Zero-field Transition Research Articles

CPT and Lorentz symmetry tests with hydrogen using a novel in-beam hyperfine spectroscopy method applicable to antihydrogen experiments

We present a Rabi-type measurement of two ground-state hydrogen hyperfine transitions performed in two opposite external magnetic field directions. This puts first constraints at the level of ▪ on a set of coefficients of the Standard Model Extension, which were not measured by previous experiments. Moreover, we introduce a novel method, applicable to antihydrogen hyperfine spectroscopy in a beam, that determines the zero-field hyperfine transition frequency from the two transitions measured at the same magnetic field. Our value, ▪, is in agreement with literature at a relative precision of 0.44 ppb. This is the highest precision achieved on hydrogen in a beam, improving over previous results by a factor of 6.

Tunable Rydberg–Rydberg transitions in helium with reduced sensitivity to dc electric fields by two-colour microwave dressing

The difference in the static electric dipole polarizabilities of the and Rydberg levels in helium has been eliminated by dressing the atom with a microwave field near resonant with the single-photon transition. For an amplitude dressing field, detuned by from the zero-field transition frequency, the dc Stark shift of the two-photon transition between these states remained within for electric fields up to . This transition was probed by single-color two-photon microwave spectroscopy, and by two-color two-photon spectroscopy with one strong additional dressing field and a weak probe field. For all measurements, the transition frequencies and Stark shifts were compared, and found to be in excellent quantitative agreement with the results of Floquet calculations of the energy-level structure of the Rydberg states in the presence of the dressing fields and applied dc electric fields. The two-color microwave dressing scheme demonstrated, with one field applied to null the differential polarizability of the Rydberg–Rydberg transition, and the second exploited to allow the two-photon transition to be employed to achieve tunable absorption of single-photons from a weak probe field, will facilitate improved coherence times and tunable single-photon absorption in hybrid cavity QED experiments with Rydberg atoms and superconducting microwave circuits.

Hilbert Space in Isotopologue Dy(III) SMM Dimers: Dipole Interaction Limit in [163/164Dy2(tmhd)6(tape)]0 Complexes.

Single-molecule magnets are molecular complexes proposed to be useful for information storage and quantum information processing applications. In the quest for multilevel systems that can act as Qudits, two dysprosium-based isotopologues were synthesized and characterized. The isotopologues are [164Dy2(tmhd)6(tape)] (1(I=0)) and [163Dy2(tmhd)6(tape)] (2(I=5/2)), where tmhd = 2,2,6,6-tetramethylheptandionate and tape = 1,6,7,12-tetraazaperylene. Both complexes showed slow relaxation at a zero applied magnetic field with dominant Orbach and Raman relaxation mechanisms. μSQUID studies at milli-Kelvin temperatures reveal quasi-single ion loops, in contrast with the expected S-shape (near zero field) butterfly loops, characteristic of antiferromagnetically coupled dimeric complexes. Through analysis of the low-temperature data, we find that the interaction operating between Dy(III) is small, leading to a small exchange biasing from the zero-field transition. The resulting indirectly coupled nuclear states are degenerate or possess a small energy difference between them. We, therefore, conclude that for the creation of Qudits with enlarged Hilbert spaces, shorter Dy(III)···Dy(III) distances are deemed essential.

Ferroelectric Nematic-Isotropic Liquid Critical End Point

A critical end point above which an isotropic phase continuously evolves into a polar (ferroelectric) nematic phase with an increasing electric field is found in a ferroelectric nematic liquid crystalline material. The critical end point is approximately 30 K above the zero-field transition temperature from the isotropic to nematic phase and at an electric field of the order of 10 V/μm. Such systems are interesting from the application point of view because a strong birefringence can be induced in a broad temperature range in an optically isotropic phase.

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.

Weak itinerant magnetic phases of La2Ni7

La2Ni7 is an intermetallic compound that is thought to have itinerant magnetism with a small moment ordering below 65 K. A recent study of single crystal samples by Ribeiro et. al. [Phys. Rev. B 105, 014412 (2022)] determined detailed anisotropic H-T phase diagrams and revealed three zero-field magnetic phase transitions at T1 ~ 61.0 K, T2 ~ 56.5K, and T3 ~ 42 K. In that study only the highest temperature phase is shown to have a clear ferromagnetic component. Here we present a single crystal neutron diffraction study determining the propagation vector and magnetic moment direction of the three magnetically ordered phases, two incommensurate and one commensurate, as a function of temperature. The higher temperature phases have similar, incommensurate propagation vectors, but with different ordered moment directions. At lower temperatures the magnetic order becomes commensurate with magnetic moments along the c direction as part of a first-order magnetic phase transition. We find that the low-temperature commensurate magnetic order is consistent with a proposal from earlier DFT calculations.

Competing correlated states around the zero-field Wigner crystallization transition of electrons in two dimensions.

The competition between kinetic energy and Coulomb interactions in electronic systems leads to complex many-body ground states with competing orders. Here we present zinc oxide-based two-dimensional electron systems as a high-mobility system to study the low-temperature phases of strongly interacting electrons. An analysis of the electronic transport provides evidence for competing correlated metallic and insulating states with varying degrees of spin polarization. Some features bear quantitative resemblance to quantum Monte Carlo simulation results, including the transition point from the paramagnetic Fermi liquid to Wigner crystal and the absence of a Stoner transition. At very low temperatures, we resolve a non-monotonic spin polarizability of electrons across the phase transition, pointing towards a low spin phase of electrons, and a two-order-of-magnitude positive magnetoresistance that is challenging to understand within traditional metallic transport paradigms. This work establishes zinc oxide as a platform for studying strongly correlated electrons in two dimensions.

Precise measurement of the D2 S1(0) vibrational transition frequency

We report the absolute frequency of the vibrational transition in the electronic ground state of D. Deuterium molecules in the state in a molecular beam are excited to using a narrow-linewidth continuous-wave laser stabilised to an optical frequency comb; a strong static electric field in the excitation region induces a transition dipole moment, greatly enhancing the transition strength. Molecules excited to are state-selectively ionised using a pulsed ultraviolet laser and mass-selectively detected. Spectra recorded by scanning the infrared laser frequency are fit with a multi-Gaussian model that accounts for the intensities and relative frequencies of all hyperfine components at the static electric field strength used in the measurement. Zero-field absolute transition frequencies were determined from fits of 28 spectra recorded at nine different field strengths. Statistical uncertainty and systematic uncertainties due to the field plate spacing and relative intensities of the hyperfine components comprise the bulk of the overall uncertainty. We determine an absolute S(0) transition frequency of 94,925,100,487(17) kHz after correcting for the photon recoil and second-order Doppler shifts. This value agrees with other recent experimental measurements and a theoretical prediction, but with a lower fractional uncertainty of only 0.2 ppb.

Low Field Nano-NMR via Three-Level System Control.

Conventional control strategies for nitrogen-vacancy centers in quantum sensing are based on a two-level model of their triplet ground state. However, this approach fails in regimes of weak bias magnetic fields or strong microwave pulses, as we demonstrate. To overcome this limitation, we propose a novel control sequence that exploits all three levels by addressing a hidden Raman configuration with microwave pulses tuned to the zero-field transition. We report excellent performance in typical dynamical decoupling sequences, opening up the possibility for nano-NMR operation in low field environments.

Effect of electric field on defect generation and migration in HfO2

Understanding the effect of electric fields on defect creation and diffusion in metal oxides is of fundamental importance for developing accurate models of oxide degradation in electronic devices and dielectric breakdown. We use the Berry phase operator method within density functional theory to calculate how an applied electric field affects barriers for the creation of oxygen vacancy-interstitial defect pairs (DPs) and diffusion of interstitial O ions in monoclinic ($m$-)${\mathrm{HfO}}_{2}$. The results demonstrate that even close to breakdown fields, barriers for DP generation exceed 6 eV in the perfect $m\ensuremath{-}{\mathrm{HfO}}_{2}$ lattice. Simulated injection of extra electrons from electrodes significantly lowers barriers for the creation of DPs, which are further reduced by the field to around 1 eV. Thus, bias application facilitates the injection of electrons into the oxide; these extra electrons reduce energy barriers for the creation of O vacancies, and these barriers as well as those for O ion diffusion are further lowered by the field. We find that, within a linear regime, the electric field modulates the barrier height by a dot product between the electric field and the electric dipole at the zero-field transition state to good accuracy.

Unveiling the Physics of the Mutual Interactions in Paramagnets

In real paramagnets, there is always a subtle many-body contribution to the system’s energy, which can be regarded as a small effective local magnetic field (Bloc). Usually, it is neglected, since it is very small when compared with thermal fluctuations and/or external magnetic fields (B). Nevertheless, as both the temperature (T) → 0 K and B → 0 T, such many-body contributions become ubiquitous. Here, employing the magnetic Grüneisen parameter (Γmag) and entropy arguments, we report on the pivotal role played by the mutual interactions in the regime of ultra-low-T and vanishing B. Our key results are: i) absence of a genuine zero-field quantum phase transition due to the presence of Bloc; ii) connection between the canonical definition of temperature and Γmag; and iii) possibility of performing adiabatic magnetization by only manipulating the mutual interactions. Our findings unveil unprecedented aspects emerging from the mutual interactions.

Physical observables of the Ising spin glass in 6−ε dimensions: Asymptotical behavior around the critical fixed point

The asymptotical behavior of physical quantities, like the order parameter, the replicon and longitudinal masses, is studied around the zero-field spin glass transition point when a small external magnetic field is applied. An effective field theory to model this asymptotics contains a small perturbation in its Lagrangian which breaks the zero-field symmetry. A first order renormalization group supplemented by perturbational results provides the scaling functions. The perturbative zero of the scaling function for the replicon mass defines a generic Almeida-Thouless surface stemming from the zero-field fixed point.

The influence of magnetic order on the magnetoresistance anisotropy of Fe1 + δ–x CuxTe

We performed resistance measurements on CuxTe with in the presence of in-plane applied magnetic fields, revealing a resistance anisotropy that can be induced at a temperature far below the structural and magnetic zero-field transition temperatures. The observed resistance anisotropy strongly depends on the field orientation with respect to the crystallographic axes, as well as on the field-cooling history. Our results imply a correlation between the observed features and the low-temperature magnetic order. Hysteresis in the angle-dependence indicates a strong pinning of the magnetic order within a temperature range that varies with the Cu content. The resistance anisotropy vanishes at different temperatures depending on whether an external magnetic field or a remnant field is present: the closing temperature is higher in the presence of an external field. For the resistance anisotropy closes above the structural transition, at the same temperature at which the zero-field short-range magnetic order disappears and the sample becomes paramagnetic. Thus we suggest that under an external magnetic field the resistance anisotropy mirrors the magnetic order parameter. We discuss similarities to nematic order observed in other iron pnictide materials.

Classical Monte Carlo Study for Antiferro Quadrupole Orders in a Diamond Lattice

We investigate antiferro quadrupole orders in a diamond lattice under magnetic fields by Monte Carlo simulations for two types of classical effective models. One is an XY model with Z_3 anisotropy, and the other is a two-component phi^4 model with a third-order anisotropy. We confirm that the universality class of the zero-field transition is that for the three-dimensional XY model. Magnetic field corresponds to a Z_3 field in the effective model, and under this field, we find that collinear and canted antiferro-quadrupole orders compete. Each phase is characterized by symmetry breaking in the sector of (sublattice Z_2)x(reflection Z_2 for the order parameter). When Z_3 anisotropy and magnetic field vary, it turns out that this system is a good playground for various multicritical points; bicritical and tetracritical points emerge in a finite field. Another important finding is about the scaling of parasitic ferro quadrupole order at the zero-field critical point. This is the secondary order parameter induced by the primary antiferro order, and its critical exponent beta'=0.815 clearly differs from the expected value that is twice the value for the primary order parameter. The corresponding correlation length exponent is also different, nu'=0.597(12). We also discuss relation of the present effective quadrupole models with the 3-state Potts model as well as implication to undertanding of orbital orders in Pr-based 1-2-20 compounds.

Magnetic relaxation dynamics driven by the first-order character of magnetocaloric La(Fe,Mn,Si)13.

Here, we study the temporal evolution of the magnetic field-driven paramagnetic to ferromagnetic transition in the La(Fe,Mn,Si)13 material family. Three compositions are chosen that show varying strengths of the first-order character of the transition, as determined by the relative magnitude of their magnetic hysteresis and temperature separation between the zero-field transition temperature Tc and the temperature Tcrit, where the transition becomes continuous. Systematic variations in the fixed field, isothermal rate of relaxation are observed as a function of temperature and as a function of the degree of first-order character. The relaxation rate is reduced in more weakly first-order compositions and is also reduced as the temperature is increased towards Tcrit At temperatures above Tcrit, the metastability of the transition vanishes along with its associated temporal dynamics.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

Second-order magnetic critical points at finite magnetic fields: Revisiting Arrott plots

The so-called Arrott plot, which consists in plotting $H/M$ against ${M}^{2}$, with $H$ the applied magnetic field and $M$ the magnetization, is used to extract valuable information in second-order magnetic phase transitions. Besides, it is widely accepted that a negative slope in the Arrott plot is indicative of a first-order magnetic transition. This is known as the Banerjee criterion. In consequence, the zero-field transition temperature ${T}^{*}$ is reported as the characteristic first-order transition temperature. By carefully analyzing the mean-field Landau model used for studying first-order magnetic transitions, we show in this work that ${T}^{*}$ corresponds in fact to a triple point where three first-order lines meet. More importantly, this analysis reveals the existence of two symmetrical second-order critical points at finite magnetic field $({T}_{c},\ifmmode\pm\else\textpm\fi{}{H}_{c})$. We then show that a modified Arrott plot can be used to obtain information about these second-order critical points. To support this idea we analyze experimental data on ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ and discuss an estimate for the location of the triple point and the second-order critical points.

Zero-field and time-reserval-symmetry-broken topological phase transitions in graphene

We propose a quantum electronic device based on strained graphene nanoribbon. Mechanical strain, internal exchange field and spin-orbit couplings (SOCs) have been exploited as principle parameters to tune physical properties of the device. We predict a remarkable zero-field topological quantum phase transition between the time-reversal-symmetry broken quantum spin hall (QSH) and quantum anomalous hall (QAH) states, which was previously thought to take place only in the presence of finite magnetic field. We illustrate as intrinsic SOC is tuned, how two different helicity edge states located in the opposite edges of the nanoribbon exchange their locations. Our results indicates that pseudomagnetic field induced by the strain could be coupled to the spin degrees of freedom through the SOC responsible for the stability of QSH state. The controllability of this zero-field phase transition with strength and direction of the strain is also demonstrated. Our prediction offers a tempting prospect of strain, electric and magnetic manipulation of the QSH effect.

Critical point scaling of Ising spin glasses in a magnetic field

Critical point scaling in a field $H$ applies for the limits $t\to 0$, (where $t=T/T_c-1$) and $H\to 0$ but with the ratio $R=t/H^{2/\Delta}$ finite. $\Delta$ is a critical exponent of the zero-field transition. We study the replicon correlation length $\xi$ and from it the crossover scaling function $f(R)$ defined via $1/(\xi H^{4/(d+2-\eta)}) \sim f(R)$. We have calculated analytically $f(R)$ for the mean-field limit of the Sherrington-Kirkpatrick model. In dimension d=3 we have determined the exponents and the critical scaling function $f(R)$ within two versions of the Migdal-Kadanoff (MK) renormalization group procedure. One of the MK versions gives results for $f(R)$ in d=3 in reasonable agreement with those of the Monte Carlo simulations at the values of R for which they can be compared. If there were a de Almeida-Thouless (AT) line for $d \le 6$ it would appear as a zero of the function $f(R)$ at some negative value of R, but there is no evidence for such behavior. This is consistent with the arguments that there should be no AT line for $d \le 6$, which we review.

Transport and pinning properties of Ag-doped FeSe0.94

We investigated the superconducting transition and the pinning properties of undoped and Ag-doped FeSe0.94 at magnetic fields up to 14 T. We established that, due to Ag addition, the hexagonal phase formation in melted FeSe0.94 samples is suppressed and the grain connectivity is strongly improved. The obtained superconducting zero-field transition becomes sharp, with a transition width below 1 K. Tc and the upper critical field were found to increase, while the normal-state resistivity was significantly reduced, becoming comparable with that of FeSe single crystals. In addition, a considerable magnetoresistance was observed due to Ag doping. The resistive transition of undoped and Ag-doped FeSe0.94 is dominated by a thermally activated flux flow. From the activation energy U versus H dependence, we found a crossover from single-vortex pinning to a collective-creep pinning behavior by increasing the magnetic field.

Thermodynamics around the first-order ferromagnetic phase transition ofFe2Psingle crystals

The specific heat and thermodynamics of ${\rm Fe}_2{\rm P}$ single-crystals around the first order paramagnetic (PM) to ferromagnetic (FM) phase transition at $T_{\rm C} = 217 \,{\rm K}$ are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived H-T phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the $c$-axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields, the phase transition is split into two, with an intermediate FM phase where there is no spontaneous magnetization along the $c$-axis. The zero-field transition displays a text-book example of a first order transition with different phase stability limits on heating and cooling. The results have special significance since ${\rm Fe}_2{\rm P}$ is the parent material to a family of compounds with outstanding magnetocaloric properties.