Itinerant Antiferromagnetism Research Articles

Phenomenological model of longitudinal spin fluctuations in itinerant antiferromagnets

We present the phenomenological analysis of the spectrum of longitudinal spin fluctuations in isotropic itinerant electron antiferromagnets with account of spin anharmonicity giving rise to coupling of transverse and longitudinal normal modes. The spectrum consists of a quasielastic part forming a central peak or a dip, depending on temperature and the Landau relaxation rate. Effects of spin fluctuation coupling also give rise to an inelastic part of the spectrum which has a form of resonances or antiresonances near the magnon frequencies related to non-propagating longitudinal excitations.

Quasiaverages, symmetry breaking and irreducible Green functions method

The development and applications of the method of quasiaverages to quantum statistical physics and to quantum solid state theory and, in particular, to quantum theory of magnetism, were considered. It was shown that the role of symmetry (and the breaking of symmetries) in combination with the degeneracy of the system was reanalyzed and essentially clarified within the framework of the method of quasiaverages. The problem of finding the ferromagnetic, antiferromagnetic and superconducting “symmetry broken” solutions of the correlated lattice fermion models was discussed within the irreducible Green functions method. A unified scheme for the construction of generalized mean fields (elastic scattering corrections) and self-energy (inelastic scattering) in terms of the equations of motion and Dyson equation was generalized in order to include the “source fields”. This approach complements previous studies of microscopic theory of antiferromagnetism and clarifies the concepts of Neel sublattices for localized and itinerant antiferromagnetism and “spin-aligning fields” of correlated lattice fermions.

Spin excitations in the excitonic spin-density-wave state of the iron pnictides

Motivated by the iron pnictides, we examine the spin excitations in an itinerant antiferromagnet where a spin-density wave (SDW) originates from an excitonic instability of nested electronlike and holelike Fermi pockets. Using the random-phase approximation, we derive the Dyson equation for the transverse susceptibility in the excitonic SDW state. The Dyson equation is solved for two different two-band models, describing an antiferromagnetic insulator and metal, respectively. We determine the collective spin-wave dispersions and also consider the single-particle continua. The results for the excitonic models are compared with each other and also contrasted with the well-known SDW state of the Hubbard model. Despite the qualitatively different SDW states in the two excitonic models, their magnetic response shows many similarities. We conclude with a discussion of the relevance of the excitonic SDW scenario to the iron pnictides.

Observation of Josephson coupling through an interlayer of antiferromagnetically ordered chromium

The supercurrent transport in metallic Josephson tunnel junctions with an additional interlayer made up by chromium, being an itinerant antiferromagnet, was studied. Uniform Josephson coupling was observed as a function of the magnetic field. The supercurrent shows a weak dependence on the interlayer thickness for thin chromium layers and decays exponentially for thicker films. The diffusion constant and the coherence length in the antiferromagnet were estimated. The antiferromagnetic state of the barrier was indirectly verified using reference samples. Our results are compared to macroscopic and microscopic models.

Crossover from non-Fermi liquid to Fermi liquid behavior close to a quantum critical point: A brief review

Abstract The nesting of the Fermi surfaces of an electron and a hole pocket separated by a nesting vector Q and the interaction between electrons gives rise to itinerant antiferromagnetism. The order can gradually be suppressed by mismatching the nesting and a quantum critical point is obtained as the Neel temperature tends to zero. We review our results on the specific heat, the quasi-particle linewidth, the electrical resistivity, the amplitudes of de Haas–van Alphen oscillations and the dynamical spin susceptibility.

Breakdown of the Bardeen–Cooper–Schrieffer ground state at a quantum phase transition

Advances in solid-state and atomic physics are exposing the hidden relationships between conventional and exotic states of quantum matter. Prominent examples include the discovery of exotic superconductivity proximate to conventional spin and charge order, and the crossover from long-range phase order to preformed pairs achieved in gases of cold fermions and inferred for copper oxide superconductors. The unifying theme is that incompatible ground states can be connected by quantum phase transitions. Quantum fluctuations about the transition are manifestations of the competition between qualitatively distinct organizing principles, such as a long-wavelength density wave and a short-coherence-length condensate. They may even give rise to 'protected' phases, like fluctuation-mediated superconductivity that survives only in the vicinity of an antiferromagnetic quantum critical point. However, few model systems that demonstrate continuous quantum phase transitions have been identified, and the complex nature of many systems of interest hinders efforts to more fully understand correlations and fluctuations near a zero-temperature instability. Here we report the suppression of magnetism by hydrostatic pressure in elemental chromium, a simple cubic metal that demonstrates a subtle form of itinerant antiferromagnetism formally equivalent to the Bardeen-Cooper-Schrieffer (BCS) state in conventional superconductors. By directly measuring the associated charge order in a diamond anvil cell at low temperatures, we find a phase transition at pressures of approximately 10 GPa driven by fluctuations that destroy the BCS-like state but preserve the strong magnetic interaction between itinerant electrons and holes. Chromium is unique among stoichiometric magnetic metals studied so far in that the quantum phase transition is continuous, allowing experimental access to the quantum singularity and a direct probe of the competition between conventional and exotic order in a theoretically tractable material.

Itinerant antiferromagnetism inBaCr2As2: Experimental characterization and electronic structure calculations

We report single-crystal synthesis, specific-heat and resistivity measurements and electronic structure calculations for ${\text{BaCr}}_{2}{\text{As}}_{2}$. This material is a metal with itinerant antiferromagnetism, similar to the parent phases of Fe-based high-temperature superconductors, but differs in magnetic order. Comparison of bare band-structure density of states and the low-temperature specific heat implies a mass renormalization of $\ensuremath{\sim}2$. ${\text{BaCr}}_{2}{\text{As}}_{2}$ shows stronger transition-metal--pnictogen covalency than the Fe compounds, and in this respect is more similar to ${\text{BaMn}}_{2}{\text{As}}_{2}$. This provides an explanation for the observation that Ni and Co doping is effective in the Fe-based superconductors, but Cr or Mn doping is not.

Quantum Phase Transition in the Itinerant Antiferromagnet(V0.9Ti0.1)2O3

Quantum-critical behavior of the itinerant electron antiferromagnet (V0.9Ti0.1)2O3 has been studied by single-crystal neutron scattering. By directly observing antiferromagnetic spin fluctuations in the paramagnetic phase, we have shown that the characteristic energy depends on temperature as c1 +c2T3/2, where c1 and c2 are constants. This T3/2 dependence demonstrates that the present strongly correlated d-electron antiferromagnet clearly shows the criticality of the spin-density-wave quantum phase transition in three space dimensions.

Evolution from Itinerant Antiferromagnet to Unconventional Superconductor with Fluorine Doping in LaFeAs(O1-xFx) Revealed by 75As and 139La Nuclear Magnetic Resonance

We report experimental results of 75 As and 139 La nuclear magnetic resonance (NMR) in the iron-based layered LaFeAs(O 1- x F x ) ( x = 0.0, 0.04, and 0.11). In the undoped LaFeAsO, 1/ T 1 of 139 La exhibits a distinct peak at T N ∼142 K below which the spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the 4% F-doped sample, 1/ T 1 T exhibits a Curie–Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the 11% F-doped sample, in contrast, pseudogap behavior is observed in 1/ T 1 T both at the 75 As and 139 La site with a gap value of Δ PG ∼172 K. The spin dynamics vary markedly with F doping, which is ascribed to the Fermi-surface structure. As for the superconducting properties for the 4 and 11% F-doped samples, 1/ T 1 in both compounds does not exhibit a coherence peak just below T c and follows a T 3 dependence at low temperatures, which suggests unconventional super...

Chromium at high pressures: Weak coupling and strong fluctuations in an itinerant antiferromagnet

The spin- and charge-density-wave order parameters of the itinerant antiferromagnet chromium are directly measured with nonresonant x-ray diffraction as the system is driven toward its quantum critical point with high pressure using a diamond anvil cell. The exponential decrease of the spin and charge diffraction intensities with pressure confirms the harmonic scaling of spin and charge, while the evolution of the incommensurate ordering vector provides important insight into the difference between pressure and chemical doping as means of driving quantum phase transitions. Measurement of the charge density wave over more than two orders of magnitude of diffraction intensity provides the clearest demonstration to date of a weakly coupled BCS-type ground state. Evidence for the coexistence of this weakly coupled ground state with high-energy excitations and pseudogap formation above the ordering temperature in chromium, the charge-ordered perovskite manganites, and the blue bronzes, among other such systems, raises fundamental questions about the distinctions between weak and strong coupling.

De Haas–van Alphen oscillations close to a quantum critical point

The amplitudes of de Haas–van Alphen oscillations are calculated for a system of heavy fermions close to a quantum critical point (QCP). The Lifshitz-Kosevich expression is modified by the quasiparticle self-energy, which has been evaluated previously. The model consists of nested Fermi surfaces of an electron pocket and a hole pocket separated by a wave vector Q, which give rise to itinerant antiferromagnetism for a repulsive interaction between the particles. The order is gradually suppressed by mismatching the nesting and a QCP is obtained as TN tends to zero.

Fincher-Burke spin excitations andω∕Tscaling in insulatingLa1.95Sr0.05CuO4

Insulating ${\mathrm{La}}_{1.95}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_{4}$ shares with superconducting cuprates the same Fincher-Burke spin excitations, which usually are observed in itinerant antiferromagnets. The local spectral function satisfies $\ensuremath{\omega}∕T$ scaling above $\ensuremath{\sim}16\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for this incommensurate insulating cuprate, together with previous results in commensurate insulating and incommensurate superconducting cuprates, further supporting the general prediction for square-lattice quantum spin $S=1∕2$ systems. The width of incommensurate peaks in ${\mathrm{La}}_{1.95}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_{4}$ scales to a similar finite value as at optimal doping, strongly suggesting that they are similarly distant from a quantum critical point. They might both be limited to a finite correlation length by partial spin-glass freezing.

Pressure-Tuned Spin and Charge Ordering in an Itinerant Antiferromagnet

Elemental chromium orders antiferromagnetically near room temperature, but the ordering temperature can be driven to zero by applying large pressures. We combine diamond anvil cell and synchrotron x-ray diffraction techniques to measure directly the spin and charge order in the pure metal at the approach to its quantum critical point. Both spin and charge order are suppressed exponentially with pressure, well beyond the region where disorder cuts off such a simple evolution, and they maintain a harmonic scaling relationship over decades in scattering intensity. By comparing the development of the order parameter with that of the magnetic wave vector, it is possible to ascribe the destruction of antiferromagnetism to the growth in electron kinetic energy relative to the underlying magnetic exchange interaction.

Anisotropic spin fluctuations and anomalies of nuclear quadrupole interactions in the itinerant antiferromagnetNpCoGa5:Co59NMR andGa69,71NMR/NQR studies

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments have been performed on a single crystal of tetragonal $\mathrm{Np}\mathrm{Co}{\mathrm{Ga}}_{5}$, an itinerant antiferromagnet with a N\'eel temperature ${T}_{N}=47\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The antiferromagnetic phase is inverted to a field-induced ferromagnetic (FIF) phase with an applied field $({H}_{0})$ above ${H}_{m}$ $(T\ensuremath{\rightarrow}0)$ $\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ oriented along the $c$ axis. NMR spectra have been measured above and below ${T}_{N}$ with ${H}_{0}\ensuremath{\Vert}c$ and $a$ axes and have been assigned to $^{69,71}\mathrm{Ga}$ nuclei on two crystallographically inequivalent $1c$ and $4i$ sites and to $^{59}\mathrm{Co}$ nuclei on the $1b$ site. Using second-order perturbation calculations, Knight shift $(K)$, electric field gradient (EFG), frequency $({\ensuremath{\nu}}_{\mathrm{Q}})$, and asymmetry parameter $(\ensuremath{\eta})$ of the EFG are deduced for each site. These parameters for the $^{69}\mathrm{Ga}(1c)$ and $^{69,71}\mathrm{Ga}(4i)$ sites are confirmed by NQR measurements in zero field. The Knight shifts obtained in the paramagnetic (PM) state obey a Curie-Weiss law, which scales with the bulk susceptibility $(\ensuremath{\chi})$. Hyperfine tensors for each site are deduced from $K\ensuremath{-}\ensuremath{\chi}$ plots with temperature as an implicit parameter. Antiferromagnetic NMR spectra in zero field were also observed, finding an internal field of $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ at the $\mathrm{Ga}(4i)$ site at the lowest temperature. The ordered moment can be estimated from this to be $0.81\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}∕\mathrm{Np}$. The nuclear quadrupolar parameters (${\ensuremath{\nu}}_{\mathrm{Q}}$ and $\ensuremath{\eta}$) are found to exhibit an anomaly just below ${T}_{N}$ in the FIF phase. ${T}_{1}$ and ${T}_{2}$ have been measured for each site. For ${H}_{0}\ensuremath{\Vert}c$, ${T}_{1}\ensuremath{\sim}\text{constant}$ behavior suggests localized $5f$ character for $T>100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and itinerant $(1∕{T}_{1}\ensuremath{\propto}T)$ behavior for ${T}_{N}<T<\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $\mathrm{Np}\mathrm{Co}{\mathrm{Ga}}_{5}$. ${T}_{2}$ measurements in the case of ${H}_{0}\ensuremath{\Vert}c$ clearly define a phase crossover between PM and FIF phases. A sharp anisotropy for spin fluctuations in $\mathrm{Np}\mathrm{Co}{\mathrm{Ga}}_{5}$ has also been demonstrated.

Rational Synthetic Tuning between Itinerant Antiferromagnetism and Ferromagnetism in the Complex Boride Series Sc2FeRu5−nRhnB2 (0≤n≤5)

Single crystals of the complex boride series Sc(2)FeRu(5-n)Rh(n)B(2) (n=1, 3, 4) were synthesized by arc-melting the elements in water-cooled copper crucibles under argon atmospheres and were chemically characterized by single-crystal XRD and EDX analyses. The new compounds are isotypic and crystallize in the tetragonal space group P4/mbm with Z=2, adopting a substitutional variant of the Ti(3)Co(5)B(2)-type structure. The magnetically active iron atoms are arranged in chains with intra- and interchain distances of about 3.02 and 6.60 A, respectively. Strong ferromagnetic interactions are observed for both Sc(2)FeRuRh(4)B(2) (64 valence electrons (VE), TC approximately 350 K, mu(a)=3.1 mu(B)) and Sc(2)FeRu(2)Rh(3)B(2) (63 VE, T(C) approximately 300 K, mu(a)=3.0 mu(B)), whereas antiferromagnetic interactions are found in the case of Sc(2)FeRu(4)RhB(2) (61 VE, T(N) approximately 10 K, mu(eff)=3.2): The magnetism of the entire Sc(2)FeRu(5-n)Rh(n)B(2) (0<or=n<or=5) series is discussed based on the variable valence-electron count and the resulting Fe-Fe bonding character, and it is also compared with other isoelectronic systems.

Dynamical spin-susceptibility of itinerant heavy electrons with nested Fermi surface

Heavy fermion systems frequently display non-Fermi liquid behavior due to a nearby quantum critical point. A nested Fermi surface together with the remaining interaction between the carriers after the heavy particles are formed may give rise to itinerant antiferromagnetism. The model under consideration consists of an electron pocket and a hole pocket, separated by a wave vector $\mathbf{Q}$ and Fermi momenta ${k}_{F1}$ and ${k}_{F2}$, respectively. The order can gradually be suppressed by increasing the mismatch of the Fermi momenta, defined as $2\ensuremath{\delta}={v}_{F}\ensuremath{\mid}{k}_{F1}\ensuremath{-}{k}_{F2}\ensuremath{\mid}$, and a quantum critical point is obtained as ${T}_{N}\ensuremath{\rightarrow}0$ at a mismatch value ${\ensuremath{\delta}}_{0}$. The dynamical spin susceptibility, ${\ensuremath{\chi}}^{\ensuremath{''}}(\ensuremath{\omega},\mathbf{Q}+\mathbf{q})$, which is of relevance to inelastic neutron scattering, is calculated. The mismatch of the Fermi vectors, for $\ensuremath{\delta}\ensuremath{\geqslant}{\ensuremath{\delta}}_{0}$, has the following consequences: (i) For the tuned quantum critical point (QCP), the specific heat $\ensuremath{\gamma}$ coefficient and the magnetic susceptibility increase with the logarithm of the temperature as $T$ is lowered. (ii) For the tuned QCP the linewidth of the quasiparticles is sublinear in $T$ and $\ensuremath{\omega}$. (iii) The specific heat and the linewidth display a crossover from non-Fermi liquid $(\ensuremath{\sim}T)$ to Fermi liquid $(\ensuremath{\sim}{T}^{2})$ behavior with increasing nesting mismatch and decreasing temperature. (iv) For the tuned QCP, the dynamical susceptibility has a quasielastic peak with a linewidth proportional to $T$. For small ${v}_{F}q$, the central peak becomes inelastic. (v) For noncritical Fermi vector mismatch, the peak is inelastic. (vi) In all cases for large ${v}_{F}q$, a broad quasielasticlike peak develops. (vii) While the specific heat, the homogeneous susceptibility, and the quasiparticle linewidth are only weakly dependent on the geometry of the nested Fermi surfaces, the momentum-dependent dynamical susceptibility is expected to be affected by the shape of the Fermi surface.

Strongly enhanced thermal expansion of low-dimensional itinerant weak antiferromagnets near the quantum critical points

The thermal expansion of low-dimensional itinerant antiferromagnetic metals is discussed along with the recent study on the magneto-volume effect by Takahashi and Nakano [J. Phys.: Condens. Matter 16, 4505 (2004)]. In this study, we find that the thermal volume expansion derived by Takahashi and Nakano shows a stronger enhancement of the T-linear slope of the thermal expansion coefficient proportional to y−1∕2 and ln(1∕y), respectively, for one-dimensional and two-dimensional (2D) systems than that of three dimensions as we approach the magnetic instability point (y→0), where y is the inverse of the reduced staggered magnetic susceptibility. On the other hand, the same coefficient for the conventional term by Moriya and Usami [Solid State Commun. 34, 95 (1980)] is suppressed proportional to 1∕ln(1∕y) for 2D systems. The present result suggests that the temperature dependence of low-dimensional weak antiferromagnets is generally dominated by the strongly enhanced T2-linear thermal expansion from the magnetic origin.

Dynamical spin response of itinerant electrons close to a quantum critical point

The dynamical spin susceptibility χ″(ω,Q), which is of relevance to inelastic neutron scattering, is calculated for an itinerant antiferromagnet (AF) close to a quantum critical point (QCP). The model consists of nested Fermi surfaces of an electron pocket and a hole pocket separated by a wave vector Q, which give rise to itinerant AF for a repulsive interaction between the carriers. The order can gradually be suppressed by mismatching the nesting and a QCP is obtained as TN→0. For the tuned QCP a quasielastic peak in χ″(ω,Q) with a linewidth proportional to T is obtained.

Anomalies in the dynamical resistivity due to a quantum critical point

Non-Fermi-liquid (NFL) properties are observed in many heavy fermion systems and frequently attributed to a nearby quantum critical point (QCP). A QCP can arise by suppressing the transition temperature ${T}_{N}$ of an antiferromagnetic phase to zero, e.g., by chemical or applied pressure or an external magnetic field. We consider a QCP arising from the nesting of Fermi surfaces of an electron pocket and a hole pocket separated by a wave vector $\mathbf{Q}$. The nesting together with the remaining interaction between the carriers after the heavy particles are formed give rise to itinerant antiferromagnetism. The order can gradually be suppressed by mismatching the nesting and a QCP is obtained as ${T}_{N}\ensuremath{\rightarrow}0$. The NFL contribution to the dynamical resistivity is calculated using the Kubo formula and a memory function formalism. The mismatched nesting has the following consequences. (i) The specific heat $\ensuremath{\gamma}$ coefficient and the magnetic susceptibility increase with the logarithm of the temperature as $T$ is lowered. (ii) For the tuned QCP the linewidth of the quasiparticles is sublinear in $T$ and $\ensuremath{\omega}$. (iii) The specific heat and the linewidth display a crossover from non-Fermi-liquid $(\ensuremath{\sim}T)$ to Fermi-liquid $(\ensuremath{\sim}{T}^{2})$ behavior with increasing nesting mismatch and decreasing temperature. (iv) For the tuned QCP the electrical resistivity acquires a quasilinear temperature dependence. This behavior is exponentially suppressed at very low $T$ if the QCP is not tuned. (v) For the tuned QCP and at low $T$ the dynamical conductivity deviates from the usual Drude behavior.

Thermal expansion of itinerant nearly and weakly antiferromagnetic materials

Magnetic thermal expansions of itinerant electron antiferromagnets are discussed along the Grüneisen's approach to the thermal expansions due to lattice vibrations. We show that the thermal expansion consists of two terms, ω 0 ( T ) , that corresponds to the Moriya–Usami's result, and a new term ω t ( T ) derived for ferromagnets recently. Comparison of these terms shows that the second term becomes dominant at low temperatures around the magnetic instability point, leading to the T 2 -linear behavior there for nearly antiferromagnetic compounds.