line for the like-sign pair must rotate in 3D, whereas for the opposite-sign defect pair the director lies in the xy-plane parallel to the substrate. For a negative dielectric anisotropy nematic, an electric field applied normal to the substrate drives the director into the xy-plane, forcing the arch of the disclination line of the like-sign pair to become extended along the z-axis. For sufficiently large field the arch splits, resulting in two nearly parallel disclination lines traversing the cell from one substrate to the other. The opposite-sign defect pair is largely unaffected by the electric field as the director already lies in the xy-plane. Experimental results are presented, which are consistent with numerical simulations.

Anomalous Quantum Metal in a 2D Crystalline Superconductor with Electronic Phase Nonuniformity.

The details of the superconducting to quantum metal transition (SQMT) at T = 0 are an open problem that invokes great interest in the nature of this exotic and unexpected ground state (Ephron et al., 1996; Mason and Kapitulnik, 1999; Chervenak and Valles, 2000). However, the SQMT was not yet investigated in a crystalline 2D superconductor with coexisting and fluctuating quantum orders. Here, we report the observation of a SQMT in 2D ion-gel-gated 1T-TiSe2 (Li et al., 2016) driven by a magnetic field. A field-induced crossover between Bose quantum metal and vortex quantum creeping with an increasing field is observed. We discuss the interplay between superconducting and CDW fluctuations (discommensurations) and their relation to the anomalous quantum metal (AQM) phase. From our findings, gate-tunable 1T-TiSe2 emerges as a privileged platform to scrutinize, in a controlled way, the details of the SQMT, the role of coexisting fluctuating orders and, ultimately, to obtain a deeper understanding of the fate of superconductivity in strictly two-dimensional crystals near zero temperature.

Toggle-switch-like crossover between two types of isolated skyrmions within the conical phase of cubic helimagnets

We investigate the field-induced crossover between two types of isolated skyrmions that exist within the conical phase of cubic helimagnets and orient themselves either along or perpendicular to the field. Such a crossover takes place for the same value of the field, at which the closely packed skyrmion lattice was predicted to stabilize in the A-phase region. The clusters and a skyrmion lattice comprised by the skyrmions perpendicular to the field, however, are unfavorable and lose their stability as compared with the skyrmions parallel to the field. We also followed transformation of perpendicular skyrmions into pairs of merons that rupture the helical state. An attractive interactions between different types of isolated skyrmions make it feasible to construct complex cluster states with the cubic arrangement of skyrmions.

Quadrupole-driven non-Fermi-liquid and magnetic-field-induced heavy fermion states in a non-Kramers doublet system

Orbital degrees of freedom in condensed matters could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focussed on a purely quadrupole-active system PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole order at TQ = 0.11 K. We observed non-Fermi liquid (NFL) behaviors emerging in the electrical resistivity and the 4f contribution to the specific heat, C_4f, in the paramagnetic state at T > TQ. Moreover, in magnetic fields below 6 T, all data set of the electrical resistivity and C_4f(T) are well scaled with characteristic temperatures T0's. This is the first observation of the NFL state in the nonmagnetic quadrupole-active system, whose origin is intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons. Below 0.13 K, the electrical resistivity and C_4f(T) exhibit anomalies as B approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.

Magnetic field-tuned Fermi liquid formation in [formula omitted

Low temperature ground state properties of Mn0.75Fe0.25Si are investigated based on temperature and magnetic field dependent behavior of specific heat and resistivity. Paramagnon spin fluctuations assisted non-Fermi liquid is suppressed by magnetic fields and gradual evolution of Fermi liquid is demonstrated. The tendency of magnetic field-induced crossover from non-Fermi to Fermi liquid behavior is illustrated by suitable magneto-specific heat scaling which reveals unusual quantum critical phenomenon in a 3d transition metal derived paramagnetic compound. In the absence of magnetic fields, Kadowaki–Wood's Ratio (KWR) A/γ2 is about 8.5μΩcmmol2K2J−2 which is 85% close to the originally proposed KWR and reaches 100% under the moderate magnetic fields of 0.7T.

Type-I superconductivity in KBi2 single crystals

We report on the detailed transport, magnetic, thermodynamic properties and theoretical calculation of KBi2 single crystals in superconducting and normal states. KBi2 exhibits metallic behavior at a normal state and enters the superconducting state below K. Moreover, KBi2 exhibits low critical fields in all measurements, field-induced crossover from second- to first-order phase transition in specific heat measurements, the typical magnetization isotherms of type-I superconductors, and a small Ginzburg–Landau parameter . These results clearly indicate that KBi2 is a type-I superconductor with a thermodynamic critical field Oe.

Determination of the entropy via measurement of the magnetization: application to the spin ice Dy2Ti2O7

The residual entropy of spin ice and other frustrated magnets is a property of considerable interest, yet the usual way of determining it, by integrating the heat capacity, is generally ambiguous. Here we note that a straightforward alternative method based on Maxwell’s thermodynamic relations can yield the residual entropy on an absolute scale. The method utilizes magnetization measurements only and hence is a useful alternative to calorimetry. We confirm that it works for the spin ice Dy2Ti2O7, which recommends its application to other systems. The analysis described here also gives an insight into the dependence of entropy on magnetic moment, which plays an important role in the theory of magnetic monopoles in spin ice. Finally, we present evidence of a field-induced crossover from correlated spin ice behaviour to ordinary paramagnetic behaviour with increasing applied field, as signalled by a change in the effective Curie constant.

Effect of A-site La and Ba doping on threshold field and characteristic temperatures of PbSc0.5Ta0.5O3 relaxor studied by acoustic emission

The structural transitions in Pb1−xLaxSc(1+x)/2Ta(1−x)/2O3, x = 0.08 (PLST) relaxor crystals were studied by means of acoustic emission (AE) under an external electric field (E) and compared with those observed in pure PbSc0.5Ta0.5O3 (PST) and Pb0.78Ba0.22Sc0.5Ta0.5O3 (PBST) [E. Dul'kin et al., EPL 94, 57002 (2011)]. Similar to both the PST and PBST compounds, in zero field PLST exhibits AE corresponding to a para-to-antiferroelectric incommensurate phase transition at Tn = 276 K, lying in the vicinity of dielectric temperature maximum (Tm). This AE signal exhibits a nontrivial behavior when applying E resembling the electric-field-dependence of Tn previously observed for both the PST and PBST, namely, Tn initially decreases with the increase of E, attains a minimum at a threshold field Eth = 0.5 kV/cm, accompanied by a pronounced maximum of the AE count rate Ṅ = 12 s−1, and then starts increasing as E enhances. The similarities and difference between PST, PLST, and PBST with respect to Tn, Eth, and Ṅ are discussed from the viewpoint of three mechanisms: (i) chemically induced random local electric field due to the extra charge on the A-site ion, (ii) disturbance of the system of stereochemically active lone-pair electrons of Pb2+ by the isotropic outermost electron shell of substituting ion, and (iii) change in the tolerance factor and elastic field to the larger ionic radius of the substituting A-site ion due to the different radius of the substituting ion. The first two mechanisms influence the actual values of Tn and Eth, whereas the latter is shown to affect the normalized Ṅ, indicating the fractions undergoing a field-induced crossover from a modulated antiferroelectric to a ferroelectric state. Creation of secondary random electric field, caused by doping-induced A-site-O ionic chemical bonding, is discussed.

Field-induced crossover from phonon to field assisted hopping conductivity in organic materials

Recently the change in temperature dependence of conductivity observed in polythiophene field-effect transistors has been attributed to field-induced metal-insulator transition. Under a high source-drain voltage, the conductivity becomes independent from temperature and depends exponentially on the inverse of the square root of electric field. In this paper we present a theoretical interpretation of this behavior in the framework of hopping mechanism. The ingredient of this model, is the energy gained from the electrical field is always larger than the energy difference between the localized states involved into the charge transitions, which induces a crossover from phonon-assisted hopping to field-assisted hopping.

Low-Temperature Magnetization of the Metamagnetic Heavy Fermion Compound YbIr2Zn20

DC magnetization of YbIr 2 Zn 20 was measured in magnetic fields H (∥[100]) up to 14.5 T at low temperatures down to 0.09 K, focusing attention on the metamagnetic behavior. At the base temperature of T =0.09 K, three sharp structures were observed in the differential susceptibility d M /d H at 6.2, 9.7 and 11.2 T, with the peak at 9.7 T being the largest. The amplitudes of these anomalies are strongly T dependent and those at 6.2 and 11.2 T disappear above 0.7 and 0.4 K, respectively. The peak value of d M /d H of the main peak at 9.7 T exhibits an unusual temperature variation on cooling below 1 K, with a divergent tendency persisting towards the lowest temperature. This feature is different from what one would expect for the normal Fermi liquid systems, and suggests the existence of underlying quantum critical behavior. The results are discussed in relation to the recently proposed field-induced valence crossover scenario.

Magnetic field induced coherence-incoherence crossover in the interlayer conductivity of a layered organic metal

The angle-dependent interlayer magnetoresistance of the pressurized (to the normal metallic state) layered organic metal $\ensuremath{\alpha}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}\text{KHg}{(\text{SCN})}_{4}$ is found to change from the conventional behavior at low magnetic fields to an anomalous one at high fields. The dependence of this field-induced crossover on the sample purity and temperature reveals parallel contribution of the classical Boltzmann and incoherent channels in the interlayer conductivity. The latter channel, having a metallic temperature dependence but being insensitive to an in-plane magnetic field, may be responsible for magnetoresistance anomalies observed in a number of layered metals. We propose a possible mechanism for the incoherent channel combining interlayer tunneling via local hopping centers and intralayer diffusion.

Temperature- and field-dependent conductivity in disordered materials

We present a model to describe electrical transport in disordered materials. Solving the rate equation by the Green function technique and taking into account the electric field effect on the effective dimension of the transport path, we obtain a different law for the conductivity field dependence. This law could be confounded with the Poole-Frenkel law in the range of field, corresponding to the experimental observations in organic materials and conjugated polymers. We show that this behavior is a common feature of all disordered systems. We find a field-induced crossover on the temperature dependence of the conductivity.

Nonlocal Magnetic Field-Tuned Quantum Criticality in CubicCeIn3−xSnx(x=0.25)

We show that antiferromagnetism in lightly (approximately 8%) Sn-doped CeIn3 terminates at a critical field mu0H(c) = 42 +/- 2 T. Electrical transport and thermodynamic measurements reveal the effective mass m* not to diverge, suggesting that cubic CeIn3 is representative of a critical spin-density wave (SDW) scenario, unlike the local quantum critical points reported in anisotropic systems such as CeCu(6-x)Au(x) and YbRh2Si(2-x)Ge(x). The existence of a maximum in m* at a lower field mu0H(x) = 30 +/- 1 T may be interpreted as a field-induced crossover from local moment to SDW behavior as the Néel temperature falls below the Fermi temperature.

Manganese(II) and copper(II) hexafluoroacetylacetonate 1:1 complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine: regiochemical parity analysis for exchange behavior of complexes between radicals and paramagnetic cations.

Mn(hfac)(2) and Cu(hfac)(2) form coordination complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine, PyrimPh-NIT. (Mn[PyrimPh-NIT](hfac)(2))(2) and (Cu[PyrimPh-NIT](hfac)(2))(2), 1 and 2, respectively, are cyclic M(2)L(2) dimers that exhibit strong exchange coupling between the coordinated paramagnetic dication (M) and nitroxide (NIT) unit. The M-NIT exchange is strongly antiferromagnetic (AFM) in 1 and strongly ferromagnetic (FM) in 2. Magnetic susceptibility measurements for 1 were fitted to an AFM spin pairing model with J/k = -0.25 K between Mn-NIT spin sites units. Complex 2 also exhibits AFM spin pairing between S = 1 Cu-NIT spin units that is somewhat field dependent at low temperature. The fit of corrected paramagnetic susceptibility chi(T) to an AFM spin pairing model at 200 Oe yields J/k = (-)3.8 K, quite similar to earlier measurements at 1000 Oe yielding J/k = (-)5.0 K. At 1.40 K, the magnetization of 2 does not approach saturation until somewhat above 170 kOe, giving an S-shaped curve; at 0.55 K, the magnetization curve shows steps characteristic of field-induced crossover between the S = 0 ground state and excited spin states. From the steps in the 0.55 K data, we estimate J/k = (-)3.8-4.0 K for 2, in good agreement with the analysis of chi(T).

Kondo screening ind-wave superconductors in a Zeeman field and implications for STM spectra of Zn-doped cuprates

We consider the screening of an impurity moment in a d-wave superconductor under the influence of a Zeeman magnetic field. Using the Numerical Renormalization Group technique, we investigate the resulting pseudogap Kondo problem, in particular the field-induced crossover behavior in the vicinity of the zero-field boundary quantum phase transition. The impurity spectral function and the resulting changes in the local host density of states are calculated, giving specific predictions for high-field STM measurements on impurity-doped cuprates.