Cascade of Magnetic-Field-Induced Lifshitz Transitions in the Ferromagnetic Kondo Lattice Material YbNi_{4}P_{2}.

A ferromagnetic quantum critical point is thought not to exist in two- and three-dimensional metallic systems yet is realized in the Kondo lattice compound YbNi_{4}(P,As)_{2}, possibly due to its one-dimensionality. It is crucial to investigate the dimensionality of the Fermi surface of YbNi_{4}P_{2} experimentally, but common probes such as angle-resolved photoemission spectroscopy and quantum oscillation measurements are lacking. Here, we study the magnetic-field dependence of transport and thermodynamic properties of YbNi_{4}P_{2}. The Kondo effect is continuously suppressed, and additionally we identify nine Lifshitz transitions between 0.4 and 18 T. We analyze the transport coefficients in detail and identify the type of Lifshitz transitions as neck or void type to gain information on the Fermi surface of YbNi_{4}P_{2}. The large number of Lifshitz transitions observed within this small energy window is unprecedented and results from the particular flat renormalized band structure with strong 4f-electron character shaped by the Kondo lattice effect.

Localised Ag(+) vibrations at the origin of ultralow thermal conductivity in layered thermoelectric AgCrSe2.

In materials science, the substructure approach consists in imagining complex materials in which a particular property is associated with a distinct structural feature, so as to combine different chosen physical characteristics, which otherwise have little chance to coexist. Applied to thermoelectric materials, it has been used to achieve simultaneously phonon-glass and electron-crystal properties. Mostly studied for its superionic conductivity, AgCrSe2 is a naturally layered compound, which achieves very low thermal conductivity, ~0.4 W.K(-1).m(-1) at RT (room temperature), and is considered a promising thermoelectric. The Cr atoms of the [CrSe2]∞ layer bear a spin S = 3/2, which orders below TN = 55 K. Here we report low temperature inelastic neutron scattering experiments on AgCrSe2, alongside the magnetic field evolution of its thermal and electrical transport. We observe a very low frequency mode at 3 meV, ascribed to large anharmonic displacements of the Ag(+) ions in the [Ag]∞ layer, and 2D magnetic fluctuations up to 3 TN in the chromium layer. The low thermal conductivity of AgCrSe2 is attributed to acoustic phonon scattering by a regular lattice of Ag(+) oscillating in quasi-2D potential wells. These findings highlight a new way to achieve localised phonon modes in a perfectly crystalline solid.

Searching for new thermoelectric materials: some examples among oxides, sulfides and selenides.

Different families of thermoelectric materials have been investigated since the discovery of thermoelectric effects in the mid-19th century, materials mostly belonging to the family of degenerate semi-conductors. In the last 20 years, new thermoelectric materials have been investigated following different theoretical proposals, showing that nanostructuration, electronic correlations and complex crystallographic structures (low dimensional structures, large number of atoms per lattice, presence of 'rattlers'…) could enhance the thermoelectric properties by enhancing the Seebeck coefficient and/or reducing the thermal conductivity. In this review, the different strategies used to optimize the thermoelectric properties of oxides and chalcogenides will be presented, starting with a review on thermoelectric oxides. The thermoelectric properties of sulfides and selenides will then be discussed, focusing on layered materials and low dimensional structures (TiS2 and pseudo-hollandites). Some sulfides with promising ZT values will also be presented (tetrahedrites and chalcopyrites).

Invited article: A round robin test of the uncertainty on the measurement of the thermoelectric dimensionless figure of merit of Co0.97Ni0.03Sb3.

A round robin test aiming at measuring the high-temperature thermoelectric properties was carried out by a group of European (mainly French) laboratories (labs). Polycrystalline skutterudite Co0.97Ni0.03Sb3 was characterized by Seebeck coefficient (8 labs), electrical resistivity (9 labs), thermal diffusivity (6 labs), mass volume density (6 labs), and specific heat (6 labs) measurements. These data were statistically processed to determine the uncertainty on all these measured quantities as a function of temperature and combined to obtain an overall uncertainty on the thermal conductivity (product of thermal diffusivity by density and by specific heat) and on the thermoelectric figure of merit ZT. An increase with temperature of all these uncertainties is observed, in agreement with growing difficulties to measure these quantities when temperature increases. The uncertainties on the electrical resistivity and thermal diffusivity are most likely dominated by the uncertainty on the sample dimensions. The temperature-averaged (300-700 K) relative standard uncertainties at the confidence level of 68% amount to 6%, 8%, 11%, and 19% for the Seebeck coefficient, electrical resistivity, thermal conductivity, and figure of merit ZT, respectively. Thermal conductivity measurements appear as the least accurate. The moderate value of the temperature-averaged relative expanded (confidence level of 95%) uncertainty of 17% on the mean of ZT is essential in establishing Co0.97Ni0.03Sb3 as a high temperature standard n-type thermoelectric material.

Thermodynamic evidence for valley-dependent density of states in bulk bismuth.

Electron-like carriers in bismuth are described by the Dirac Hamiltonian, with a band mass becoming a thousandth of the bare electron mass along one crystalline axis. The existence of three anisotropic valleys offers electrons an additional degree of freedom, a subject of recent attention. Here, we map the Landau spectrum by angle-resolved magnetostriction, and quantify the carrier number in each valley: while the electron valleys keep identical spectra, they substantially differ in their density of states at the Fermi level. Thus, the electron fluid does not keep the rotational symmetry of the lattice at low temperature and high magnetic field, even in the absence of internal strain. This effect, reminiscent of the Coulomb pseudogap in localized electronic states, affects only electrons in the immediate vicinity of the Fermi level. It presents the most striking departure from the non-interacting picture of electrons in bulk bismuth.

Interplay between Kondo suppression and Lifshitz transitions in YbRh2Si2 at high magnetic fields.

We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity, and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands.

Fermi-surface reconstruction by stripe order in cuprate superconductors.

The origin of pairing in a superconductor resides in the underlying normal state. In the cuprate high-temperature superconductor YBa(2)Cu(3)O(y) (YBCO), application of a magnetic field to suppress superconductivity reveals a ground state that appears to break the translational symmetry of the lattice, pointing to some density-wave order. Here we use a comparative study of thermoelectric transport in the cuprates YBCO and La(1.8-x)Eu(0.2)Sr(x)CuO(4) (Eu-LSCO) to show that the two materials exhibit the same process of Fermi-surface reconstruction as a function of temperature and doping. The fact that in Eu-LSCO this reconstruction coexists with spin and charge modulations that break translational symmetry shows that stripe order is the generic non-superconducting ground state of hole-doped cuprates.

Nernst and Seebeck coefficients of the cuprate superconductor YBa2Cu3O6.67: a study of Fermi surface reconstruction.

The Seebeck and Nernst coefficients S and nu of the cuprate superconductor YBa{2}Cu{3}O{y} (YBCO) were measured in a single crystal with doping p=0.12 in magnetic fields up to H=28 T. Down to T=9 K, nu becomes independent of field by H approximately 30 T, showing that superconducting fluctuations have become negligible. In this field-induced normal state, S/T and nu/T are both large and negative in the T-->0 limit, with the magnitude and sign of S/T consistent with the small electronlike Fermi surface pocket detected previously by quantum oscillations and the Hall effect. The change of sign in S(T) at T approximately 50 K is remarkably similar to that observed in La2-xBaxCuO4, La{2-x-y}Nd{y}Sr_{x}CuO{4}, and La{2-x-y}Eu{y}Sr{x}CuO{4}, where it is clearly associated with the onset of stripe order. We propose that a similar density-wave mechanism causes the Fermi surface reconstruction in YBCO.

Broken rotational symmetry in the pseudogap phase of a high-T(c) superconductor.

The nature of the pseudogap phase is a central problem in the effort to understand the high-transition-temperature (high-T(c)) copper oxide superconductors. A fundamental question is what symmetries are broken when the pseudogap phase sets in, which occurs when the temperature decreases below a value T*. There is evidence from measurements of both polarized neutron diffraction and the polar Kerr effect that time-reversal symmetry is broken, but at temperatures that differ significantly from one another. Broken rotational symmetry was detected from both resistivity measurements and inelastic neutron scattering at low doping, and from scanning tunnelling spectroscopy at low temperature, but showed no clear relation to T*. Here we report the observation of a large in-plane anisotropy of the Nernst effect in YBa(2)Cu(3)O(y) that sets in precisely at T* throughout the doping phase diagram. We show that the CuO chains of the orthorhombic lattice are not responsible for this anisotropy, which is therefore an intrinsic property of the CuO(2) planes. We conclude that the pseudogap phase is an electronic state that strongly breaks four-fold rotational symmetry. This narrows the range of possible states considerably, pointing to stripe or nematic order.

Enhancement of the Nernst effect by stripe order in a high-T(c) superconductor.

The Nernst effect in metals is highly sensitive to two kinds of phase transition: superconductivity and density-wave order. The large, positive Nernst signal observed in hole-doped high-T(c) superconductors above their transition temperature (T(c)) has so far been attributed to fluctuating superconductivity. Here we report that in some of these materials the large Nernst signal is in fact the result of stripe order, a form of spin/charge modulation that causes a reconstruction of the Fermi surface. In La(2-x)Sr(x)CuO(4) (LSCO) doped with Nd or Eu, the onset of stripe order causes the Nernst signal to change from being small and negative to being large and positive, as revealed either by lowering the hole concentration across the quantum critical point in Nd-doped LSCO (refs 6-8) or by lowering the temperature across the ordering temperature in Eu-doped LSCO (refs 9, 10). In the second case, two separate peaks are resolved, respectively associated with the onset of stripe order at high temperature and superconductivity near T(c).

Electron pockets in the Fermi surface of hole-doped high-Tc superconductors.

High-temperature superconductivity in copper oxides occurs when the materials are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear, and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of the electronic states in the underdoped 'YBCO' materials YBa2Cu3O(y) and YBa2Cu4O8 was recently shown to include small pockets, in contrast with the large cylinder that characterizes the overdoped regime, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic-field-induced normal state of YBa2Cu3O(y) and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order. Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-transition-temperature (T(c)) superconductors.

Continuous evolution of the Fermi surface of CeRu2Si2 across the metamagnetic transition.

We present new, high resolution Hall effect and magnetoresistance measurements across the metamagnetic transition in the heavy fermion compound CeRu2Si2 . The results, and ambiguities in the interpretation of de Haas-van Alphen data, force us to rethink the notion that the transition is accompanied by an abrupt f-electron localization. Instead, we explain our data assuming a continuous evolution of the Fermi surface, which sees one of the spin-split sheets of the heaviest surface shrink to a point.

[Cholecystectomy using a minilaparotomy]. / Cholécystectomie par mini-laparotomie.

Recent randomized studies have shown that laparoscopic cholecystectomy has little or even no advantage when compared to minilaparotomy cholecystectomy. The authors report the results of a prospective study of minilaparotomy performed at Dahr el Bacheq governmental hospital where laparoscopic equipment was not available. From July 1994 to July 1997 minilaparotomy cholecystectomy was performed on one hundred consecutive patients (75 women and 25 men with an age varying between 26 and 93 years). However, the cholecystectomy could be accomplished through the mini-incision in only 88 cases. Lengthening of the incision was necessary in 12 cases: common duct stones (8 cases), cancer (2 cases), cholecystoduodenal fistula (2 cases). Intraoperative cholangiography was not performed in 3 cases: very thin cystic duct (2 cases), technical problem (1 case). Two patients operated for acute cholecystitis had wound infection. Postoperative course of the 88 completed minilaparotomy cholecystectomies was uneventful: no mortality, no biliary complications, little pain with low analgesia requirement, oral intake on day 1, discharge from hospital on day 2, return to normal activity between day 8 and day 14. Results of minilaparotomy cholecystectomy compare favorably with those of laparoscopic cholecystectomy. It should be an alternative to laparoscopic cholecystectomy especially when cost is a problem or when laparoscopic equipment is not available and an alternative to conventional open cholecystectomy in the case of contraindication to laparoscopic cholecystectomy.

[Hyperthyroidism and cancer of the thyroid]. / Hyperthyroïdie et cancer de la thyroïde.

STUDY AIM: The idea that thyrotoxicosis is insurance against thyroid cancer has prevailed for a long time. However this association has been reported frequently in the recent literature. The aim of this study is to report our experience concerning this association and to discuss its incidence and its consequences in the light of the present data. PATIENTS AND METHOD: Among the 734 patients who underwent a thyroidectomy between January 1983 and January 1998, there were 125 patients with hyperthyroidism, ten with associated cancer (8%). All patients were female (mean age: 43.8 years) with Graves' disease (n = 2), a toxic adenoma (n = 5), or a multi nodular toxic goiter (n = 3). The cancer was recognized intraoperatively in eight patients. A total thyroidectomy was performed in five cases and a total lobectomy associated with a subtotal contralateral lobectomy in five cases, completed by a treatment with l131 in nine cases. RESULTS: Concurrent carcinoma was present in 10% of the patients with toxic nodular goiter and in 4.4% in those with Graves' disease. Papillary carcinoma presented in all cases. The size of the cancer was between 0.2 and 1.5 cm. Nodal involvement was present in one patient. With a follow-up of 1 to 15 years, there was no local recurrence and no metastasis. CONCLUSION: The diagnosis of hyperthyroidism does not preclude concurrent thyroid carcinoma. The prognostic and therapeutic consequences of this association are still debated.

[Thyroidectomy without drainage]. / Thyroïdectomie sans drainage.

Recent randomized series did not support routine prophylactic drainage after thyroidectomy. We undertook a prospective study in order to evaluate the effectiveness and the morbidity of a non drainage strategy after thyroidectomy. Between april 1993 and may 1995, one hundred fifty consecutive patients underwent thyroidectomy without drainage. During this period, two thyroid cancers were treated by total thyroidectomy with a modified radical neck dissection and drainage; they are not included in the study. Age range was 16 to 72 years. Sex ratio was 126F/124M. Indication for surgery was: solitary nodule (16), multinodular goiter (56), Graves' disease (21), toxic nodular goiter (34), cancer (8), retrosternal goiter (13), thyroiditis (2). The surgery done was: total lobectomy + isthmusectomy (15), total lobectomy + subtotal controlateral thyroidectomy (42), bilateral subtotal thyroidectomy (84), total thyroidectomy (9). Surgical technique was identical to that used previously by the author when drainage was installed routinely. Patients left the hospital on the first or second postoperative day and were reexamined on day 7 and day 30. There was no mortality, no suffocating hematoma, no reoperation and no laryngeal nerve paralysis. One patient developed a transient hypocalcemia that regressed one month later. Two patients developed a minor hematoma of which one disappeared after two weeks and the other drained spontaneously through the surgical incision on the seventh postoperative day. We conclude that drainage after thyroidectomy has no adverse effects and can be avoided if meticulous surgery is done. Absence of drainage simplifies the early postoperative course, improves the comfort of the patient, decreases hospital stay and reduces hospital cost. However, drainage may be of value in case of hemostatic problems or associated cervical neck dissection.

[Papillary carcinoma developed in the wall of a thyroglossal duct cyst]. / Carcinome papillaire développé dans la paroi d'un kyste du tractus thyréoglosse.

A case of papillary carcinoma arising in the wall of a thyroglossal duct cyst is described. This is a rare occurrence (1% of cases). Controversies exist concerning its nature (cancer of the thyroid or primary cancer of the thyroglossal cyst) and its treatment (Sistrunk's operation alone or combined with thyroidectomy).

[Retro-costo-xyphoid hernia in adults. Apropos of 3 cases]. / Hernie rétro-costo-xyphoïdienne de l'adulte. A propos de 3 observations.

Three cases of retrosternal diaphragmatic hernia (Morgagni hernia) are reported. Diagnosis have been made by conventional radiology (Chest X-ray, Upper G-I series) in 2 cases: the third one was found incidentally during a laparotomy. Peritoneography, CT scan, NMR are recommended in the difficult cases. ONe case presented as acute gastric outlet obstruction secondary to an intra-thoracic volvulus of the herniated stomach. Surgical treatment is indicated in all cases of retrosternal diaphragmatic hernia because of the high-risk of complications (gastric volvulus, colonic obstruction). Surgery through an abdominal approach is preferred and post-operative course is benign.

[Substernal goiters]. / Les goîtres plongeants.

In a series of 363 patients operated for goiter, 60 were retrosternal (16.5%). The female incidence although less than in cervical goiters remains predominant (2,5:1). Most patients are in older age group, 68% being above 40 years. The majority of goiters are multinodular and of long duration. Bilateral (37%) and left sided location (43%) were common. The incidence of cancer was 7%, thyrotoxicosis was noted in 5 patients (8%). Most patients were symptomatic (97%). Diagnosis is easily done by physical examination, chest X-Ray and thyroid scintigraphy. Computed tomography might be helpful. Retrosternal goiter is an indication to surgery except in high risk patients. The cervical approach has been used in 98% of cases. There were no post-operative death and no major complications (compressive hematoma, laryngeal nerve paralysis, hypocalcemia).