Fluctuations Of Magnetic Moments Research Articles

Measuring the Magnetic Dipole Moment and Magnetospheric Fluctuations of SXP 18.3 with a Kalman Filter

The magnetic dipole moment μ of an accretion-powered pulsar in magnetocentrifugal equilibrium cannot be inferred uniquely from time-averaged pulse period and aperiodic X-ray flux data, because the radiative efficiency η 0 of the accretion is unknown, as are the mass, radius, and distance of the star. The degeneracy associated with the radiative efficiency is circumvented if fluctuations of the pulse period and aperiodic X-ray flux are tracked with a Kalman filter, whereupon μ can be measured uniquely up to the uncertainties in the mass, radius, and distance. Here, the Kalman filter analysis is demonstrated successfully in practice for the first time on Rossi X-ray Timing Explorer observations of the X-ray transient SXP 18.3 in the Small Magellanic Cloud (SMC), which is monitored regularly. The analysis yields μ=8.0−1.2+1.3×1030Gcm3 and η0=0.04−0.01+0.02 , compared to μ=5.0−1.0+1.0×1030Gcm3 as inferred traditionally from time-averaged data assuming η 0 = 1. The analysis also yields time-resolved estimates of two hidden state variables, the mass accretion rate and the Maxwell stress at the disk–magnetosphere boundary. The success of the demonstration confirms that the Kalman filter analysis can be applied in the future to study the magnetic moments and disk–magnetosphere physics of accretion-powered pulsar populations in the SMC and elsewhere.

Structure of the normal state and origin of the Schottky anomaly in the correlated heavy-fermion superconductor UTe2

The recently discovered $\mathrm{U}{\mathrm{Te}}_{2}$ superconductor is regarded as a heavy-fermion mixed-valence system with very peculiar properties within the normal and superconducting (SC) states. It shows no signs of magnetic order, but has a strong anisotropy of magnetic susceptibility and SC critical field. In addition to a heavy-fermion-like behavior in the normal state, it exhibits a distinctive Schottky-type anomaly at $\ensuremath{\sim}12$ K and a characteristic excitations gap $\ensuremath{\sim}35\ensuremath{-}40$ K. Here, we show, by virtue of dynamical mean-field theory calculations with a quasiatomic treatment of electron correlations, that ab initio-derived crystal-field splitting of the $5{f}^{2}$ ionic configuration yields agreement with these experimental observations. A close analogy of the normal paramagnetic state of $\mathrm{U}{\mathrm{Te}}_{2}$ to that of $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$ in the Kondo arrest scenario is revealed. We consider the impact of anisotropic superexchange interactions within the U-U structural dimer on magnetization and development of strong orbital and magnetic moment fluctuations in $\mathrm{U}{\mathrm{Te}}_{2}$.

Simulation of thermodynamic properties of magnetic transition metals from an efficient tight-binding model: The case of cobalt and beyond

Atomic scale simulations at finite temperature are an ideal approach to study the thermodynamic properties of magnetic transition metals. However, the development of interatomic potentials explicitly taking into account magnetic variables is a delicate task. In this context, we present a tight-binding model for magnetic transition metals in the Stoner approximation. This potential is integrated into a Monte Carlo structural relaxations code where trials of atomic displacements as well as fluctuations of local magnetic moments are performed to determine the thermodynamic equilibrium state of the considered systems. As an example, the Curie temperature of cobalt is investigated while showing the important role of atomic relaxations. Furthermore, our model is generalized to other transition metals highlighting a local magnetic moment distribution that varies with the gradual filling of the d states. Consequently, the successful validation of the potential for different magnetic configurations indicates its great transferability makes it a good choice for atomistic simulations sampling a large configuration space.

Low-dimensional antiferromagnetic fluctuations in the heavy-fermion paramagnetic ladder compound UTe2

Inelastic-neutron-scattering measurements were performed on a single crystal of the heavy-fermion paramagnet UTe$_2$ above its superconducting temperature. We confirm the presence of antiferromagnetic fluctuations with the incommensurate wavevector $\mathbf{k}_1=(0,0.57,0)$. A quasielastic signal is found, whose momentum-transfer dependence is compatible with fluctuations of magnetic moments $\mu\parallel\mathbf{a}$, with a sine-wave modulation of wavevector $\mathbf{k}_1$ and in-phase moments on the nearest U atoms. Low dimensionality of the magnetic fluctuations, consequence of the ladder structure, is indicated by weak correlations along the direction $\mathbf{c}$. These fluctuations saturate below the temperature $T_1^*\simeq15$~K, in possible relation with anomalies observed in thermodynamic, electrical-transport and nuclear-magnetic-resonance measurements. The absence or weakness of ferromagnetic fluctuations, in our data collected at temperatures down to 2.1 K and energy transfers from 0.6 to 7.5 meV, is emphasized. These results constitute constraints for models of magnetically-mediated superconductivity in UTe$_2$.

Electronic correlation-driven orbital polarization transitions in the orbital-selective Mott compound Ba2CuO4−δ

The electronic states near the Fermi level of recently discovered superconductor ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ consist primarily of the Cu ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals. We investigate the electronic correlation effect and the orbital polarization of an effective two-orbital Hubbard model mimicking the low-energy physics of ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ in the hole-rich regime by utilizing the dynamical mean-field theory with the Lanczos method as the impurity solver. We find that the hole-overdoped ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ with $3{d}^{8}$ (${\mathrm{Cu}}^{3+}$) is in the orbital-selective Mott phase (OSMP) at half-filling, and the typical two-orbital feature remains in ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ when the electron filling approaches ${n}_{e}\ensuremath{\sim}2.5$, which closely approximates to the experimental hole doping for the emergence of the high-${T}_{c}$ superconductivity. We also obtain that the orbital polarization is very stable in the OSMP, and the multiorbital correlation can drive orbital polarization transitions. These results indicate that in hole-overdoped ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ the OSMP physics and orbital polarization, local magnetic moment, and spin or orbital fluctuations still exist. We propose that our present results are also applicable to ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ and other two-orbital cuprates, demanding an unconventional multiorbital superconducting scenario in hole-overdoped high-${T}_{c}$ cuprates.

Modeling of magnetization dynamics and thermal magnetic moment fluctuations in nanoparticle-enhanced magnetic resonance detection

This study presents a systematic numerical modeling investigation of magnetization dynamics and thermal magnetic moment fluctuations of single magnetic domain nanoparticles in a configuration applicable to enhancing inductive magnetic resonance detection signal to noise ratio (SNR). Previous proposals for oriented anisotropic single magnetic domain nanoparticle amplification of magnetic flux in a magnetic resonance imaging (MRI) coil focused only on the coil pick-up voltage signal enhancement. In this study, the numerical evaluation of the SNR has been extended by modeling the inherent thermal magnetic noise introduced into the detection coil by the insertion of such anisotropic nanoparticle-filled coil core. The Landau–Lifshitz–Gilbert equation under the Stoner–Wohlfarth single magnetic domain (macrospin) assumption was utilized to simulate the magnetization dynamics due to AC drive field as well as thermal noise. These simulations are used to evaluate the nanoparticle configurations and shape effects on enhancing SNR. Finally, we explore the effect of narrow band filtering of the broadband magnetic moment thermal fluctuation noise on the SNR. It was observed that for a particular shape of a single nanoparticle, the SNR could be increased up to ∼8 and the choice of an appropriate number of the nanoparticles increases the SNR by several orders of magnitude and could consequently lead to the detectability of a very small field of ∼10 pT. These results could provide an impetus for relatively simple modifications to existing MRI systems for achieving enhanced detection SNR in scanners with modest polarizing magnetic fields.

Development of highly sensitive MI element by controlling anisotropy and noise analysis of MI element

In order to further improve noise performance and achieve higher sensitivity, we have investigated a highly sensitive MI element by controlling anisotropy, and have analyzed noise components of a MI sensor by using a femto-tesla MI sensor system with a synchronized peak-to-peak detector. According to the results, sensitivity of a MI element decreases when tension increases. Meanwhile, sensitivity of a MI element is also linearly proportional to the ratio of impedance change and anisotropy field (∆Z/Hk). The theoretical model shows the same effect as measurement results. Furthermore, the noise of a MI sensor is considered to be mainly due to circuit noise, fluctuation of wire magnetic moment (thermal magnetic noise), and irreversible movement of a domain wall trapped by impurities and scratches on a wire surface (Barkhausen noise). The lowest thermal magnetic noise of amorphous wire calculated in our study is approximately 60 fT. Meanwhile, thermal magnetic noise and magnetic noise due to irreversible movement of a domain wall have been demonstrated as a function of the magnetic anisotropy.

From magnetic order to quantum disorder in the Zn-barlowite series of S\u2009=\u20091/2 kagom\xe9 antiferromagnets

We report a comprehensive muon spectroscopy study of the Zn-barlowite series of S=frac{1}{2} kagomé antiferromagnets, ZnxCu4−x(OH)6FBr, for x = 0.00 to 0.99(1). By combining muon spin relaxation and rotation measurements with state-of-the-art density-functional theory muon-site calculations, we observe the formation of both μ–F and μ–OH complexes in Zn-barlowite. From these stopping sites, implanted muon spins reveal the suppression of long-range magnetic order into a possible quantum spin liquid state upon the increasing concentration of Zn-substitution. In the parent compound (x = 0), static long-range magnetic order below TN = 15 K manifests itself in the form of spontaneous oscillations in the time-dependent muon asymmetry signal consistent with the dipolar fields expected from the calculated muon stopping sites and the previously determined magnetic structure of barlowite. Meanwhile, in the x = 1.0 end-member of the series—in which antiferromagnetic kagomé layers of Cu2+S=frac{1}{2} moments are decoupled by diamagnetic Zn2+ ions—we observe that dynamic magnetic moment fluctuations persist down to at least 50 mK, indicative of a quantum disordered ground state. We demonstrate that this crossover from a static to dynamic magnetic ground state occurs for compositions of Zn-barlowite with x > 0.5, which bears resemblance to the dynamical behaviour of the widely studied Zn-paratacamite series that contains the quantum spin liquid candidate herbertsmithite.

Spin treacle in a frustrated magnet observed with spin current

By means of spin current, the flow of spin angular momentum, we find a regime of "spin treacle" in a frustrated magnetic system. To establish its existence, we have performed spin transport measurements in nanometer-scale spin glasses. At temperatures high enough that the magnetic moments fluctuate at high frequencies, the spin Hall angle, the conversion yield between spin current and charge current, is independent of temperature. The spin Hall angle starts to decrease at a certain temperature $T^{*}$ and completely vanishes at a lower temperature. We argue that the latter corresponds to the spin freezing temperature $T_{\rm f}$ of the nanometer-scale spin glass, where the direction of conduction electron spin is randomized by the exchange coupling with the localized moments. The present experiment \textit{quantitatively} verifies the existence of a distinct "spin treacle" between $T_{\rm f}$ and $T^{*}$. We have also quantified a time scale of fluctuation of local magnetic moments in the spin treacle from the spin relaxation time of conduction electrons.

Magnetic Relaxation in a Viscoelastic Ferrocolloid

A method has been proposed for calculating linear dynamic magnetization of a viscoelastic ferrocolloid in a constant magnetic field (displacement field). The magnetic phase of the colloid consists of Brownian ferromagnetic nanoparticles placed into a Jeffry’s fluid. Therefore, each particle, upon its rotation induced by an alternating (probe) field, dissipates energy via two friction “channels” operating in parallel. The usual (Newtonian) viscosity prevails at short times, while the retarded (Maxwellian) dissipative interaction plays the main role at long times. It has been shown that the retarded friction on the Jeffry’s medium gives rise to a slow magnetization relaxation mode, which must be most pronounced in ferrocolloids having substantial elasticity. As the displacement field is enhanced, this mode weakens and the friction relevant to the Newtonian viscosity becomes prevailing, because it causes small-angle orientational fluctuations of particle magnetic moments. The proposed method yields an exact solution of the model, and the results obtained using method prove that previous approximate calculations are substantially limited.

The ε-AlxFe2-xO3 nanomagnets as MRI contrast agents: Factors influencing transverse relaxivity

Most magnetic materials studied as negative contrast agents for magnetic resonance imaging (MRI) enter the superparamagnetic state with decreasing size of their particles, which is distinguished by thermal fluctuations of particle magnetic moment. In the present study, a novel type of contrast agents based on non-superparamagnetic nanoparticles (nanomagnets) of aluminum-doped epsilon polymorph of Fe2O3 is suggested and used as a model system to elucidate the role of magnetic blocking and other factors in the transverse relaxation of water. Specifically, the dependence of the transverse relaxivity r2 on the magnetic field (0.47–11.75 T), temperature (278−348 K), magnetization, surface modification (silica or citrate) and thickness of silica coating (6−21 nm) is analyzed for the ε-AlxFe2-xO3 nanoparticles with x = 0.23 and the median size of 21 nm. These nanoparticles evinced higher magnetization than the undoped material and were in the blocked state in the temperature range under study. Further, irreversible magnetization processes were revealed by SQUID magnetometry in aqueous suspension of coated clusters of magnetic crystallites that resulted in the field dependence of r2. The temperature dependence of r2 was interpreted by the combination of two different regimes – motional averaging and static dephasing regimes. Slow diffusion of water molecules inside the silica shells was suggested to explain only moderate decrease of r2 with increasing the coating thickness. Moreover, the performance of the contrast agent was demonstrated not only in ultra-high-field MRI at 11.75 T but also by imaging in vivo on a mouse model at 1 T. Finally, preliminary evaluation of cytotoxicity on rat mesenchymal stem cells did not reveal any significant effects. Due to their low toxicity and high relaxivity, ε-AlxFe2-xO3 nanoparticles present a promising starting material for further biomedical applications involving MRI, multimodal contrast agents and theranostic carriers.

Perovskite magnet with quantum mechanical glassiness

Magnetic materials of strongly correlated origin are at the forefront of research in the exploration of new quantum mechanical properties involving spin and charges. Quantum magnetic material with certain intrinsic functionalities, such as continuously fluctuating spins to the lowest accessible temperature - reminiscing a liquid pattern, is argued to form a new phase of matter. Envisaging a phenomenon where the continuously fluctuating spins are also randomly frozen in space, can diminish the distinction between the two opposite extremes of quantum and classical magnetism. Here, we report new evidence in this regard where the coexistence of quantum spin continuum with a spin glass order in Co-doped CaRuO3 perovskite is demonstrated. In experimental investigations of Co-doped CaRuO3 perovskite, we find that (a) a continuum spectrum in the energy-momentum space, due to the uncorrelated spin fluctuations, persists across the weak spin glass transition at TG≃ 23 K and (b) the quantum fluctuation of magnetic moment is spatially confined to individual sites only, thereby making it an extremely local event. Thus, the fluctuating spins, at a given time, are also randomly frozen macroscopically. The experimental observations suggest the discovery of a new magnetic phase at the cross-road of classical and quantum physics in strongly correlated perovskite materials.

Intrinsic Low-Temperature Magnetism in SmB_{6}.

By means of new muon spin relaxation experiments, we disentangle extrinsic and intrinsic sources of low-temperature bulk magnetism in the candidate topological Kondo insulator (TKI) SmB_{6}. Results on Al-flux-grown SmB_{6} single crystals are compared to those on a large floating-zone-grown ^{154}Sm ^{11}B_{6} single crystal in which a 14meV bulk spin exciton has been detected by inelastic neutron scattering. Below ∼10 K, we detect the gradual development of quasistatic magnetism due to rare-earth impurities and Sm vacancies. Our measurements also reveal two additional forms of intrinsic magnetism: (1)underlying low-energy (∼100 meV) weak magnetic moment (∼10^{-2} μ_{B}) fluctuations similar to those detected in the related candidate TKI YbB_{12} that persist down to millikelvin temperatures, and (2)magnetic fluctuations consistent with a 2.6meV bulk magnetic excitation at zero magnetic field that appears to hinder surface conductivity above ∼4.5 K. We discuss potential origins of the magnetism.

Triple-Kagomé-Layer Slabs of Mixed-Valence Rare-Earth Ions Exhibiting Quantum Spin Liquid Behaviors: Synthesis and Characterization of Eu9MgS2B20O41.

We prepared a new rare-earth compound, Eu9MgS2B20O41 (EMSBO), and characterized its structural and physical properties. EMSBO consists of triple-Kagomé-layer slabs separated by nonmagnetic ions and groups. Within each slab, intervalence charge transfer has been found to occur between the Eu2+ and Eu3+ Kagomé layers, a new channel for quantum fluctuation of magnetic moments. The measured magnetic susceptibilities and the specific heat capacity exhibit very similar features characteristic of quantum spin liquid behaviors observed in other materials.

Structural, optical and magnetic properties of Tm3+ substituted cobalt spinel ferrites synthesized via sonochemical approach

Co-Tm nano-spinel ferrite with chemical formula CoTmxFe2−xO4 (0.0 ≤ x ≤ 0.08) NPs were prepared via sonochemical approach. X-ray powder diffraction patterns, microscopic images (SEM and TEM) and infrared spectra proved the formation of Co spinel ferrite. The effect of Tm3+ substituted on spinal structure was evaluated by lattice parameters, tetrahedral and octahedral bond length and cationic distribution. The band gap energy (Eg) of samples were estimated by performing UV–Vis percent diffuse reflectance (% DR) and applying the Kubelka-Munk theory. Eg values are in an interval between 1.33 eV and 1.64 eV. The analyses of magnetization were performed at room (300 K; RT) and low (10 K) temperatures. Different magnetic parameters including coercivity Hc, saturation magnetization Ms, remanence Mr, squareness ratio (SQR = Mr/Ms) and magnetic moment nB were deduced and discussed. The results showed superparamagnetic (SPM) nature at RT for x = 0.00 and 0.02 samples. However, the other products exhibit ferromagnetic (FM) nature. At 10 K, all synthesized NPs display FM behavior. An amazing increase in the magnitudes of Ms, Mr and Hc was observed at 10 K in comparison to RT, which is principally due to the reduced thermal fluctuations of magnetic moments at lower temperatures. The Tm3+ substitution affects considerably the magnetizations data. An enhancement in the Ms, Mr, and nB was detected on increasing the Tm3+ concentration. The SQR values at RT are found to be smaller than 0.5 postulating a single domain nature with uniaxial anisotropy for all produced ferrites. However, SQRs are in the range 0.66–0.76 at 10 K, suggesting the multi magnetic domain at low temperature, except the x = 0.02 product where the SQR = 0.47 indicating the single magnetic domain. The obtained magnetic results were investigated deeply with relation to structural and microstructural properties.

Thermodynamic stability of magnetic states of monovacancy in graphene revealed by ab initio molecular dynamics simulations

The stability of magnetic states is essential for potential spintronic applications. Here we report on the thermal stability of magnetic states of monovacancy graphene using ab initio molecular dynamics simulations. At room temperature, thermal fluctuations of the graphene lattice induce a rapid magnetic switching between two states with a high and low magnetic moment, indicating that due to the instability of the atomic structure of the vacancy, the associated magnetic moment is thermodynamically unstable. Lowering the temperature can significantly reduce the rate of the switching process and enhance the resident time on the high magnetic state. It stabilizes in the high magnetic state at as low as 30 K. Analyzing the atomic trajectories and the instant electronic structures confirms that these two magnetic states in MD simulations correspond to the magnetic and nonmagnetic states reported in the literatures. Such fluctuations of local magnetic moments are associated with the vertical displacement of the carbon atoms with the unsaturated dangling bond. This study reveals the dynamical correlation between atomic movement and the magnetic switching, and a comprehensive picture of vacancy magnetism in graphene. It has implications in graphene based spintronic devices.

Anomalous freezing of dielectric polarons near magnetic ordering in multiferroic La0.5Bi0.5FeO3

A single-phase multiferroic (MF) La0.5Bi0.5FeO3 (LBFO) ceramic sample was prepared by the conventional solid-state reaction method. The crystallographic phase formation and morphological studies were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The temperature-dependent magnetization exhibits weak ferromagnetic ordering near TC = 220 K. Complex dielectric and impedance properties of LBFO as a function of temperature and frequency were performed to understand the correlation between magnetic and dielectric properties. The analysis of electrical modulus spectrum reveals that the magnetic ordering imposes a change in the activation energy of dielectric polarons. The thermal fluctuations of magnetic moments activate the freezing of dielectric polarons with enhancement in temperature.

Temperature dependent electronic transport in concentrated solid solutions of the 3d -transition metals Ni, Fe, Co and Cr from first principles

An approach previously developed for the calculation of transport coefficients via the Mott relations is applied to the calculation of finite temperature transport properties of disordered alloys---electrical resistivity and the electronic part of thermal conductivity. The coherent-potential approximation is used to treat chemical disorder as well as other sources of electron scattering, i.e., temperature induced magnetic moment fluctuations and lattice vibrations via the alloy analogy model. This approach, which treats all forms of disorder on an equal first-principles footing, is applied to the calculation of transport properties of a series of fcc concentrated solid solutions of the $3d$-transition metals Ni, Fe, Co, and Cr. For the nonmagnetic alloys ${\mathrm{Ni}}_{0.8}{\mathrm{Cr}}_{0.2}$ and ${\mathrm{Ni}}_{0.33}{\mathrm{Co}}_{0.33}{\mathrm{Cr}}_{0.3}$, the combined effects of chemical disorder and electron-lattice vibrations scattering result in a monotonic increase in the resistivity as a function of temperature from an already large, $T=0$, residual resistivity. For magnetic ${\mathrm{Ni}}_{0.5}{\mathrm{Co}}_{0.5},{\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_{0.5}$, and ${\mathrm{Ni}}_{0.33}{\mathrm{Fe}}_{0.33}{\mathrm{Co}}_{0.33}$, the residual resistivity of which is small, additional electron scattering from temperature induced magnetic moment fluctuations results in a further rapid increase of the resistivity as a function of temperature. The electronic part of the thermal conductivity in nonmagnetic ${\mathrm{Ni}}_{0.8}{\mathrm{Cr}}_{0.2}$ and ${\mathrm{Ni}}_{0.33}{\mathrm{Co}}_{0.33}{\mathrm{Cr}}_{0.33}$ monotonically increases with temperature. This behavior is a result of the competition between a reduction in the conductivity due to electron-lattice vibrations scattering and temperature induced increase in the number of carriers. In the magnetic alloys, electron scattering from magnetic fluctuations leads to an initial rapid decrease in thermal conductivity until this is overcome by an increasing number of carriers at temperatures slightly below the Curie temperature. Similar to the resistivity above ${T}_{C}$, the electronic parts of the thermal conductivities are close to each other in all alloys studied.

Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu2Si2 versus YbRh2Si2

ABSTRACTIn this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu2Si2, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu2Si2. In YbRh2Si2, superconductivity appears to be suppressed at T ⪆ 10 mK by AF order (TN = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at TA slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at Tc = 2 mK. Like the pressure – induced QCP in CeRhIn5, the magnetic field – induced one in YbRh2Si2 is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-T unconventional heavy – fermion superconductors and other families of unconventional superconductors with higher Tcs, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.

Universal fluctuations in orbital diamagnetism

Bohr–van Leuween theorem has attracted the notice of physicists for more than 100 years. The theorem states about the absence of magnetisation in classical systems in thermal equilibrium. In this paper, we discuss about fluctuations of magnetic moment in classical systems. In recent years, this topic has been investigated intensively and it is not free from controversy. We have considered a system consisting of a single particle moving in a plane. A magnetic field is applied perpendicular to the plane. The system is in contact with a thermal bath. We have considered three cases: (a) particle moving in a homogeneous medium, (b) particle moving in a medium with space-dependent friction and (c) particle moving in a medium with space-dependent temperature. For all the three cases, the average magnetic moment and fluctuations in magnetic moment have been calculated. Average magnetic moment saturates to a finite value in the case of free particle but goes to zero when the particle is confined by a 2D harmonic potential. Fluctuations in magnetic moment shows universal features in the presence of arbitrary friction inhomogeneity. For this case, the system reaches equilibrium asymptotically. In the case of space-dependent temperature profile, the stationary distribution is non-Gibbsian and fluctuations deviate from universal value for the bounded system only.